GB2630290A

GB2630290A - Light sources for colour assessment

Info

Publication number: GB2630290A
Application number: GB2307444.6A
Authority: GB
Inventors: Butterworth Michael; John Styles Ian
Original assignee: Verivide Ltd
Current assignee: Verivide Ltd
Priority date: 2023-05-18
Filing date: 2023-05-18
Publication date: 2024-11-27
Also published as: GB202307444D0

Abstract

The light source 10, particularly for colour assessment of samples 20, comprises a first LED 2 that emits both ultraviolet and visible light and a filter that selectively transmits ultraviolet in preference to visible wavelengths. Optionally, a second LED 4 emits light at visible wavelengths. There may be a shield to prevent direct illumination of the second LED by the first LED. Also claimed is a colour assessment apparatus (e.g., a cabinet 14 with walls 18) comprising the light source, wherein the LEDs are arranged to illuminate a viewing space 16 and can be selectively activated or deactivated by control means 17. Optionally, the first LED can be switched on and off while the second LED remains on. A camera or sensor may be arranged to record colour information from samples in the viewing space. A method is disclosed wherein samples are inspected under substantially identical visible lighting conditions with and without additional UV light.

Description

TITLE

Light sources for colour assessment

DESCRIPTION

Technical field

The invention relates to the field of assessing the colour of a sample under carefully controlled lighting conditions. In particular, it relates to light sources used to illuminate the sample at both visible and ultraviolet wavelengths. The spectral power distribution of the light source may be designed to represent a defined illuminant. The sample may be any product, such as fabrics or printed media, the colour of which needs to be assessed or compared against a standard. The illuminated sample may be viewed by eye or by a camera or sensor.

Background of the invention

Colour assessment is important for determining the appearance of samples under known lighting conditions in a diverse range of industries such as textiles, printed media and paints. The assessment may be to check the quality of a single sample under prescribed lighting conditions, for example a photograph in a magazine, or may be to compare different samples, for example to ensure that two samples of dyed fabric that are to be used in a garment match one another reliably in different lighting conditions. It is essential that the conditions under which the samples are assessed should be standardized, for example so that a supplier and a purchaser can compare sample products consistently using their respective apparatus.

An "illuminant" is a light source that has a defined spectral power distribution, i.e. the intensity of the light at each wavelength over a specified range. It may or may not exist as a real light source but may be replicated to sufficient accuracy to be used as a practical standard. D50 and D65 are examples of standard illuminants that are not realisable as sources. A reference illuminant may represent natural daylight, typical store lighting or other light sources, as defined by an international body such as the Commission Internationale de l'Eclairage (CIE) or specified by a commercial or industrial body. For example, the CIE has defined a particular spectral power -2 -distribution D65, described as "average north sky daylight", which is illustrated in Figure 1 using arbitrary units for intensity. The range of wavelengths in the spectrum will include visible light but for some applications may also include wavelengths in the ultraviolet or infrared because light in those regions can generate a visible response in certain colourants. In this patent specification, "spectrum" is used as an abbreviation of "spectral power distribution".

A "source" is a physical light source that represents a defined standard. It may not spectrally match the defined standard but in practical terms is a good enough approximation for practical use. Some sources may also be illuminants, e.g. illuminants "B" & "C". In recent years it has become common to use sources that comprise sets of light-emitting diodes (LEDs) of different colours. Although each LED typically emits light in a narrow spectrum about a specific wavelength, the intensities of the respective LEDs in a set may be controlled so that in combination they provide reasonable approximations to a number of different reference illuminants. LEDs provide advantages over traditional incandescent or fluorescent light sources, which include low power consumption and rapid switching.

Visual quality inspection under standardised illuminants is the widely accepted method for ensuring consistency and making pass/reject decisions. Where variance occurs, visual mismatches may be visible to an end customer. As such, quality inspection procedures are required to check all aspects of colour and appearance.

Traditional requirements for colour assessment equipment are: * Quality of light source -accuracy to various standard and real-world illuminants.

* Seamless switching between illuminants -such that the eye is able to see the difference between the sample when viewed under the first and second illuminants without a period of darkness between them.

* Provision of a standardised, neutral viewing area. -3 -

Processes such as garment dyeing, printing, or paint mixing may include elements with an optical brightening agent. Optical brightening agents are added to enhance the perceived brightness of substrates including textiles, paints, papers etc. This perception of brightness is created when they absorb invisible ultraviolet light and emit it as visible light in the blue region. Optical brighteners impart a colour tint which is typically green or purple. They may cause the appearance of the sample to differ under normal assessment conditions (e.g. daylight vs non daylight). It is difficult to visually assess the effect of the optical brightening agent on the appearance of a sample by comparing it under two visually different light sources, as the general appearance will be different.

Instead, it is necessary to make this assessment under one light source, both with the correct amount of ultraviolet and without ultraviolet.

Summary of the invention

The invention provides a light source comprising a first LED that emits light of both ultraviolet wavelengths and visible wavelengths; and a filter arranged to filter light from the first LED, wherein the filter selectively transmits light of ultraviolet wavelengths in preference to light of visible wavelengths. The light source preferably further comprises a second LED that emits light at visible wavelengths.

The inventors have found that ultraviolet LEDs, which nominally emit ultraviolet light at wavelengths close to the boundary of the visible part of the spectrum, in fact emit spectra of light that are broad enough to overlap the boundary. They therefore illuminate the sample in visible light, in addition to ultraviolet, and contribute a purple glow that interferes with assessment of the tint due to the optical brighteners. Using a filter in accordance with the invention to exclude visible light generated by the ultraviolet component of the source enables accurate assessment of the colour tint on substrates that is attributable to the optical brightening agent.

A skilled person in this field will be familiar with the fact that spectra from different components of the light source might overlap; and will be aware of algorithms for adjusting the intensities of the respective components to optimize the combined spectrum to match a target illuminant. However, that natural approach will not work -4 -in solving the present problem because of the desire to assess the effects of optical brighteners, which requires the ultraviolet components of the source to be switched on and off independently from the visible components. If the violet light emitted by a nominally ultraviolet first LED is compensated by, for example, reducing the intensity of a blue second LED then, when the ultraviolet LED is turned off, the violet part of the visible spectrum will be under-represented in the spectrum. For this reason, the present inventors propose the different approach of using a filter to exclude contributions from the ultraviolet LED to the visible part of the spectrum.

This allows the following key innovations: 1. Creation of an otherwise invisible ultraviolet light, free from the visible tint typical of ultraviolet emitters.

2. To allow the inclusion of the correct amount of ultraviolet light to standard daylight illuminants without influencing or modifying the visible part of the defined spectral power distribution.

3. To allow additional ultraviolet light to be added to standard non-daylight sources without affecting their visible colour characteristics. This allows the effect of background ultraviolet light on the perceived colour of an article to be visualised for UV-deficient light sources.

4. To allow the removal of the ultraviolet component of standard daylight sources without materially degrading the quality of the visual component. This allows the effect of the ultraviolet component of the source to be visualised for UV-included light sources.

Innovation I allows isolated examination of ultraviolet-reactive samples, e.g. those including optical brighteners.

Innovations 3 and 4 provide methods of demonstrating whether fluorescent properties of a sample (e.g., clothing or paint) are present: * Under the same visible lighting conditions.

* Without visual interruption to the viewing area because there is no cut-to-black between the viewing conditions. -5 -

* By comparing samples under a sample illuminant and under a visually identical illuminant with additional ultraviolet light, the visual impact of the optical brightening agent is isolated and can be assessed.

These innovations would not be possible by simply accounting for the visible section of the ultraviolet light in the mix of standard illuminants.

Innovation 2 allows for greater fidelity in the reproduction of the effects of full spectrum daylight, including non-visible ultraviolet, while meeting the standard spectral power distribution.

The light source may additionally comprise a shield arranged to prevent light being transmitted directly from the first LED to the second LED.

The second LED, which emits light at visible wavelengths, may comprise a phosphor coating to alter the spectrum of the light emitted by the device compared with that emitted by its internal diode. In some cases, exposure of the second LED to light from the first LED may cause its phosphor coating to fluoresce, thereby altering the visible spectrum emitted by the second LED according to whether or not the first LED is switched on. This will result in a further discrepancy in the assessment of the effect of optical brighteners. The interaction between the two LEDs may also result in their combined output differing from that which would be expected by simply adding their respective emission spectra, with the result that the light source fails to match the reference illuminant as closely as it should. A physical shield blocks light from the first LED from reaching the second LED so this problem is avoided.

The invention further provides a colour assessment apparatus comprising first and second LEDs and a filter as previously defined; and further comprising a viewing space arranged to receive light from the first and second LEDs; and control means for selectively switching on or off the first and second LEDs. Preferably, the control means is configured to allow the first LED to be switched on and off while the second LED remains on. -6 -

For the purposes of the present specification, a wavelength of 400nm is considered to be the boundary between visible light (at longer wavelengths) and ultraviolet light (at shorter wavelengths). However, the precise location of the boundary is not critical because the emission spectrum of every LED and the absorption spectrum of every filter varies continuously over at least a narrow range of wavelengths.

The drawings Figure 1 is a plot of intensity against wavelength for the D65 standard illuminant.

Figure 2 is a schematic illustration of a light source according to the present invention.

Figure 3 is a schematic illustration of a colour assessment apparatus incorporating a light source according to the present invention.

Figure 4 is a plot of intensity against wavelength for light emitted by a first example of an ultraviolet LED, with and without a visible light filter.

Figure 5 is a plot of intensity against wavelength for light emitted by a second example of an ultraviolet LED, with and without a visible light filter.

Figure 6 is a plot of relative intensity against wavelength for light emitted by three LEDs, without a visible light filter.

Figure 7 is a plot of relative intensity against wavelength for light emitted by the three LEDs of Figure 6, with a visible light filter.

Figure 2 is a schematic illustration of a light source 10, which comprises at least one ultraviolet LED 2, which emits light principally at wavelengths in the ultraviolet range, less than 400nm; and at least at least one visible LED 4, which emits light wholly or principally at wavelengths in the visible range, greater than 400nm. In Figures 2 and 3, visible light is represented by solid arrows and ultraviolet light is represented by dashed arrows. The two LEDs 2,4 are mounted on a common substrate 6 and are connected to an electrical circuit (not illustrated) by which they may be powered and independently switched on or off It should be understood that Figure 2 is purely schematic, being intended to illustrate the relative configuration of the components of the light source 10. It is not drawn to -7 -scale and the components do not necessarily have the form shown. A practical light source is likely to comprise multiple LEDs emitting at different wavelengths in the visible and ultraviolet parts of the spectrum, in order to be able to represent a range of reference illuminants. Additionally, the chosen set of LEDs may be repeated a number of times, either within a single unit or by using a number of similar units to build up the desired light source for a given application.

A filter 8 is mounted adjacent to the ultraviolet LED 2 so that light emitted by the LED 2 in directions away from the substrate 6 must strike the filter 8. The filter 8 is a visible light filter, which selectively transmits light of ultraviolet wavelengths in preference to light of visible wavelengths. Preferably, the filter 8 allows the passage of most light with a wavelength below a threshold of, e.g., 400nm but blocks the passage of most light with a wavelength above that threshold.

The filter 8 is mounted on a frame 12, which supports it in relation to the substrate 6 and the ultraviolet LED 2. Preferably, the frame 12 is opaque -at least in the region between the ultraviolet LED 2 and the visible LED 4 -so that it acts as a shield to block the transmission of both ultraviolet and visible light from the ultraviolet LED 2 to the visible LED 4. This avoids the problem that ultraviolet light from the ultraviolet LED 2 might cause a phosphor of the visible LED 4 to fluoresce and thereby change the spectrum emitted by the visible LED 4 In preferred embodiments of the invention, the frame may be wholly opaque and may enclose the ultraviolet LED 2 on all sides so that light from the ultraviolet LED 2 can escape only by passing through the filter. In other embodiments (not illustrated) an opaque shield may be provided as a component distinct from the frame 12 and positioned between the ultraviolet LED 2 and the visible LED 4.

Figure 3 is a schematic illustration of a colour assessment apparatus incorporating a light source 10 like that in Figure 2. The apparatus includes a cabinet 14, the interior of which defines a viewing space 16. The light source 10 is arranged inside the cabinet 14 to illuminate the viewing space 16. An interface 17 allows an operator to control the light source 10, for example to select different illuminants or to switch the -8 -ultraviolet LEDs 2 on or off. The apparatus may additionally or alternatively be controlled remotely. Interior walls 18 of the cabinet 14 are typically coated in a matt, neutral grey to distribute light from the source 10 around the viewing space without changing its colour. One or more samples 20 can be placed or mounted in the viewing space 16 to be illuminated by the light source 10 and viewed for assessment or comparison of their colour. The cabinet 14 may be in the form of a booth with an open front to allow visual assessment of the samples 20. Alternatively, the cabinet 14 may be closed, with a camera or other sensor (not illustrated) mounted inside for recording images or colour data from the samples 20. It is not essential for the invention that the light source 10 should be installed in a cabinet 14 or be part of a complete colour assessment apparatus: for example, it could be embodied in a simple luminaire for illumination of a working surface.

Figure 4 is a plot of absolute intensity (irradiance) against wavelength for light emitted by a first example of an ultraviolet LED 2, before and after passing through a visible light filter 8. The filter 8 has a nominal threshold wavelength of 400nm, indicated by a dashed line 21. In practice, the transmission spectrum of the filter 8 will be an S-shaped curve centred on the nominal value so that some light of longer (visible) wavelengths will be transmitted while some light of shorter (ultraviolet) wavelengths will be blocked. Curve 22 shows the unfiltered output of the LED 2, which has a peak at approximately 397nm and a spectrum extending across both visible and ultraviolet wavelengths. Curve 24 shows the spectrum of the same light after passing through the visible filter 8, which causes a reduction in intensity at all wavelengths but substantially eliminates transmission at wavelengths above 400nm.

Figure 5 is a plot similar to Figure 4 but relating to a second example of an ultraviolet LED 2, which has a peak output at approximately 389nm. Even before filtering, its spectrum 26 displays only a small "tail" extending into visible wavelengths above 400nm. After passing through the same filter as in Figure 4, shown by curve 28, the tail of the spectrum is substantially eliminated.

As already noted in relation to Figure 4, passing the light emitted by the ultraviolet LED 2 through the filter 8 may significantly reduce the intensity of its output even in the ultraviolet range, especially at wavelengths close to the threshold. To compensate for this and maintain the desired contribution of the LED 2 to the spectrum of the illuminant being approximated, the power or duty cycle of the LED 2 may be increased (or additional, similar LEDs may be provided). Figures 6 and 7 show that boosting the intensity of the output at ultraviolet wavelengths does not cancel out the effects of the filter 8 in reducing the intensity at visible wavelengths.

Figures 6 and 7 show output spectra of three ultraviolet LEDs 2, which have been normalized so that each spectrum has a peak intensity of 1.0 (in arbitrary units). In Figure 6, curves 28, 29 and 30 show the unfiltered outputs of the LEDs in relation to the threshold wavelength of the filter 8, indicated by the dashed line 21. The LEDs 2 represented by curves 29 and 30 make significant contributions in the visible range above 400nm. In Figure 7, the corresponding curves 32, 33 and 34 shows the effects of the filter 8, followed by normalization so that their peak intensities are the same as in Figure 6. It will be seen that the peaks of curves 33 and 34 have been shifted to lower wavelengths and their contributions above 400nm are significantly reduced compared with curves 29 and 30 in Figure 4.

Claims

-10 -CLAIMS1. A light source comprising: a first LED that emits light of both ultraviolet wavelengths and visible wavelengths; and a filter arranged to filter light from the first LED, wherein the filter selectively transmits light of ultraviolet wavelengths in preference to light of visible wavelengths.
2 A light source according to claim 1, further comprising a second LED that emits light at visible wavelengths.
3. A light source according to claim 2, further comprising a shield arranged to prevent light being transmitted directly from the first LED to the second LED.
Colour assessment apparatus comprising: a light source according to claim 2 or claim 3; a viewing space arranged to receive light from the first and second LEDs; and control means for selectively switching on or off the first and second LEDs.
5. Colour assessment apparatus according to claim 4, wherein the control means is configured to allow the first LED to be switched on and off while the second LED remains on.
6. Colour assessment apparatus according to claim 4 or claim 5, further comprising a camera or sensor arranged to record colour information from samples that may be placed in the viewing space during use of the apparatus.