WO2009107082A1

WO2009107082A1 - Apparatus and method for measuring chromaticity of light

Info

Publication number: WO2009107082A1
Application number: PCT/IB2009/050769
Authority: WO
Inventors: Ian Ashdown
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2008-02-28
Filing date: 2009-02-25
Publication date: 2009-09-03
Anticipated expiration: 2010-08-28

Abstract

Disclosed herein is a method and apparatus for measuring chromaticity of light emitted by a light source including an electroluminescent light-emitting element (202) and a photoluminescent material (204) optically coupled thereto. The apparatus includes a sensing system (210) configured to supply a first measurement indicative of intensity of light emitted by the electroluminescent light emitting element and a second measurement indicative of intensity of light emitted by the photoluminescent material; an initial chromaticity measurement system (240) configured to supply a first chromaticity measurement of light emitted by the electroluminescent light-emitting element; and a processing system (230), operatively coupled to the sensing system (210) and the initial chromaticity measurement system (240), configured to determine an overall chromaticity measurement of light emitted by the light source using the first measurement, the second measurement, the first chromaticity measurement, and prespecified information regarding chromaticity of light emitted by the photoluminescent material.

Description

APPARATUS AND METHOD FOR MEASURING CHROMATICITY OF LIGHT

FIELD OF THE INVENTION

[0001] The present invention is directed generally to illumination. More particularly, various inventive methods and apparatus disclosed herein relate to measuring particular attributes of light in an illumination system.

BACKGROUND

[0002] Digital lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full- spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects.

[0003] It is known that light of a desired spectral composition or, in photometric terms, a desired chromaticity and luminous flux, can be generated by intermixing adequate amounts of light from differently colored LEDs. When light from LEDs of different colors is intermixed; the chromaticity of the mixed light can be sufficiently accurately determined via several attributes such as the intensities, center wavelengths and spectral bandwidths of the LEDs.

[0004] Another possibility for generating light of a desired spectral composition is to include photoluminescent materials in the intermixing process. Photoluminescent materials, such as phosphor coatings or quantum dots, can operate to absorb and re-radiate light from LEDs, the re-radiated light potentially having different qualities than those of the incident light. Photoluminescent materials may therefore be used in conjunction with LEDs to produce light of a desired spectral composition by intermixing, and can be integrated into a single package, such as in phosphor-coated white light LEDs.

[0005] In operation of a typical phosphor-coated white light LED, a pump LED generates blue light, a portion of which is absorbed by the phosphor and re-emitted as yellow light to produce cool white light. If a second red-emitting phosphor is employed, a combination of blue, yellow and red light can produce warm white light. The chromaticity of this light can depend on the relative amounts of blue, yellow, and optionally red light. In other lighting applications, additional LEDs generating narrowband visible radiation, for example green, blue, red, or amber light, may be employed to maintain or controllably vary the chromaticity of the light generated by phosphor-coated white light LEDs. One drawbacks of this approach, however, is that it requires precisely measuring the chromaticity of the resultant white light. One such conventional apparatus includes a spectroradiometer configured to accurately measure the spectral power distribution of the white light and calculate its chromaticity based on that distribution. This apparatus is expensive, and requires considerable processing power to calculate the chromaticity for real-time applications such as optical feedback.

[0006] Another approach for measuring chromaticity includes three photosensors with optical filters whose spectral responsivities approximate the CIE color matching functions as defined in CIE 15:2004. It is impractical however to manufacture optical filters such that the photosensor spectral responsivities precisely match the CIE color matching functions. Consequently, the apparatus is capable of generating only approximate chromaticity values.

SUMMARY

[0007] An object of the present invention is to provide a cost-efficient apparatus for accurately measuring chromaticity of light while addressing at least some of the shortcomings of conventional approaches. The present invention stems from the realization that chromaticity of the combined light, for example, formed by an electroluminescent component light and a photoluminescent component light can be determined from relative intensities of the electroluminescent and the photoluminescent lights. Moreover, the relative intensities of the electroluminescent and the photoluminescent lights can be determined by analyzing the light itself.

[0008] Generally, in one aspect, the invention focuses on an apparatus for measuring chromaticity of combined light formed by a first component light emitted by an electroluminescent light-emitting element and a second component light emitted by a photoluminescent material. The first component light has a chromaticity characterized by a first chromaticity measurement. The apparatus includes a sensing system configured to respond to the combined light by supplying an intensity measurement indicative of relative intensities of the first component light and the second component light; and a processing system configured to determine the chromaticity of the combined light based at least in part on the intensity measurement and the first chromaticity measurement.

[0009] In some embodiments, the sensing system is further configured to sense an intensity of the first component light indicated by a first measurement, and an intensity of the second component light indicated by a second measurement. The intensity measurement includes the first measurement and the second measurement. In some versions of the embodiment, the first measurement is determined using a sensor configured to sense only the first component light, and the second measurement is determined using a sensor configured to sense only the second component light.

[0010] In one embodiment, the second component light has a chromaticity indicated by a second chromaticity measurement, and the processing system is further configured to determine the chromaticity of the light based on the second chromaticity measurement. For example, the processing system determines the second chromaticity measurement based on the intensity measurement, and the first chromaticity measurement.

[0011] In another embodiment, the first chromaticity measurement of the electroluminescent chromaticity is determined by an initial chromaticity measurement system. For example, in one version of the embodiment, the initial chromaticity measurement system includes a temperature sensor configured to measure a temperature of the electroluminescent light-emitting element; and a translator configured to translate the temperature into the first chromaticity measurement.

[0012] In accordance with another aspect of the present invention, the invention relates to an apparatus for measuring chromaticity of light emitted by a light source, the light source including an electroluminescent light-emitting element and a photoluminescent material optically coupled thereto. The apparatus includes a sensing system configured to respond to the light emitted by the light source by supplying a first measurement indicative of intensity of a first component light emitted by the electroluminescent light-emitting element and a second measurement indicative of intensity of a second component light emitted by the photoluminescent material; an initial chromaticity measurement system configured to supply a first chromaticity measurement of the first component light; and a processing system configured to determine a chromaticity of the light emitted by the light source using the first measurement, the second measurement, and the first chromaticity measurement.

[0013] In some embodiments, the second component light has a chromaticity indicated by a second chromaticity measurement, and the processing system is further configured to determine the chromaticity of the light using the second chromaticity measurement. For example, the processing system determines the second chromaticity measurement based on the intensity measurement, and the first chromaticity measurement. Alternatively, the second chromaticity measurement can be determined based on a temperature of the light emitted by the photoluminescent material.

[0014] In one embodiment, the sensing system includes a first sensor configured to sense only the first component light emitted, and a second sensor configured to sense only the second component light. In another embodiment, the sensing system includes a bandpass electronic filter configured to represent the intensity of the first component light, and a lowpass electronic filter configured to represent the intensity of the second component light.

[0015] In some embodiments, the initial chromaticity measurement system includes a temperature sensor configured to measure a temperature of the electroluminescent light- emitting element, and a translator configured to translate the temperature into the first chromaticity measurement. In another embodiment, the initial chromaticity measurement system is configured to measure a forward voltage of the electroluminescent light-emitting element and further determine the first chromaticity measurement based on the forward voltage. Other embodiments contemplate determining the first chromaticity measurement based on a package temperature and/or a peak wavelength of the electroluminescent light- emitting element. In yet another embodiment, the initial chromaticity measurement system includes two or more photosensors having substantially different spectral responsivities, wherein the initial chromaticity measurement system is configured to determine the first chromaticity measurement based on outputs of the two or more photosensors.

[0016] In accordance with yet another aspect, the invention relates to a method for measuring chromaticity of light emitted by a light source, the light source comprising an electroluminescent light-emitting element and a photoluminescent material optically coupled thereto. The method includes the steps of (i) obtaining, in response to the light emitted by the light source, a first measurement indicative of intensity of a first component light emitted by the electroluminescent light-emitting element and a second measurement indicative of intensity of a second component light emitted by the photoluminescent material and (ii) obtaining an initial chromaticity measurement of the first component light; and processing the first measurement, the second measurement, and the first chromaticity measurement to determine a chromaticity of the light emitted by the light source.

[0017] As used herein for purposes of the present disclosure, the term "light-emitting element" (LEE) should be understood to include a device that emits radiation in a region or combination of regions of the electromagnetic spectrum, for example the visible region, infrared and/or ultraviolet region. Therefore a light-emitting element can have monochromatic, quasi-monochromatic, polychromatic or broadband spectral emission characteristics. One example of the LEE is a "light-emitting diode" (LED), which is used herein to refer to any electroluminescent diode or other type of carrier injection/junction- based system that is capable of generating radiation in response to an electric signal. Thus, the term LED includes, but is not limited to, various semiconductor-based structures that emit light in response to current, light emitting polymers, organic light emitting diodes (OLEDs), electroluminescent strips, and the like. In particular, the term LED refers to light emitting diodes of all types (including semi-conductor and organic light emitting diodes) that may be configured to generate radiation in one or more of the infrared spectrum, ultraviolet spectrum, and various portions of the visible spectrum (generally including radiation wavelengths from approximately 400 nanometers to approximately 700 nanometers). Some examples of LEDs include, but are not limited to, various types of infrared LEDs, ultraviolet LEDs, red LEDs, blue LEDs, green LEDs, yellow LEDs, amber LEDs, orange LEDs, and white LEDs (discussed further below). It also should be appreciated that LEDs may be configured and/or controlled to generate radiation having various bandwidths (e.g., full widths at half maximum, or FWHM) for a given spectrum (e.g., narrow bandwidth, broad bandwidth), and a variety of dominant wavelengths within a given general color categorization.

[0018] For example, one implementation of an LED configured to generate essentially white light (e.g., a white LED) may include a number of dies which respectively emit different spectra of electroluminescence that, in combination, mix to form essentially white light. In another implementation, a white light LED may be associated with a phosphor material that converts electroluminescence having a first spectrum to a different second spectrum. In one example of this implementation, electroluminescence having a relatively short wavelength and narrow bandwidth spectrum "pumps" the phosphor material, which in turn radiates longer wavelength radiation having a somewhat broader spectrum.

[0019] It should also be understood that the term LED does not limit the physical and/or electrical package type of an LED. For example, as discussed above, an LED may refer to a single light emitting device having multiple dies that are configured to respectively emit different spectra of radiation (e.g., that may or may not be individually controllable). Also, an LED may be associated with a phosphor that is considered as an integral part of the LED (e.g., some types of white LEDs). In general, the term LED may refer to packaged LEDs, non- packaged LEDs, surface mount LEDs, chip-on-board LEDs, T-package mount LEDs, radial package LEDs, power package LEDs, LEDs including some type of encasement and/or optical element (e.g., a diffusing lens), etc. [0020] The term "electroluminescent light-emitting element" is used to describe a primary LEE which emits a certain colour or spectral distribution of light. The electroluminescent LEE typically emits light in response to an electrical stimulus, such as voltage or current. For example, a blue pump LED used to illuminate a phosphor is an electroluminescent LEE.

[0021] The term "photoluminescent material" is used to describe a material which can absorb and re-radiate electromagnetic radiation to emit light in response to incident light. The emitted light can be of a different nature than the incident light, for example with respect to the wavelengths of light. Examples of photoluminescent materials include phosphor coatings, such as YAG:Ce³⁺, TAG:Ce³⁺, YAG:Cr, and quantum dots, or a combination of multiple phosphor coatings or other materials having desired qualities, such as colour of light.

[0022] The term "optical sensor" is used to define an optical device having a measurable sensor parameter in response to a characteristic of incident light, such as its luminous flux or radiant flux. For example, an optical sensor may generate an electrical signal in response to a characteristic of incident light. More specifically, the term "broadband optical sensor" is used to define an optical sensor that is responsive to light within a wide range of wavelengths, such as the visible spectrum, while the term "narrowband optical sensor" is used to define an optical sensor that is responsive to light within a narrow range of wavelengths, such as the red region of the visible spectrum.

[0023] The term "chromaticity" is used to define the perceived colour impression of light according to standards of the Illuminating Engineering Society of North America. Chromaticity can be represented by a vector quantity according to these or other standards or other methods as would be readily understood by a worker skilled in the art.

[0024] The term "luminous flux" is used to define the instantaneous quantity of visible light emitted by a light source according to standards of the Illuminating Engineering Society of North America.

[0025] The term "spectral radiant flux" is used to define the instantaneous quantity of electromagnetic power emitted by a light source at a specified wavelength according to standards of the Illuminating Engineering Society of North America.

[0026] The term "spectral radiant power distribution" is used to define the distribution of spectral radiant flux emitted by a light source over a range of wavelengths, such as the visible spectrum for example.

[0027] The term "radiant flux" is used to define the sum of spectral radiant flux emitted by a light source over a specified range of wavelengths.

[0028] The term "intensity of light" is used to define a general quantity representative of radiant flux, for example a radiant flux or luminous flux or time-averaged representation thereof over one or more predetermined time periods.

[0029] The term "light source" should be understood to refer to any one or more of a variety of radiation sources, including, but not limited to, LED-based sources (including one or more LEDs as defined above), incandescent sources (e.g., filament lamps, halogen lamps), fluorescent sources, phosphorescent sources, high-intensity discharge sources (e.g., sodium vapor, mercury vapor, and metal halide lamps), lasers, other types of electroluminescent sources, pyro-luminescent sources (e.g., flames), candle-luminescent sources (e.g., gas mantles, carbon arc radiation sources), photo-luminescent sources (e.g., gaseous discharge sources), cathode luminescent sources using electronic satiation, galvano-luminescent sources, crystallo-luminescent sources, kine-luminescent sources, thermo-luminescent sources, triboluminescent sources, sonoluminescent sources, radioluminescent sources, and luminescent polymers.

[0030] The term "spectrum" should be understood to refer to any one or more frequencies (or wavelengths) of radiation produced by one or more light sources. Accordingly, the term "spectrum" refers to frequencies (or wavelengths) not only in the visible range, but also frequencies (or wavelengths) in the infrared, ultraviolet, and other areas of the overall electromagnetic spectrum. Also, a given spectrum may have a relatively narrow bandwidth (e.g., a FWHM having essentially few frequency or wavelength components) or a relatively wide bandwidth (several frequency or wavelength components having various relative strengths). It should also be appreciated that a given spectrum may be the result of a mixing of two or more other spectra (e.g., mixing radiation respectively emitted from multiple light sources).

[0031] The term "controller" is used herein generally to describe various apparatus relating to the operation of one or more light sources. A controller can be implemented in numerous ways (e.g., such as with dedicated hardware) to perform various functions discussed herein. A "processor" is one example of a controller which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform various functions discussed herein. A controller may be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions.

[0032] In various implementations, a processor or controller may be associated with one or more storage media (generically referred to herein as "memory," e.g., volatile and nonvolatile computer memory such as RAM, PROM, EPROM, and EEPROM, floppy disks, compact disks, optical disks, magnetic tape, etc.). In some implementations, the storage media may be encoded with one or more programs that, when executed on one or more processors and/or controllers, perform at least some of the functions discussed herein. Various storage media may be fixed within a processor or controller or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller so as to implement various aspects of the present invention discussed herein.

[0033] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. [0035] Figure 1 illustrates the spectral radiant power distribution of a cool white light

LED, comprising a blue pump LED and a yellow phosphor, wherein the contribution of the blue pump LED is essentially disjoint from the contribution of the phosphor.

[0036] Figure 2 illustrates a block diagram of an apparatus for measuring chromaticity of light according to one embodiment of the present invention.

[0037] Figure 3 illustrates a block diagram of an apparatus for measuring chromaticity of light according to another embodiment of the present invention.

[0038] Figure 4 illustrates a block diagram of a portion of an apparatus for measuring chromaticity of light according to another embodiment of the present invention.

DETAILED DESCRIPTION

[0039] In its various embodiments and implementations, the present invention relates to a method and apparatus for measuring chromaticity of light emitted by a light source. Such a light source includes at least an electroluminescent light-emitting element, e.g., an LED, configured to emit an electroluminescent light and a photoluminescent material, e.g., a phosphor coating, optically coupled to the electroluminescent light-emitting element, and configured to emit an electroluminescent light. An apparatus for measuring chromaticity includes a sensing system optically coupled to the light source, and a processing system. The apparatus may also include an initial chromaticity measurement system operatively coupled to the light source.

[0040] In one embodiment, the sensing system is configured to respond to the light source by supplying an intensity measurement indicative of relative intensities of the electroluminescent light and the photoluminescent light. According to some embodiments, the intensity measurement includes a first measurement indicative of the intensity of a first component light emitted by the electroluminescent light-emitting element and a second measurement indicative of the intensity of second component light emitted by the photoluminescent material. Thus, the sensing system is configured to respond to the light source by supplying the first and the second measurements.

[0041] The electroluminescent light has an electroluminescent chromaticity indicated by a first chromaticity measurement. In one embodiment, the initial chromaticity measurement system is configured to supply the first chromaticity measurement. Various embodiments also provide for measurements of the intensity and chromaticity of light emitted by other light-emitting elements which are optionally present in the light source.

[0042] The processing system is operatively coupled to the sensing system and, optionally, to the initial chromaticity measurement system. The processing system is configured to determine a chromaticity of the light using the intensity measurement, and the first chromaticity measurement. In some embodiments, the processing system is configured to determine an overall chromaticity measurement of light emitted by the light source using the first measurement, the second measurement, the first chromaticity measurement, and prespecified information regarding chromaticity of light emitted by the photoluminescent material.

[0043] In one embodiment, operation of the processing system further includes determination, from information such as prespecified information or information collected by some of the modules of the apparatus, of a chromaticity value for the photoluminescent material indicated by a second chromaticity measurement and of the relative intensities of light emitted by the electroluminescent light-emitting element and the photoluminescent material. The determined chromaticities and relative intensities of the electroluminescent light-emitting element and the photoluminescent material, respectively, are used to determine an overall chromaticity measurement of light emitted by the light source according to Grassman's law of additivity. For example, if ( x_L, y_L ) and ( x_P, y_P ) are vectors representing chromaticity of the light emitted by the electroluminescent LEE and the photoluminescent material, respectively, and I_L and I_P are values representing the relative intensities of light of the electroluminescent LEE and the photoluminescent material, respectively (where (I_L + I_P = I)), then a vector ( x_w, y_w ) representing chromaticity of the resultant white light can be calculated by Grassman's Law of Additivity, which can be represented as follows:

^XW ^~ * L ^XL + ^■* P^XP , ,. >

JV = hyL ^{+ I}py_P Sensing System

[0044] A sensing system is optically coupled to the light source comprising the electroluminescent LEE and photoluminescent material, and is configured to measure the intensity of light formed by an first component light emitted by the electroluminescent LEE, the electroluminescent light having an electroluminescent chromaticity indicated by a first chromaticity measurement, and a second component light emitted by the photoluminescent material. In one embodiment, the sensing system includes one or more photosensors, for example silicon photodiodes, configured for this purpose. An object of the sensing system, or of a corresponding method of sensing light, is to measure relative intensities of portions of light emitted by one or more of the photoluminescent materials and electroluminescent LEEs of the light source.

[0045] In some embodiment, the light source includes an electroluminescent LEE and a photoluminescent material, and the sensing system includes two optically filtered photosensors configured as follows. A first photosensor can be equipped with a broadband optical filter such that the spectral responsivity of the photosensor substantially spans a region of the electromagnetic spectrum that excludes the spectral power distribution of the photoluminescent material. For example, if the electroluminescent LEE is a pump LED emitting blue light with a peak wavelength of about 460 nm and a spectral bandwidth of about 30 nm, the spectral responsivity of the first photosensor may extend from about 500 nm to about 700 nm or some other range substantially excluding the blue LED light. The output of the first photosensor can thus be substantially directly proportional to the intensity of the light emitted by the photoluminescent material, such as the non-blue light emitted by a phosphor. A second photosensor is equipped with a narrowband optical filter such that the spectral responsivity of the second photosensor substantially spans a region of the electromagnetic spectrum that includes the spectral power distribution of the electroluminescent LEE while substantially excluding the light emitted by the photoluminescent material. The output of this sensor is substantially directly proportional to the intensity of the electroluminescent LEE, for example the blue light emitted by a pump LED. [0046] In other embodiments, the sensing system includes a photosensor equipped with a broadband optical filter such that the spectral responsivity of the photosensor substantially spans a region of the visible spectrum that includes the spectral power distribution of both the electroluminescent LEE and the photoluminescent material. The output of the photosensor is operatively coupled to a lowpass electronic filter and to a bandpass electronic filter, and the electroluminescent LEE is modulated at or near a prespecified frequency. Since photoluminescent materials such as phosphors can continue to emit light for a time period following removal of incident light, light from the photoluminescent material (arising from photoluminescent response to the electroluminescent LEE) behaves in the manner of an optical low-pass filter such that the temporal modulation of the light from the photoluminescent material is reduced in comparison to the modulation of the light generated by the electroluminescent LEE. By configuring, in a manner such as is specified below, the modulation frequency of the electroluminescent LEE, the lowpass electronic filter cutoff frequency, and the bandpass electronic filter cutoff frequencies, the output of the lowpass electronic filter and the bandpass electronic filter is representative of the intensity of the photoluminescent material and the electroluminescent LEE, respectively.

[0047] For example, it is observed that the intensity of light emitted by photoluminescent material including common single phosphors at time t after removal of incident light can be modelled by an exponential decay equation which can be defined as follows:

W = I(0)e-"^T (2) where /(0) is the initial intensity immediately following removal of the excitation radiation, /_t is the intensity of light at time t, and r is a decay time constant. It is known that for many common single phosphors, I₀ will generally decrease with increasing temperature and phosphor aging, but r will generally remain substantially constant.

[0048] As another example, for a photoluminescent material including a mixture of two different phosphors, the intensity of light may be substantially modelled by the sum of two equations of the form of Equation (2) with different values of /(0) and rfor each equation. This can often be modeled as hyperbolic decay which can be defined as follows:

where a and /?are constants. Analogous models can be derived for photoluminescent material comprising three or more phosphors.

[0049] The decay time constant rfor photoluminescent materials such as modeled above in Equations (2) and (3) is defined as the time for the phosphor emission /(t) to decay from /(0) to /(0)/e, or about 0.368 /(0). Typical time constants for current phosphor-based photoluminescent materials can range from about 30 μsec to about 100 μsec for Eu-based phosphors and from about 10 nsec to about 30 nsec for Ce-based phosphors. For example, YAG:Ce³⁺ phosphors have decay time constants less than about 100 nsec. This corresponds to a frequency of greater than about 10 MHz.

[0050] There are, however, some phosphors (for example, YAG:Cr) with time constants on the order of milliseconds or longer. While such phosphors are not currently used for commercial light-emitting diodes, the state-of-the-art in phosphor research is widely acknowledged as being behind that of semiconductor LEDs. It is therefore a reasonable expectation that suitable phosphors with long time constants may eventually be developed. For such phosphors, a decay time constant of for example about one millisecond corresponds to a frequency of about 1 kHz.

[0051] In some embodiments, therefore, a light source in an illumination system includes an electroluminescent LEE such as an LED, and a photoluminescent material such as a phosphor, wherein the LED is modulated with a frequency substantially greater than the inverse of the phosphor time constant τ. For example, the LED may be modulated with a frequency of at least about ten times the inverse of the phosphor time constant. In one embodiment therefore, the intensity of light emitted by the phosphor exhibits substantially small fluctuation while the LED exhibits substantially large fluctuation. For this configuration, according to an embodiment of the present invention, a single photosensor, equipped with a broadband optical filter such that the spectral responsivity of the photosensor substantially spans a region of the visible spectrum that includes the spectral power distribution of the LED and the phosphor, is employed in the sensing system. The output of this photosensor is operatively coupled to a lowpass electronic filter having a cutoff frequency substantially less than the LED modulation frequency, and to a band pass electronic filter having a center frequency substantially equal to the LED modulation frequency. The output of the lowpass filter can therefore be substantially directly proportional to the intensity of the light emitted by the phosphor plus the time-averaged intensity of the light emitted by the LED, while the time-averaged output of the bandpass filter can be substantially directly proportional to the intensity of the light emitted by the LED. If desired, the output of the band pass filter can be subtracted from the output of the low pass filter to obtain the intensity of light emitted by the phosphor.

[0052] The sensing system can be configured to account for absorbing properties of photoluminescent materials when measuring intensity of light. For example, the characteristics of an electroluminescent LEE optically coupled to a photoluminescent material can be different than the characteristics of an electroluminescent LEE not so optically coupled, since the photoluminescent material absorbs a substantial portion of radiation from the electroluminescent LEE. The sensing system and the processing system can be configured to account for this in a measurement or correlation by methods as would be known to a worker skilled in the art. Similarly, other inherent absorption, reflection, scattering or other effect as would be readily understood by a worker skilled in the art inherent in an embodiment of the present invention may be similarly compensated for.

Initial Chromaticity Measurement System

[0053] Typically, the chromaticity of an electrolumious LEE can be expected to vary over time, due to factors such as temperature variation and device aging, among other factors as would be readily understood by a worker skilled in the art. An apparatus according to the present invention is therefore further configured, according to one embodiment, to provide a first initial chromaticity measurement system to measure, approximate, or otherwise infer the current chromaticity of the electrolumious LEE. In various embodiments, chromaticity measurement is done directly, or by indirect methods such as measuring one or more attributes which can be correlated with chromaticity, such as temperatures, voltages, peak wavelengths, device ages, or other attributes as would be known to a worker skilled in the art.

[0054] In one embodiment, chromaticity of the electroluminescent LEE is correlated, for example experimentally, with an associated temperature value, and the chromaticity thus indirectly measured by measuring said temperature. For example it is known that the chromaticity of an LED, such as a blue pump LED commonly used to irradiate a phosphor coating, can vary with LED package or junction temperature. Accordingly, in some embodiments, the initial chromaticity measurement system includes a temperature sensor operatively coupled to the electroluminescent LEE, and a processor or other means which translates temperature measurements into chromaticity values according to a predetermined translation operation. For example, in one embodiment, the translation operation interpolates experimentally determined or model-based correlations between temperature and chromaticity to infer chromaticity of the electroluminescent LEE.

[0055] In another embodiment, chromaticity of an electroluminescent LEE is correlated with a peak wavelength of the spectral power distribution of light emitted thereby, and said peak wavelength is further correlated with electroluminescent LEE temperature. For example, the peak wavelength of certain types of pump LED is temperature-dependent, and can typically vary by about 0.04 nm/°C during typical operation. Moreover, spectral bandwidth of the spectral power distribution generally increases with the peak wavelength. Therefore, a spectral bandwidth value can be inferred from the peak wavelength. For many LEDs, the spectral power distribution can be accurately modeled by a family of distributions parameterized by a peak wavelength value and a spectral bandwidth value. Since chromaticity can be determined from spectral power distribution, LED chromaticity can therefore be substantially accurately estimated from the measured LED junction temperature. This can be determined for example by measuring the LED forward voltage or the LED package temperature. [0056] In one embodiment, correlation of peak wavelength with chromaticity is performed experimentally by measuring, for various LED temperatures, various chromaticities and peak wavelengths of light emitted by said LED using for example a spectroradiometer. In at least one embodiment, this correlation is pre-defined, for example according to laboratory measurements, and interpolated to determine a chromaticity value corresponding to a range of peak wavelengths.

[0057] In another embodiment, correlation of peak wavelength with chromaticity is performed by a combination of experimental measurements and LED modeling, for example computer, physical, or mathematical modeling. The correlation is represented as a mathematical or functional relationship, or as a collection of tabulated correlations, possibly amenable to interpolation.

[0058] In yet another embodiment, the initial chromaticity measurement system includes two or more photosensors with substantially different spectral responsivities. The relative outputs of the photosensors is employed to measure attributes of light from the light source, and thereby determine the LEE chromaticity by correlating chromaticity with said measured attributes for a given LEE. For example, the relative outputs of two or more photosensors can be used to determine the intensity of light for a multiplicity of predetermined spectral ranges, with predetermined sensitivity at each frequency, corresponding to the configuration of the photosensors. By a suitable parameterization of light emitted by the LEE, said intensity measurements can be correlated with an appropriate spectral power distribution or with an appropriate chromaticity. If a greater number of photosensors with different spectral responsivities are used, the initial chromaticity measurement system, in one embodiment, relies less on parameterization and more on the relative photosensor outputs, since the photosensor measurements is combined in this case to substantially comprise a spectroradiometer.

[0059] In a further embodiment, the photosensor arrangement discussed above includes one or more photosensors of the sensing system, for example the second narrowband photosensor measuring the intensity of light emitted by the electroluminescent LEE. In this manner the number of components of the invention is reduced. [0060] In one embodiment, additional initial chromaticity measurement systems are provided to measure chromaticity of additional light-emitting elements within the light source. These systems operate similarly to the first initial chromaticity measurement system, and can possibly share system components, such as photosensors and processors, where convenient. The output of each additional chromaticity measurement system is optionally coupled to the processing system for determination of the chromaticity of the light emitted by the light source as described below.

Processing System

[0061] An apparatus according to many embodiments of the present invention includes a processing system operatively coupled to the sensing system, as well as, optionally, the first initial chromaticity measurement system, and, if present, additional initial chromaticity measurement systems. The processing system is configured to determine an overall chromaticity measurement of light emitted by the light source using, depending on embodiments, the intensity measurement, e.g., the first measurement and the second measurement, the first chromaticity measurement, any additional chromaticity measurements, and prespecified information regarding chromaticity of light emitted by the photoluminescent material.

[0062] In many embodiments, the processing system is configured to determine an overall chromaticity measurement of light emitted by the light source substantially according to Grassman's law of additivity, such as in Equation (1). Such a determination requires, in addition to the information above, a chromaticity value of the photoluminescent light, which can be obtained in several ways, for example using techniques discussed below.

[0063] In some embodiments, chromaticity of light emitted by the photoluminescent material is measured directly using a photosensor. For photoluminescent materials, spectral power distribution can be substantially independent of temperature. For example, it is known that the spectral power distributions and hence chromaticity of many broadband phosphors such as YAG:Ce³⁺ and TAG:Ce³⁺ are substantially independent of their temperature. The chromaticities of such photoluminescent materials can therefore be substantially accurately measured with for example a silicon photodiode equipped with an optical filter. For example, such a photosensor may be provided having a spectral responsivity substantially the same as the CIE V-lambda luminous efficiency function as defined in CIE 15:2004. The output of the photosensor may be correlated with chromaticity of the photoluminescent material in a predetermined manner, thereby supplying a chromaticity measurement of the photoluminescent material.

[0064] In other embodiments, chromaticity of light emitted by the photoluminescent material is measured indirectly. For example, it is known that the chromaticity of many phosphor-coated white light LEDs is dependent upon several factors, including phosphor emission spectrum; phosphor excitation spectrum; phosphor temperature; phosphor aging; and pump LED spectral power distribution, among other factors as would readily be understood by a worker skilled in the art. Therefore, in various embodiments, one or more of the above factors are measured and correlated with chromaticity of a photoluminescent material to obtain a chromaticity measurement thereof. Alternatively, in one embodiment, chromaticity of the photoluminescent material is prespecified, for example as a fixed or variable value or range of values substantially independent of the above-mentioned factors.

[0065] In one particular embodiment, chromaticity of light emitted by the photoluminescent material is measured indirectly by correlation with a spectral power distribution of an electroluminescent LEE, as measured for example by the initial chromaticity measurement system. For example, the relative amounts of blue, yellow, and optionally red light emitted by a typical phosphor-coated white light LED above can vary depending on the spectral power distribution of light incident on the phosphor, for example as emitted by a pump LED. As mentioned in discussing the initial chromaticity measurement system, as the junction temperature of a typical pump LED increases, the peak wavelength may shift towards longer wavelengths, and the spectral bandwidth may broaden. Other types of LEEs may have analogous characteristics which may be similarly exploited. Depending on the phosphor excitation spectra (which can be different for different phosphors, for example yellow-emitting and red-emitting phosphors), the conversion efficiency of the phosphor may change, for example the conversion efficiency may decrease, thereby changing the relative amounts of light of various wavelengths, for example of blue, yellow, and optionally red light. Therefore, a spectral power measurement, chromaticity measurement, or temperature measurement of the electroluminescent LEE, for example as supplied by the sensing system or an initial chromaticity measurement system, or other related measurement, can be used to provide a chromaticity measurement of the photoluminescent material through a correlation operation.

[0066] In another embodiment, chromaticity of light emitted by the photoluminescent material can be measured indirectly by correlation with temperature of the photoluminescent material. For example, the conversion efficiency of typical phosphors can also depend on their temperature, an effect known as "thermal quenching." For example, the photon conversion efficiency of cerium-activated yttrium and terbium aluminum garnet phosphors (YAG:Ce³⁺ and TAG:Ce³⁺) typically used to generate yellow light steadily decreases with increasing temperature. Their relative efficiencies at 150⁵C in typical operation can be approximately 85% and 60% respectively of their efficiencies at 25⁵C. This again results in changes in the relative amounts of blue, yellow, and optionally red light. Therefore, a temperature measurement of the electroluminescent LEE or the photoluminescent material, for example as supplied by the sensing system or an initial chromaticity measurement system, or other related measurement, can be used to provide a chromaticity measurement of the photoluminescent material through a correlation operation.

[0067] In one embodiment of the present invention, the chromaticity of the phosphor is determined by direct measurement in the laboratory or by measuring the spectral power distribution of a white light LED and subtracting the contribution of the blue pump LED. Figure 1 illustrates the spectral power distribution of a cool white light LED, wherein the contribution of the blue pump LED is essentially disjoint from the yellow emission SPD of the phosphor. The spectral power distribution of the blue LED can be accurately estimated or modeled using for example a "double Gaussian" model. The double Gaussian model for LED emission spectra includes two Gaussian distributions with different peak wavelengths and full-width half-maximum (FWHM) widths. In one embodiment of the present invention the LED emission spectra is defined as follows:

where /₀₁ and I₀₂ are peak intensities, A₀₁ and A₀₂ are peak wavelengths, and AA₁ and AA₂ are FWHM spectral bandwidths.

[0068] Computation of Equation (1) by the processing system or related method requires measurement of the relative intensities of the electroluminescent LEE and the photoluminescent material, whereas the sensing system, in embodiments such as described above, provides absolute intensity measurements. In this regard, for example, suppose S₈ and S_N represent the outputs of the first broadband and second narrowband sensors of the sensing system, respectively. Then I_L can be represented as l_L=aS_N and I_P can be represented as

where « and /?are the sensor constants of proportionality such that: aS_N + βS_B = l (5)

For example, a=S_N/(S_N + S₈) and P=S₈Z(S_N + S₈). Equivalently, having solved Equation (5), the overall chromaticity measurement ( x_w, yw ) of light emitted by the light source can be accurately determined as follows: x_w = ccS_Nx_L + βS_BXp y_w = aS_Ny_L + βS_By_P

[0069] Over the range of 0 to 1 for the relative values of I_L and I_P, Equation (1) defines a straight line between the chromaticities ( x^, y_L ) and ( xp, yp) plotted on the CIE 1931 xy chromaticity diagram. Alternatively, over the range of 0 to 1 for the relative values of aS_N and 6S₈, Equation (6) defines a straight line between the chromaticities ( x^, y_L ) and ( x_P, y_P ) plotted on the CIE 1931 xy chromaticity diagram. In practice, the chromaticity of a phosphor-coated white light LED may not coincide with a point on this line. If so, the endpoints ( x_L, y_L ) and ( x_P, y_P) are adjusted as required to substantially ensure coincidence. [0070] In one embodiment, Grassman's Law of Additivity is applied to one or more light sources whose light is mixed, so that the above method is extended to include additional LEEs, for example narrowband LEDs employed to maintain or controllably vary the chromaticity of the light generated by phosphor-coated white light LEDs. For example, the combination of blue, green, and warm white LEDs can require narrowband photosensors for blue and green LEDs with outputs S_NB and S_NG, so that ( x_w, y_w ) can be accurately determined as follows: x_w = aS_NBx_B + βS_NGx_G + γS_Bx_P JV = aS_NBy_B + βS_my_G + γS_By_P where ( x_B, y_B ) is the chromaticity of the blue LEDs, ( x_G, y_G ) is the chromaticity of the green LEDs, S_NB represents the output of a narrowband photosensor for the blue LEDs, S_NG represents the output of a narrowband photosensor for the green LEDs, S_B represents the output of the broadband photosensor, and a, /?and γ are sensor constants of proportionality such that aS_NB +βS_m + χS_B = l (8)

[0071] Various inventive concepts disclosed herein will now be described with reference to specific examples. It will be understood that the following examples are intended to describe embodiments of the invention and are not intended to limit the invention in any way.

EXAMPLES EXAMPLE 1:

[0072] Figure 2 illustrates an apparatus for measuring chromaticity of a light source according to one embodiment of the present invention. The light source 200 includes a first LED 202 optically coupled to a phosphor material 204. The light source 200 is optically coupled to a sensing system 210 that includes two optical filters 212 and 214 for discrimination of light from light source component LED 202 and phosphor material 204, respectively. A first optical filter 212 is configured to substantially pass light from the first LED 202, while substantially blocking light from other sources. A second optical filter 214 is configured to substantially pass light from the phosphor material 204, while substantially blocking light from other sources. The sensing system further includes two photosensors, 222 and 224, for example, those employing silicon photodiodes, optically coupled to the two optical filters 212 and 214. Due to the optical coupling, the first photosensor 222 is configured to supply a measurement indicative of intensity light emitted by the first LED 202, and the second photosensor 224 is configured to supply a measurement indicative of intensity light emitted by the phosphor material 204.

[0073] Figure 2 further illustrates a first initial chromaticity measurement system 240, operatively coupled to the first LED 202 of the light source 200 and configured to measure the chromaticity thereof. In the present embodiment, the first initial chromaticity measurement system 240 includes a temperature sensor 242 configured to measure a temperature of the first LED 202. The temperature measurement is processed by processing devices 244 and 246 to determine the peak wavelength and spectral bandwidth, respectively, of the spectral power distribution of the first LED 202. Processing devices 244 and 246 are configured for this purpose in a predetermined manner as described above. For example processing device 244, according to one embodiment, accepts a temperature measurement as input to a lookup table or functional relationship based on prespecified information regarding peak wavelength of the spectral power distribution of the first LED as a function of temperature, for example as obtained through laboratory measurements. Processing device 244, according to another embodiment, supplies a peak wavelength value corresponding to the input temperature measurement, or alternatively an interpolated value based on a multiplicity of corresponding peak wavelength values. Similarly, in yet another embodiment, the processing device 246 accepts a temperature measurement as input to a lookup table or functional relationship and supply a corresponding spectral bandwidth value or interpolated value. In the present embodiment, processing devices 244 and 246 are operatively coupled to processing device 248, which accepts as input the peak wavelength value and spectral bandwidth value and supplies a chromaticity measurement for the first LED 202. Processing device 248 is configured for this purpose in a predetermined manner, for example based on a prespecified lookup table or functional relationship regarding the spectral power distribution parameters of peak wavelength and spectral bandwidth, and the chromaticity of the first LED 202.

[0074] Still referring to Figure 2, in an alternative embodiment, processing devices 244, 246, and 248 can be replaced by a single processing device which supplies a chromaticity value directly in response to a temperature measurement. Equivalently, processing devices 244, 246, and 248 can be associated with different aspects of the same processor. Either case can obviate a need for intermediately determining a peak wavelength value and a spectral bandwidth value.

[0075] Figure 2 further illustrates, according to one embodiment of the present invention, a processing system 230 operatively coupled to the sensing system 210, and the first initial chromaticity measurement system 240. The processing system 230 is configured to supply a chromaticity measurement of the light source 200 given measurements indicative of intensity light emitted by the first LED 202 and the phosphor material 204, as supplied by the two photosensors 222 and 224. The processing system 230 is further configured to supply a chromaticity measurement of the light source 200 given measurements indicative of chromaticity of the first LED 202, as supplied by the first initial chromaticity measurement system 240. In the present embodiment, operation of processing system 230 includes two sub-operations. In the first sub-operation, a processing device 232 (which is part of the processing system 230, according to one embodiment) is configured to accept as input the chromaticity value of the first LED 202 and the measurements indicative of intensity of light of the first LED 202 and the phosphor material 204, and is further configured to supply as output a chromaticity value of the phosphor material 204. Processing device 232 is configured for this purpose in a predetermined manner, for example based on a prespecified lookup table or functional relationship regarding the chromaticity values and intensities of light of the first LED 202 and the phosphor material 204, for example as predetermined by experimentation or modelling or a combination thereof. In the second sub-operation, a processing device 234 (which is part of the processing system 230, according to one embodiment) is configured to accept as input the chromaticity values of the first LED 202 and the phosphor material 204, and the measurements indicative of intensity of light of the first LED 202 and the phosphor material 204, and is further configured to supply as output a chromaticity value of the light source 204 at output 236. In one embodiment, the processing device 234 operates substantially according to Grassman's law of additivity for example as presented in Equation (6). For this purpose, processing device 234 is further configured to compute weighting factors a and β, such that Equation (5) is substantially satisfied.

EXAMPLE 2:

[0076] Figure 3 illustrates an apparatus for measuring chromaticity of a light source according to one embodiment of the present invention. The light source 300 includes a first LED 302 optically coupled to a phosphor material 304, and a second LED 306 capable of adjusting chromaticity of the light source 300. The light source 300 is optically coupled to a sensing system 310 that includes three optical filters 312, 314, and 316 for discrimination of light from light source components 302, 304, and 306, respectively. A first optical filter 312 is configured to substantially pass light from the first LED 302, while substantially blocking light from other sources. A second optical filter 314 is configured to substantially pass light from the phosphor material 304, while substantially blocking light from other sources. A third optical filter 316 is configured to substantially pass light from the second LED 306, while substantially blocking light from other sources. The sensing system further includes three photosensors, 322, 324, and 326, for example comprising silicon photodiodes, optically coupled to the three optical filters 312, 314, and 316. Due to the optical coupling, the first photosensor 322 is configured to supply a measurement indicative of intensity light emitted by the first LED 302, the second photosensor 324 is configured to supply a measurement indicative of intensity light emitted by the phosphor material 304, and the third photosensor 326 is configured to supply a measurement indicative of intensity light emitted by the second LED 306.

[0077] Figure 3 further illustrates, according to one embodiment of the present invention, a first initial chromaticity measurement system 340, operatively coupled to the first LED 302 of the light source 300 and configured to measure the chromaticity thereof. The first chromaticity measurement system 340 can operate similarly to the first chromaticity measurement system as described above in relation to Figure 2, and in this respect can, for example, include a temperature sensor 342 configured to measure a temperature of the first LED 302, the temperature measurement being processed by processing devices 344 and 346 to determine the peak wavelength and spectral bandwidth, respectively, of the spectral power distribution of the first LED 302. The first initial chromaticity measurement system 340 optionally further includes a processing device 348, operatively coupled to processing devices 344 and 346, the processing device 348 accepting as input the peak wavelength value and spectral bandwidth value and supplying a chromaticity measurement for the first LED 302.

[0078] In an alternative embodiment with respect to Figure 3, processing devices 344, 346, and 348 can be replaced by a single processing device which supplies a chromaticity value directly in response to a temperature measurement. Equivalently, processing devices 344, 346, and 348 can be associated with different aspects of the same processor. Either case can obviate any need for intermediately determining a peak wavelength value and a spectral bandwidth value.

[0079] Figure 3 further illustrates, according to one embodiment of the present invention, a second initial chromaticity measurement system 360, operatively coupled to the second LED 306 of the light source 300 and configured to measure the chromaticity thereof. The second initial chromaticity measurement system 360, in one embodiment, operates analogously to the first initial chromaticity measurement system 340 described above. In such an embodiment, the second initial chromaticity measurement system 360 includes a temperature sensor 362 operatively coupled to the second LED 306 and to two processing device 364 and 366 for determining a peak wavelength value and a spectral bandwidth value, respectively, of the second LED 306. Furthermore, the second initial chromaticity measurement system 360 includes a processing device 368 operatively coupled to the two processing devices 364 and 366 and configured to determine a chromaticity measurement for the second LED 306 based on the supplied peak wavelength value and spectral bandwidth value.

[0080] Figure 3 further illustrates, according to one embodiment of the present invention, a processing system 330 operatively coupled to the sensing system 310, the first initial chromaticity measurement system 340, and the second initial chromaticity measurement system 360. The processing system 330 is configured to supply a chromaticity measurement of the light source 300 given measurements indicative of intensity light emitted by the first LED 302, the phosphor material 304, and the second LED 306, as supplied by the three photosensors 322, 324, and 326. The processing system 330 is further configured to supply a chromaticity measurement of the light source 300 given measurements indicative of chromaticity of the first LED 302 and the second LED 306, as supplied by the two initial chromaticity measurement systems 340 and 360. In the present embodiment, operation of processing system 330 includes two sub-operations. In the first sub-operation, a processing device 332 (which is part of the processing system 330, according to one embodiment) is configured to accept as input the chromaticity value of the first LED 302 and the measurements indicative of intensity of light of the first LED 302 and the phosphor material 304, and is configured to supply as output a chromaticity value of the phosphor material 304. Processing device 332 is configured for this purpose in a predetermined manner, for example based on a prespecified lookup table or functional relationship regarding the chromaticity values and intensities of light of the first LED 302 and the phosphor material 304, for example as predetermined by experimentation or modelling or a combination thereof. In the second sub-operation, a processing device 334 (which is part of the processing system 330, according to one embodiment) is configured to accept as input the chromaticity values of the first LED 302, the phosphor material 304, and the second LED 306, and the measurements indicative of intensity of light of the first LED 302, the phosphor material 304, and the second LED 306, and is further configured to supply as output a chromaticity value of the light source 304 at output 336. In one embodiment, the processing device 334 operates substantially according to Grassman's law of additivity for example as presented in Equation (7). For this purpose, processing device 334 is further configured to compute weighting factors a, 6, and y, such that Equation (8) is satisfied.

EXAMPLE 3:

[0081] Figure 4 illustrates an example of a sensing system 410 comprising a portion of one embodiment of the present invention, optically coupled to a light source 400. The sensing system or a variant thereof as would be understood by a worker skilled in the art, can, for example, be used in place of the sensing system as depicted in Figures 2 and 3. The light source includes an electroluminescent light-emitting element 402 optically coupled to a photoluminescent material 404, the photoluminescent material having a decay time constant r as discussed above, for example as described in relation to Equations (2) and (3). The intensity of the electroluminescent light-emitting element 402 is modulated at a frequency substantially of k/τ, where k is a predetermined value such that e^~k is substantially less than one, for example k=10. The light source 400 is optically coupled to the sensing system 410, the sensing system having a photosensor 411. The photosensor 411 is configured to receive light emitted by both the electroluminescent light-emitting element 402 and the photoluminescent material 4O4_J and to provide a time-varying signal indicative of the combined time-varying radiant flux thereof.

[0082] Continuing with reference to the example illustrated in Figure 4, due to the non- instantaneous response characteristics of the phosphor as discussed above, for example as described in relation to Equations (2) and (3), the time-varying signal supplied by the photosensor 411 can be temporally filtered to discriminate radiant flux emitted by the electroluminescent light-emitting element 402 from radiant flux emitted by the photoluminescent material 404. For this purpose, the photosensor 411 is operatively coupled to electronic filters 415 and 416. Electronic filter 415 is configured to discriminate a time-average radiant flux due to the combination of the electroluminescent light-emitting element 402 and the photoluminescent material 404. Electronic filter 415 includes a low- pass temporal filter with a cut-off frequency selected so as to substantially filter out modulation of the electroluminescent light-emitting element 402. For example, the cut-off frequency can be selected as k/τ, or a marginally lower value as would be understood by a worker skilled in the art. Electronic filter 415 can be further configured to supply a signal indicative of the time-average radiant flux (or intensity of light) due to the combination of the electroluminescent light-emitting element 402 and the photoluminescent material 404, and the signal can be passed to a processing system similar to 230 or 330 for further processing. Electronic filter 416 is configured to discriminate a time-varying radiant flux due to the electroluminescent light-emitting element 402. For example, electronic filter 416 includes a band pass temporal filter configured so as to substantially filter out light modulated at all frequencies except those substantially of the electroluminescent light- emitting element 402. The band pass filter may pass frequencies substantially at or near k/τ. Electronic filter 416 is further operatively coupled to a time averaging system 417, the electronic filter 416 configured to supply a time-varying signal indicative of the time-varying radiant flux due to the electroluminescent light-emitting element 402 to the time averaging system 417. The time averaging system averages said time-varying signal to produce a signal indicative of the average radiant flux (or intensity of light) due to the electroluminescent light-emitting element 402, and the signal can be passed to a processing system similar to 230 or 330 for further processing. If required, the signal indicative of the average radiant flux due to the electroluminescent light-emitting element 402 can be subtracted from the signal indicative of the time-average radiant flux due to the combination of the electroluminescent light-emitting element 402 and the photoluminescent material 404 by either the sensing system or the processing system to obtain a separate signal indicative of the time-average radiant flux due to the photoluminescent material 404 alone.

[0083] While several inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.

[0084] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.

[0085] The indefinite articles "a" and "an," as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean "at least one."

[0086] The phrase "and/or," as used herein in the specification and in the claims, should be understood to mean "either or both" of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with "and/or" should be construed in the same fashion, i.e., "one or more" of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the "and/or" clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to "A and/or B", when used in conjunction with open-ended language such as "comprising" can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. [0087] As used herein in the specification and in the claims, "or" should be understood to have the same meaning as "and/or" as defined above. For example, when separating items in a list, "or" or "and/or" shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as "only one of" or "exactly one of," or, when used in the claims, "consisting of," will refer to the inclusion of exactly one element of a number or list of elements. In general, the term "or" as used herein shall only be interpreted as indicating exclusive alternatives (i.e. "one or the other but not both") when preceded by terms of exclusivity, such as "either," "one of," "only one of," or "exactly one of." "Consisting essentially of," when used in the claims, shall have its ordinary meaning as used in the field of patent law.

[0088] As used herein in the specification and in the claims, the phrase "at least one," in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase "at least one" refers, whether related or unrelated to those elements specifically identified.

[0089] It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited.

[0090] Finally, the reference numerals in the claims are merely for convenience and are not to be read in any way as limiting.

Claims

WE CLAIM:

1. An apparatus for measuring a chromaticity of a combined light comprising (i) a first component light emitted by an electroluminescent light-emitting element, the first component light having a chromaticity characterized by a first chromaticity measurement and (ii) a second component light emitted by a photoluminescent material, the apparatus comprising: a sensing system (210) configured to respond to the combined light by supplying an intensity measurement indicative of relative intensities of the first component light and the second component light; and a processing system (230) configured to determine the chromaticity of the combined light based at least in part on the intensity measurement and the first chromaticity measurement.

2. The apparatus of claim 1, wherein the second component light has a chromaticity characterized by a second chromaticity measurement, and the processing system is configured to determine the chromaticity of the combined light based at least in part on the intensity measurement, the first chromaticity measurement, and the second chromaticity measurement.

3. The apparatus of claim 2, wherein the processing system is further configured to determine the second chromaticity measurement based on the intensity measurement, and the first chromaticity measurement.

4. The apparatus of claim 1, further comprising: an initial chromaticity measurement system (240) configured to supply the first chromaticity measurement.

5. An apparatus for measuring a chromaticity of a light emitted by a light source (200), the light source comprising an electroluminescent light-emitting element (202) and a photoluminescent material (204) optically coupled thereto, the apparatus comprising: a sensing system (210) configured to respond to the light emitted by the light source (200) by supplying a first measurement indicative of intensity of a first component light emitted by the electroluminescent light-emitting element and a second measurement indicative of intensity of a second component light emitted by the photoluminescent material; an initial chromaticity measurement system (240) configured to supply a first chromaticity measurement of the first component light; and a processing system (230) configured to determine a chromaticity of the light emitted by the light source (200) based at least in part on the first measurement, the second measurement, and the first chromaticity measurement.

6. The apparatus of claim 5, wherein the processing system is configured to determine a second chromaticity measurement of the second component light.

7. The apparatus of claim 5, wherein the sensing system further comprises: a first sensor (222) configured to sense only the first component light; and a second sensor (224) configured to sense only the second component light.

8. The apparatus of claim 5, wherein the sensing system further comprises: a bandpass electronic filter (212) configured to represent the intensity of the first component light; and a lowpass electronic filter (214) configured to represent the intensity of the second component light.

9. The apparatus of claim 8, wherein the lowpass electronic filter is configured to have a cut-off frequency lower than a frequency of the electroluminescent light-emitting element, and the bandpass electronic filter is configured to have a center frequency substantially equal to the frequency of the electroluminescent light-emitting element.

10. The apparatus of claim 5, wherein the initial chromaticity measurement system further comprises: a temperature sensor (242) configured to measure a temperature of the electroluminescent light-emitting element; and a translator configured (248) to translate the temperature into the first chromaticity measurement.

11. The apparatus of claim 5, wherein the initial chromaticity measurement system is configured to determine the first chromaticity measurement based on an element selected from the group consisted of: a forward voltage of the electroluminescent light-emitting element, a package temperature of the electroluminescent light-emitting element, and a peak wavelength of the electroluminescent light-emitting element.

12. The apparatus of claim 5, wherein the initial chromaticity measurement system further comprises: two or more photosensors having different spectral responsivities, wherein the initial chromaticity measurement system is configured to determine the first chromaticity measurement based on outputs of the two or more photosensors.

13. The apparatus of claim 5, wherein the chromaticity of the light is determined according to Grassman's law of additivity.

14. A method for measuring a chromaticity of a light emitted by a light source (200), the light source comprising an electroluminescent light-emitting element (202) and a photoluminescent material (204) optically coupled thereto, the method comprising: obtaining, in response to the light emitted by the light source, a first measurement indicative of intensity of a first component light emitted by the electroluminescent light- emitting element and a second measurement indicative of intensity of a second component light emitted by the photoluminescent material; obtaining an initial chromaticity measurement of the first component light; and processing at least one of the first measurement, the second measurement, and the first chromaticity measurement to determine the chromaticity of the light emitted by the light source.

15. The method of claim 14, wherein the processing step further includes processing at least one of the first measurement, the second measurement, the first chromaticity measurement, and the second chromaticity measurement.

16. The method of claim 14, further comprising: measuring a temperature of the first component light; and determining the initial chromaticity measurement based on the temperature.

17. The method of claim 14, further comprising: sensing only the first component light to obtain the first measurement; and sensing only the second component light to obtain the second measurement.