CN1988745A - Led illumination unit, projection display device using the same, and method of operating led illumination unit - Google Patents

Abstract

The invention provides an LED light emitting unit, a projection display device using the LED light emitting unit and a method for operating the LED light emitting unit. The LED lighting unit includes a light source having a plurality of LEDs and a lighting control unit that sequentially turns on the LEDs for a lighting period specified for each LED within a cycle time period. The light emission control unit includes a drive signal modulator for generating a modulation signal to turn on at least one of the LEDs with a light emission duty shorter than a light emission period of the at least one LED. The method includes sequentially turning on a plurality of LEDs of the LED lighting unit for a lighting time period specified for each LED within a cycle time period. The at least one of the LEDs is turned on with a lighting duty shorter than a lighting period of the at least one of the LEDs.

Description

LED lighting unit, projection display device and method of operating the lighting unit

This application claims Japanese Patent Application No. 2005-366324 filed with the Japan Intellectual Property Office on December 20, 2005 and Korean Patent Application No. 10-2006-089156 filed with the Korean Intellectual Property Office on September 14, 2006 The disclosures of both applications are hereby incorporated by reference in their entirety.

technical field

Apparatus and methods consistent with the present invention relate to light emitting diode (LED) lighting units, projection display devices using such LED lighting units, and methods of operating such LED lighting units, and more particularly, to methods for directing light to display media such as reflective screen or transmissive screen) to display images.

Background technique

In prior art projection display devices such as projectors and rear projection televisions, LEDs are used as light sources, and the LEDs are operated by a pulse lighting method.

For example, when a full-color image is projected using LEDs that emit three primary colors of light (such as red, green, and blue), the red, green, and blue lights are sequentially emitted by the time-division method, and images corresponding to the lights of the respective colors are utilized. signal to spatially modulate red, green, and blue light.

In Japanese Patent Application Laid-Open No. 2005-149132 ( FIGS. 2 and 4 ), an image processing device for picking up and processing an image and an electronic component mounting apparatus having the image processing device are disclosed. The image processing device includes: a light source having a plurality of LEDs; and an LED driving circuit for adjusting the brightness of light emitted from the LEDs by using a pulse width modulation method.

In addition, an LED driving circuit is disclosed in Japanese Patent Laid-Open No. 2005-5112 (FIG. 1, FIG. 2 and FIG. 3). The disclosed LED driving circuit generates a triangular wave current signal with an offset, and operates the LED approximately in a constant current mode by utilizing a current detector and a switch controller to convert the current signal at a frequency higher than the photoresponse frequency of the human eye.

However, the related art visible light LED light sources, projection display devices using the LED light sources, and methods of operating the LED light sources have the following disadvantages.

When an LED emits light, it simultaneously generates heat, which reduces the light output of the LED. Therefore, when the LED is operated for a long time to provide high-brightness light emission, the luminous efficiency of the LED decreases with time, and the lifetime of the LED also decreases.

For example, in a light source in which red, green, and blue LEDs are assembled to generate white light, reduction in luminance is more likely to occur because LEDs of various colors exert thermal influence on each other. Additionally, red, green and blue LEDs have different thermal properties depending on their color. Therefore, red, green and blue LEDs have different brightness and lifetimes. Therefore, it is difficult to adjust the brightness and driving time of the LED for stable operation.

In order to solve these problems, it is possible to adjust the brightness of the LED according to Japanese Patent Application Publication No. 2005-149132. However, the pulse width modulation method disclosed in this publication is designed for picking up an image with a light source. In the disclosed pulse width modulation method, the brightness of an object is measured, and the measured result is used to generate a pulse width modulation signal to obtain a desired exposure energy.

For LED light-emitting units that sequentially irradiate red light, green light, and blue light onto a display medium (such as a reflective screen or a transmissive screen) to form an image, the required exposure energy is delivered through pulse width modulation, and red light, green light, and green light are irradiated sequentially. The time periods of light and blue light include intermittent intervals. Due to the intermittent interval, the frequency at which the light color changes decreases, causing problems related to image quality such as increased flicker.

In addition, Japanese Patent Application Laid-Open No. 2005-5112 related to an LED driving circuit related to a light emitting device used for automobiles or traffic lights suggests that even when the driving current signal is a square wave, by using Frequency modulation of the drive current signal can also approximately drive the LED in a constant current mode. However, this publication does not disclose or suggest a reduced LED driving circuit and method for reducing the brightness of the LED.

Contents of the invention

Exemplary embodiments of the present invention overcome the above disadvantages and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantages described above, and an exemplary embodiment of the present invention may not overcome any of the problems described above.

The present invention provides an LED light emitting unit, when the LEDs are sequentially turned on to irradiate light onto a display medium for displaying an image on the display medium, the decrease in brightness of the visible light LEDs caused by heat generated by the LEDs can be eliminated. LED lighting unit for reliable operation. The invention also provides a projection display device using the LED light emitting unit and a method for operating the LED light emitting unit.

According to an aspect of the present invention, there is provided an LED lighting unit, the LED lighting unit includes: a light source including a plurality of LEDs; a lighting control unit, for the lighting time period specified for each LED within a cycle time period, sequentially Turning on the LEDs, wherein the light emission control unit includes a drive signal modulator that generates a modulation signal that turns on the at least one of the LEDs with a light emission duty shorter than a light emission period of at least one of the LEDs. .

According to another aspect of the present invention, a projection display device using the LED light emitting unit is provided.

According to another aspect of the present invention, there is provided a method of operating an LED lighting unit, the method comprising sequentially turning on a plurality of LEDs of the LED lighting unit for a lighting time period specified for each LED within a cycle time period , wherein the at least one of the LEDs is turned on with a lighting duty shorter than the lighting period.

According to the present invention, at least one of the visible light LEDs is turned on with a light emission duty whose width is smaller than the light emission time period. Therefore, this LED generates less heat than other LEDs that are turned on at 100% duty cycle during the light emitting period. In addition, when the LED is turned off, heat can be easily released from the LED during intermittent intervals of the lighting period. Therefore, the temperature of the light source can be lowered.

The term duty ratio means the ratio of the lighting time within a specified lighting time period to the entire lighting time period. The waveform of the modulation signal is not limited to a square wave.

In this case, although the LED is turned off for intermittent intervals within the lighting period, the LED can be adjusted so that by changing the brightness and the width of the interval of the LED during the specified lighting period, the human eye does not perceive that the LED is turned off . In an afterimage phenomenon, the photostimulus remains after the stimulus ends. Therefore, a human perceives that the LEDs are continuously turned on to display images.

For example, in an LED lighting unit of a projection display device in which LEDs are sequentially turned on to display a full-color image, red, green, and blue lights are sequentially emitted in a lighting time period of a specified time period. A time period is set such that the human eye perceives a mixed color of red, green, and blue during the time period. Therefore, when the luminance level of the LED is properly set, human eyes cannot perceive an intermittent period of time smaller than the light emitting period.

Furthermore, a sufficiently small gap between pauses increases the difference between the physically integrated value of the brightness of the LED and the brightness level of the LED as perceived by the human eye. Compared to the physical integral value of the brightness of the LED, the LED appears brighter.

The lighting time period can be set so that the integrated value of the modulated signal with a duty ratio of less than 100% is smaller than the integrated value of a modulated signal with a duty ratio of 100%. In this case, since the heat generated by the LED is reduced compared to the case where the duty ratio is 100%, the reduction in brightness of the LED can be more effectively prevented.

Description of drawings

The above and other aspects of the present invention will become more apparent by describing in detail exemplary embodiments of the present invention with reference to the accompanying drawings, in which:

1 is a schematic diagram illustrating a projection display device using an LED light emitting unit according to an exemplary embodiment of the present invention;

FIG. 2 is a functional block diagram illustrating the structure of the LED light emitting unit according to an exemplary embodiment of the present invention described in FIG. 1;

3 is a graph illustrating modulation signals for a plurality of LEDs of the LED light emitting unit according to an exemplary embodiment of the present invention described in FIG. 1;

4 is a graph illustrating a detailed waveform pattern of a modulation signal described in FIG. 3, the modulation signal being generated by a driving signal modulator of an LED light emitting unit according to an exemplary embodiment of the present invention;

Figure 5 is a graph showing the response of the human eye to light stimulation as a function of time;

FIG. 6 is a graph showing reduction in LED brightness;

7 is a graph illustrating a waveform pattern of a modulation signal generated by a driving signal modulator of an LED lighting unit according to an exemplary embodiment of the present invention;

FIG. 8 is a graph illustrating a waveform pattern of a modulation signal generated by a driving signal modulator of an LED lighting unit according to an exemplary embodiment of the present invention.

Detailed ways

The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The same reference numerals in the drawings denote the same elements, and thus their descriptions will be omitted.

An LED and a projection display device using the same according to an exemplary embodiment of the present invention will now be described.

FIG. 1 is a schematic diagram showing a projection display apparatus 1 using an LED lighting unit 2 according to an exemplary embodiment of the present invention. FIG. 2 is a functional block diagram illustrating an LED lighting unit 2 according to an exemplary embodiment of the present invention. FIG. 3 is a graph showing modulation signals for a plurality of LEDs of the LED lighting unit 2 according to an exemplary embodiment of the present invention. FIG. 4 is a graph showing a detailed waveform pattern of the modulation signal described in FIG. 3, which is generated by the driving signal modulator of the LED lighting unit 2 according to the exemplary embodiment of the present invention.

The projection display device 1 uses the LED light emitting unit 2 to project a full-color image onto a reflective screen 6 according to an image signal input.

The projection display device 1 includes an LED lighting unit 2 , a condenser lens 3 , a spatial modulator 4 , a projection lens 5 and a controller 10 , and the controller 10 controls all operations of the projection display device 1 .

In order to display full-color images, the LED light emitting unit 2 sequentially generates lights of different wavelengths corresponding to at least three primary colors such as red, green, and blue by a time division method.

The condenser lens 3 receives the light emitted from the LED light emitting unit 2 and focuses the light onto the modulation area of the spatial modulator 4 .

The spatial modulator 4 displays a color-separated image for projection by spatially modulating the light from the condensing lens 3 based on the image signal of the corresponding wavelength light. The spatial modulator 4 is driven by the controller 10 . A transmissive device such as a liquid crystal display (LCD), or a reflective device such as a digital micromirror device (DMD) and a liquid crystal on silicon (LCOS) having a micromirror array structure can be used as the spatial modulator 4 .

The projection lens 5 projects the image displayed on the spatial modulator 4 onto the reflective screen 6 in an enlarged size.

The structure of the LED lighting unit 2 will now be described in detail.

Referring to FIGS. 1 and 2 , the LED lighting unit 2 includes a light source 22 , an LED drive circuit 21 , a current detector 23 , a drive signal modulator 24 and a power supply circuit 20 that provides power to the light source 22 .

The light source 22 includes LEDs 22R, 22G, and 22B, which respectively emit light of three primary colors, such as light of red wavelength, light of green wavelength and light of blue wavelength. LEDs 22R, 22G and 22B are grouped in their mutual thermal influence area.

The LED driving circuit 21, the current detector 23 and the driving signal modulator 24 are used to sequentially turn on the LEDs 22R, 22G and 22B for their respective lighting times in each cycle time period according to the lighting clock signal of the controller 10. The LED driving circuit 21 , the current detector 23 and the driving signal modulator 24 constitute a lighting control unit 70 .

The light-emitting clock signal is a signal for obtaining light-emitting timing for sequentially turning on the LEDs in each cycle time period. The luminous signal may have the same frequency range as the prior art, such as 180Hz to 360Hz. In the current exemplary embodiment, the fundamental frequency f ₀ of light emission is 240 Hz.

In order to individually drive the LEDs 22R, 22G, and 22B, the LED driving circuit 21 converts the current from the power supply circuit 21 according to the modulation signal from the driving signal modulator 24.

The current detector 23 measures the current flowing through the LEDs 22R, 22G, and 22B of the light source 22 according to a predetermined schedule to generate a control signal for maintaining the maximum luminous output of the LEDs 22R, 22G, and 22B at a predetermined level, and The control signal is sent to the drive signal modulator 24 .

The drive signal modulator 24 generates a modulation signal that divides the cycle time period set by the lighting clock signal into lighting time periods of the LEDs 22R, 22G, and 22B of the light source 22 . During the lighting period of the specified cycle time period, a corresponding one of the LEDs 22R, 22G, and 22B is turned on for a longer lighting duty portion that is shorter than the specified lighting period. The term lighting duty refers to the portion of the lighting period in which one of the LEDs is on.

In the exemplary embodiment shown in FIG. 3 , at a period T ₀ determined by the fundamental frequency f ₀ of light emission, such as T ₀ =1/f ₀ =t ₄ -t ₁ , modulation signals 30R, 30G are generated by sequential time division method and 30B to drive LED22R, 22G and 22B. For example, during the lighting periods _TR (t ₁ -t ₂ ), T _G (t ₂ -t ₃ ) and T _B (t ₃ -t ₄ ), where t ₁ <t ₂ <t ₃ <t ₄ , Light of red wavelength, light of green wavelength and light of blue wavelength are respectively emitted.

The widths of the lighting periods T _R , T _G and _TB can be individually determined according to the thermal characteristics of the LEDs 22R, 22G and 22B. In this exemplary embodiment, since the LEDs 22G and 22B have substantially the same thermal characteristics, and the LED 22R has relatively poor thermal characteristics, the lighting time periods T _R , T _G and _TB are set as follows: _TR / T ₀ =0.2 (20%), T _G /T ₀ =0.4 (40%), T _B /T ₀ =0.4 (40%).

In the prior art lighting method, the LEDs 22R, 22G, and 22B are turned on at a duty ratio of 100% during the lighting periods _TR , _TG , and _TB . In the prior art pulse width modulation method for light emission, since light is emitted with a duty ratio of 100% within a specified light emission time period, the duty ratios of LEDs 22R, 22G, and 22B are respectively T _R / The patterns of T ₀ , T _G /T ₀ and T _B /T ₀ determine the lighting time periods T _R , T _G and T _B relative to the fundamental frequency f ₀ . In addition, the driving currents applied to the LEDs 22R, 22G, and 22B for the modulation signals 30R, 30G, and 30B are determined according to the duty cycle to obtain a desired light energy density on the reflective screen 6 .

However, in the exemplary embodiments shown in FIGS. 3 and 4 , the modulation signal 30R, 30G or 30B is _high in the burst frequency fb during the specified light-emitting period T _R , T _G or _TB of the time period T ₀ Based on the pulse width modulation signal of the luminous fundamental frequency f ₀ . In FIG. 4, _tS and _tE denote the start point and end point of the specified lighting time period _TR , _TG or _TB .

The burst frequency f _b and the duty ratio may be appropriately selected according to experiments conducted to obtain a relationship between LED luminance reduction due to heat generation and actual luminance perceived by human eyes. In particular, the burst frequency _fb can be selected from high frequencies, so that at least two pulses can be generated within a given lighting time period _TR , _TG or _TB .

For example, the pulse width modulation can be set as follows: f _b =20×f ₀ , T _b (pulse period)=1/f _b , duty ratio=50%.

In this case, the power required to drive the LEDs 22R, 22G, and 22B is reduced by half compared to the case of using a duty cycle of 100%.

Furthermore, the modulated signal 30R, 30G or 30B may have the same peak value as the pulse width modulated signal of the prior art. For example, the peak drive current for modulation signal 30R (30G, 30B) may be 1.5A.

The modulation signal 30R, 30G or 30B is sent to the controller 10 to generate timing signals for driving the spatial modulator 4 .

The function of the LED lighting unit 2 will now be described with respect to the operation of the projection display apparatus 1 .

Figure 5 is a graph showing the response of the human eye to light stimulation as a function of time. FIG. 6 is a graph showing reduction in luminance of LEDs.

When the controller 10 of the projection display apparatus 1 receives the color-separated red, green, and blue image signals, the controller 10 sends a clock signal to the LED lighting unit 2 .

Then the driving signal modulator 24 of the LED lighting unit 2 generates modulation signals 30R, 30G, and 30B and sends them to the LED driving circuit 21 . Drive currents corresponding to modulation signals 30R, 30G, and 30B may then be supplied to LEDs 22R, 22G, and 22B of light source 22 . Accordingly, the LEDs 22R, 22G, and 22B can be individually operated according to the waveform patterns of the modulation signals 30R, 30G, and 30B.

Modulation signals 30R, 30G and 30B are also sent to controller 10 to generate timing signals for spatial modulator 4 . Therefore, the driving timing of the spatial modulator 4 can be synchronized with the light of the red wavelength, the light of the green wavelength and the light of the blue wavelength emitted from the light source 22 within the emission periods _TR , T _G and T _G. In this way, the spatial modulator 4 can operate from the red, green and blue image signals in a time-divided manner.

The red light, green light and blue light emitted from the LED light emitting unit 2 are collected by the condenser lens 3 and focused onto the spatial modulator 4 . In order to display a color-separated image of red, green and blue, the spatial modulator 4 spatially modulates red, green and blue light. The color-separated image displayed on the spatial modulator 4 is enlarged by a projection lens 5 and projected onto a reflective screen 6 . Since the color-separated images are merged in human eyes, people can see a full-color image from light of sequential colors reflected by the reflective screen 6 .

In the pulse width modulation of the present invention, the pulse width is modulated at a duty ratio of less than 100% in the same lighting period as in the pulse width modulation of the related art. However, although the duty ratio in the light emitting period is less than 100%, the LED light emitting unit 2 can be used without a problem regarding a decrease in luminance. The reason for this will now be explained.

In the present exemplary embodiment, since the duty ratio is 50%, the amount of light emitted from the light source 22 is smaller than that when the duty ratio is 100%. Therefore, when using the light source 22 as a light source for a printer in which the light source is used to scan a photoconductor having a specific photosensitivity, the scanning energy decreases as the duty cycle decreases.

However, for an LED lighting unit that displays an image by irradiating visible light onto a display medium such as a reflective screen or a transmissive screen, when the LED lighting unit is operated in a predetermined manner, human eyes cannot perceive the light output by the LED lighting unit. The reduction in power results in a reduction in the brightness of the image.

When a person's eyes receive a light stimulus, the person perceives the light stimulus for a period of time after the light stimulus ends. This is called afterimage phenomenon. Referring to FIG. 5, when the eye sees a light pulse having an intensity _Ii , the perceived intensity _Iv of the light pulse decreases with time. The perceptual intensity curve 100 decreases gradually and then more sharply.

Due to this afterimage phenomenon, sequentially radiated red, green, and blue light can be perceived as white light. Light of different colors can be mixed in this way at the frequencies used by the projector. However, the perceived intensity of light is not much different from the actual intensity of light. It can be understood from the case where the white balance is broken when changing the light-emitting time periods of red, green, and blue, or the case where red, green, and blue lights are irradiated simultaneously for white light emission after sequentially irradiating red, green, and blue lights , to increase the perceptual strength of the image.

Referring to Fig. 4, when the burst frequency f _b (1/T _b ) increases above the fundamental frequency f ₀ (1/T ₀ ), due to the afterimage phenomenon, within the lighting period _TR , T _G or T _B , it is more difficult for the human eye to perceive discrete light pulses. As a result, an image can be displayed brighter compared to the amount of light emitted from the light source 22 . Modulation signals 30R, 30G, and 30B are generated based on this fact.

In addition, according to an exemplary embodiment of the present invention, since the amount of light emitted from the light source 22 can be reduced, heat generated by the light source 22 can be reduced, so that concerns about a decrease in brightness of the light source 22 due to the generated heat can be reduced. question. This will be described in more detail later.

FIG. 6 is a graph showing reduction in LED luminance due to heat generation. In FIG. 6 , the horizontal axis represents light emission time (t), and the vertical axis represents luminance (P).

When the LED continuously emits light while receiving a constant driving current, the temperature of the LED increases due to heat generation. Therefore, based on the temperature characteristics of the LED, the brightness P of the LED decreases from the original brightness _Pi , as shown by the brightness curve 101 in FIG. 6 . Then, when the temperature of the LED reaches an equilibrium state, the brightness P of the LED approaches a constant value.

For example, when the interval from 0 to t ₁₀ when the LED is turned on is long, the brightness P of the LED decreases by ΔP ₁₀ . When the LED is turned on for a long interval from z to t ₂₀ , where t ₁₀ <t ₂₀ , the brightness P of the LED decreases by ΔP ₂₀ . Furthermore, since the heat generated increases with the lighting time, less heat is generated when the LED is turned on for a short interval from 0 to _t10 than when it is turned on for a long interval from 0 to _t20 . Therefore, when the total light-emitting time is equal, the brightness of the LED is reduced less in the case of high-frequency pulse width modulation. Therefore, light emission can be efficiently performed by using high-frequency pulse width modulation.

This advantage is particularly important for projection display devices that require very bright illumination light, such as projectors and projection televisions.

As described above, according to an exemplary embodiment of the present invention, the modulation signal is pulse-width-modulated, so that in a given lighting time period, the modulation signal has a duty cycle of less than 100%, and the burst frequency _fb is significantly higher than the basic Light emission frequency f ₀ . Thus, although the amount of light emitted from light source 22 is reduced when compared to prior art pulse width modulation methods, the amount of light that can be perceived remains substantially the same as prior art pulse width modulation due to afterimage effects. same. Furthermore, the heat generated by the light source 22 and the resulting decrease in the brightness of the light source 22 can be greatly reduced. Therefore, the LEDs 22R, 22G, and 22B of the light source 22 can be stably used.

According to an exemplary embodiment of the present invention, in the LED lighting unit 2, through the above-mentioned pulse width modulation method, the driving signal modulator 24 generates a modulation signal for at least one of the LEDs 22R, 22G, and 22B, so that The wide modulated signal operates the LED for a given period of light emission.

In addition, reduction in brightness of the LED can be prevented or reduced simply by adjusting the width of the light pulse of the modulation signal that changes the duty cycle.

An LED lighting unit will now be described according to an exemplary embodiment of the present invention.

7 is a graph illustrating a waveform pattern of a modulation signal generated by the driving signal modulator 240 of the LED lighting unit 200 according to an exemplary embodiment of the present invention.

The LED lighting unit 200 of the present embodiment has the same configuration as the LED lighting unit 2 shown in FIG. 2 except that the LED lighting unit 200 includes the driving signal modulator 240 .

The driving signal modulator 240 of the LED lighting unit 200 generates a modulation signal 40R, 40G or 40B, which is different from the above modulation signal 30R, 30G or 30B.

The modulated signal 40R, 40G or 40B is generated in the same way as shown in the embodiment of FIG. 3 during the light period _TR , _TG or _TB of the time cycle.

Referring to FIG. 7, the waveform pattern is a power modulation waveform pattern having a step shape in which a signal level changes from _Ip (from _t1 to _t2 ) to _I0 ( _t2 to _t3 ). Lighting period T _R , T _G or T _B =t ₄ -t ₁ , wherein t ₁ <t ₂ <t ₃ <t ₄ , and I _p >I ₀ .

The peak value I _p and the light emission duty t ₂ -t ₁ and t ₃ -t ₂ can be set based on experimental results showing the relationship between luminance reduction and perceived luminance change by human eyes. To prevent reducing the lifetime of LEDs 22R, 22G, and 22B, the peak _Ip is limited by bounds on the magnitude and duration of the drive currents of LEDs 22R, 22G, and 22B.

Suppose I _p =2I ₀ , (t ₂ -t ₁ )=(1/3)(t ₄ -t ₁ ) and (t ₃ -t ₁ )=(2/3)(t ₄ -t ₁ ), now The function of the modulated signal 40R, 40G or 40B will be described.

Referring to FIG. 7 , the step pulse 100 specified by the above setting can cause the heat generated by the LEDs 22R, 22G, and 22B to be matched with a modulated square pulse having a signal level I ₀ and a duty cycle of 100% during a specified lighting time period. 41 produces the same amount of heat.

Curve 104 represents the reduction in brightness when LEDs 22R, 22G, and 22B are driven by the step curve 100 of the exemplary embodiment, and curve 105 represents the reduction in brightness when LEDs 22R, 22G, and 22B are driven by modulated square pulse 41 . In FIG. 7, although the brightness curves 104 and 105 are plotted only for the second period after the first period _T0 , the brightness curves 104 and 105 are the same in all periods. Therefore, for the sake of clarity, the following description of the brightness curves 104 and 105 will only be given with respect to the first period T ₀ .

Since the modulation signal 40R, _40G or _40B of the present embodiment initially has a peak value Ip, the luminance curve 104 changes from the maximum point (a ) down to point (b). Then, during the next time interval from _t2 to _t3 , the level of the modulation signal 40R, 40G or 40B drops to a value _I0 which is approximately half the peak value _Ip . Thus, the heat generated by the LED is halved, and the brightness curve 104 does not change as steeply from point (b) to point (c) in the time interval from _t2 to _t3 as it does in the time interval from _t1 to _t2 . At point (c), the brightness is P ₁ . During the time interval from _t3 to _t4 , the LED is off. However, the human eye cannot detect that the LED is off. Therefore, the brightness _P1 is maintained for a while.

For the modulated square pulse 41, since the level of the pulse 41 remains constant at a value _I0 which is half the peak value _Ip , the brightness curve ₁₀₅ is at point (e) having half the height of point (a) Initially, in the time interval t ₁ to t ₄ , in the form of the curve 101 shown in FIG. 6 , it drops to a point (f) with a brightness of P ₂ (P ₂ <P ₁ ).

Although the total heat generated by the LED is the same for the stepped pulse 100 and the square pulse 41 of this exemplary embodiment, the brightness of the LED is reduced much less for the stepped pulse 100 than for the square pulse 41 . Furthermore, the human eye perceives the light emitted from the LED to be much brighter when the LED is driven with the stepped pulse 100 instead of the square pulse 41 due to the afterimage phenomenon.

Although the heat generated by the LED is the same for the stepped pulse 100 and the square pulse 41 , the trapezoidal pulse 100 can reduce the heat generated by the LED, such as by adjusting the peak value I _p and the time points t ₂ and t ₃ of the pulse 100 .

As described above, in this exemplary embodiment, for at least one of the LEDs 22R, 22G, and _22B , the drive signal modulator 240 of the LED lighting unit _{200 generates} A power modulated signal 40R, 40G or 40B having a peak value _Ip .

When the accumulated heat is low, in the initial part of the time period _TR , _TG or _TB , the LED emits light with maximum brightness, thereby increasing the luminous efficiency of the LED. Therefore, the LED can be operated with high luminance.

Furthermore, in the present exemplary embodiment, the level of the power modulation signal 40R, 40G, or 40B initially has a peak value and then monotonously decreases with time.

In this case, the heat generated by the LED decreases monotonically, so that the reduction in brightness of the LED can be effectively reduced.

Here, the term monotonic is used to denote monotonic reduction in a broad sense, including stepwise reduction.

Another exemplary embodiment of the present invention will now be described.

8 is a graph illustrating a waveform pattern of a modulation signal generated by a driving signal modulator of an LED lighting unit according to another exemplary embodiment of the present invention.

The drive signal modulator generates modulated signal 50R, 50G or 50B instead of modulated signal 40R, 40G or 40B.

The modulating signal 50R, 50G or 50B has a trapezoidal waveform pattern in which the level of the modulating signal 50R, 50G or 50B decreases linearly from a peak value _Iq to a value _Ir within a time interval _t1 to _t5 . Here, t ₁ <t ₅ <t ₄ , and I _r <I ₀ <I _q .

I _q , I _r , and time intervals t ₁ to t ₅ can be set in accordance with experimental results showing the relationship between brightness reduction and perceived brightness change by human eyes. To prevent degrading the lifetime of LEDs 22R, 22G, and 22B, peak _Iq is limited by bounds on the magnitude and duration of the drive currents of LEDs 22R, 22G, and 22B.

Modulation signal 50R, 50G, or 50B is another example of a monotonically decreasing waveform signal. The function of the modulated signal 50R, 50G or 50B is substantially the same as that of the modulated signal 40R, 40G or 40B.

The modulation signal of this exemplary embodiment may be used for all or at least one LED of the LEDs 22R, 22G, and 22B according to heat generated by the LEDs 22R, 22G, and 22B and interference between the LEDs 22R, 22G, and 22B. The one or some of the LEDs 22R, 22G, and 22B that generate a small amount of heat and reduce reduction in brightness may be operated at a duty cycle of 100% during a specified light emission period.

In addition, although there are three LEDs emitting visible light in the above-described embodiment, the number of LEDs may be two, four or more. For example, two red LEDs, two green LEDs and two blue LEDs may be used. Additionally, multiple LEDs may be provided individually rather than assembled into a single light source.

In addition, although the LEDs are sequentially turned on for the specified time period T ₀ , two or more LEDs may be turned on simultaneously for the specified time period T ₀ . For example, red LEDs, green LEDs, and blue LEDs can be turned on at the same time to increase apparent brightness.

Also, projection display devices may display images using a transmissive screen instead of a reflective screen. For example, the projection display device may be a rear projection television using a transmissive screen.

In addition, although the LED lighting unit is used in the projection display device, the LED lighting unit can also be used in other devices that radiate light onto a display medium (eg, a reflective screen or a transmissive screen), such as a lighting device or a projector.

The assembly of the projection display device and the LED lighting unit can be used in different configurations within the scope of the invention.

As described above, for the LED lighting unit, the projection display device using the LED lighting unit, and the method of operating the LED lighting unit, at least one LED in the LED lighting unit is operated with a duty ratio of less than 100% during its lighting time period. Therefore, the heating time of the LED is reduced. Additionally, during periods of inactivity, LEDs can dissipate the heat they generate. Therefore, it is possible to reduce reduction in luminance due to heat generated by the LED, thereby improving reliability of the LED light emitting unit.

While the invention has been particularly shown and described with reference to exemplary embodiments thereof, those skilled in the art should understand that, without departing from the spirit and scope of the invention as defined by the claims and their legal equivalents, various modifications in form and detail may be made under certain circumstances.

Claims (12)

1. An LED lighting unit, comprising: a light source comprising a plurality of LEDs; the lighting control unit sequentially turns on the LEDs for the lighting time periods specified for the respective LEDs within the cycle time period, Wherein, the light emission control unit includes a driving signal modulator that generates a modulation signal that turns on at least one of the LEDs with a lighting duty shorter than a lighting time period of at least one of the LEDs. 2. The LED lighting unit according to claim 1, wherein the driving signal modulator generates the modulation signal for turning on the at least one of the LEDs during the lighting period by pulse width modulation. 3. The LED lighting unit according to claim 2, wherein the driving signal modulator performs the pulse width modulation with a predetermined frequency so as to generate at least two pulses within the lighting time period. 4. The LED lighting unit according to claim 1, wherein, in the first half of the lighting time period, the modulation signal has a peak value, and the driving signal modulator adjusts the power within the lighting time period Turning on the at least one of the LEDs. 5. The LED lighting unit according to claim 4, wherein the modulation signal has the peak value at a starting point of the lighting period. 6. The LED lighting unit according to claim 5, wherein the modulation signal has a waveform pattern that decreases monotonously with time. 7. The LED lighting unit according to claim 5, wherein the modulation signal has a waveform pattern that decreases with time in a stepwise manner. 8. A projection display device that displays an image in an enlarged size by irradiating light onto a screen, the projection display device comprising an LED light emitting unit, Wherein, the LED lighting unit includes: a light source comprising a plurality of LEDs; the lighting control unit sequentially turns on the LEDs for the lighting time periods specified for the respective LEDs within the cycle time period, Wherein, the light emission control unit includes a driving signal modulator that generates a modulation signal that turns on at least one of the LEDs with a lighting duty shorter than a lighting time period of at least one of the LEDs. 9. The projection display apparatus according to claim 8, wherein the driving signal modulator generates the modulation signal for turning on the at least one of the LEDs during the light emitting period by pulse width modulation. 10. The projection display device according to claim 8, wherein, in the first half of the lighting time period, the modulation signal has a peak value, and the driving signal modulator is in the light emitting time period through power modulation Turning on the at least one of the LEDs. 11. The projection display apparatus according to claim 10, wherein the modulation signal has the peak value at the start point of the lighting period and a waveform pattern that decreases monotonously with time. 12. A method of operating an LED lighting unit, the method comprising sequentially turning on a plurality of LEDs of the LED lighting unit for a specified lighting time period for each LED within a cycle time period, Wherein, the at least one of the LEDs is turned on with a lighting duty shorter than a lighting period of at least one of the LEDs.

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