GB2411010A

GB2411010A - Anomaloscope

Info

Publication number: GB2411010A
Application number: GB0502906A
Authority: GB
Inventors: Lionel George Ripley
Original assignee: University of Sussex
Current assignee: University of Sussex
Priority date: 2004-02-13
Filing date: 2005-02-11
Publication date: 2005-08-17
Anticipated expiration: 2025-02-11
Also published as: GB0502906D0; WO2005079656A1; GB2411010B; GB0403290D0

Abstract

An anomaloscope 1 comprises two light sources that can be viewed side-by-side. The first light source is a reference white light source 6 which is made up of a blue LED which excites a yellow phosphor on its cover and a green LED, which when viewed together present a white light. The second light source is a comparison light source 7 which comprises a blue LED and a yellow LED. A subject-adjustable mixture control 8 is provided for adjusting the proportions of yellow and blue light in the comparison mixed light source 7 to allow a subject to try to match the comparison mixed light source 7 to the reference white light source 6.

Description

241 1010 - 1 71910.211 Optical Instrument The present invention relates to

an optical instrument and in particular to an anomaloscope for assessing colour vision.

Anomaloscopes use metameric colour matches (i.e. when two colours whose spectral composition differs are subjectively matched (seen as the same colour) by a human observer) to assess colour vision. The subject being tested typically mixes light of two different wavelengths (colours) in an attempt to match the result with a third, reference colour made up of a different wavelength or wavelengths (i.e. having a different spectral composition).

The human eye has three types of cones that are used for colour vision, one type of cone being stimulated by red wavelengths of visible light, one type of cone being stimulated by green wavelengths and the other type of cone being stimulated by blue wavelengths of visible light. The signals from these cones are combined into two "colour channels" that used in colour vision. The first colour channel is the so-called "red-green" colour channel that provides a signal corresponding to the stimulus of the red cones minus the stimulus of the green cones (i.e. red - green). The second colour channel is the so-called "tritan" colour channel which is a measure of the stimulation of the blue cones minus a "brightness'' (or yellow) measure which is derived by adding the simulation of the red and green cones together (i.e. the tritan colour channel indicates: blue (red + green)).

One well-known colour vision colour match test is the Rayleigh match, which uses a mixture of red light plus green light versus a reference yellow light to test a subject's red-green colour channel. In a Rayleigh match anomaloscope, the test subject adjusts a comparison mixture of (monochromatic) red and (monochromatic) green light until they perceive the mixed colour to match a reference (monochromatic) yellow light. In this way, a metameric match is achieved, as the comparison mixed 'yellow' light source comprises red and green wavelengths, and therefore differs in spectral composition from the reference yellow light source which comprises monochromatic yellow light.

In the Rayleigh match, the relative percentages of red and green light used in making the match gives information about the colour vision of the test subject.

For example, a subject with normal colour vision will be able to make a precise colour match between a monochromatic spectral yellow and a suitable mixture of monochromatic red and monochromatic green, but an abnormal subject will fail to make a normal match. The results of the test can be used to identify those people with defective red-green vision (which may, for example, make them unsuitable for certain vocations where good colour vision is required).

There are a number of known anomaloscopes, such as the Nagel Anomaloscope, the Neitz Anomaloscope, the Besoncon Anomalometer, and the PickfordNicolson Anomaloscope. However, these instruments are relatively complicated and expensive.

The Applicants have developed a relatively cheap, robust and easy-to-use Anomaloscope for testing the Rayleigh match. This anomaloscope, known as the "Trafford Anomaloscope", uses solid state light sources in the form of monochromatic LEDs (light emitting diodes) as the red, green and yellow light sources.

Such light sources are relatively cheap and robust, and can be driven by relatively simple analogue or digital circuits and can be used with a relatively simple optical arrangement. The Applicants have been able to construct an Anomaloscope using such LEDs which is - 3 - portable and relatively easy-to-use, and has been found to give suitable colour vision testing results.

It is also desirable to be able to test the tritan colour channel. Testing of tritan vision is important because defects in tritan vision are believed to be symptoms of a number of diseases, such as diabetic retinopathy, Parkinson's disease, thyroid problems, and possibly multiple sclerosis. A reliable tritan vision test would help to detect these diseases.

There are colour match tests, similar to the Rayleigh match, that have been developed to assess tritan vision. These include the Moreland match and Engelking-Trendelenburg matches, both of which match blue plus green to blue-green, and the Pickford-Lakowski match, which matches blue plus yellow to white.

Present anomaloscopes and other instruments can perform some testing of tritan vision, but there remains a need for a relatively cheap, robust, simple to use, sensitive and efficient instrument for doing so.

The Applicants, following their success with a solid state light source, LED, based red-green, Rayleigh match, anomaloscope, have therefore considered attempting to construct a tritan, e.g. Pickford-Lakowski match, anomaloscope in a similar manner.

For the Pickford-Lakowski match, a comparison mixed blue and yellow light source, and a reference white light source, are required. The yellow and blue light sources making up the comparison mixed light source and the reference white light source should all lie on a straight line on the CIE chromaticity diagram, and this line should ideally be a tritan confusion line (i.e. a straight line passing through the tritan point on the CIE diagram).

To provide an appropriate Pickford-Lakowski colour match test, the Applicants have recognized that the reference white light source should be spectrally different from the blue and yellow light sources that - 4 make up the mixed, comparison blue and yellow light source, i.e. should be such that a metameric match, rather than an isomeric match, with the mixture of the light from the blue and yellow LEDs is achieved. (An isomeric match is a match in which the two colours being compared have the same spectral compositions (i.e. are made up of the same wavelengths of light).) In other words the white light source should be a different metamer for white light than the mixed blue and yellow LEDs.

This is because with an isomeric match (where the spectral compositions (wavelengths) making up the "colours" being compared are the same), the subject will perceive both light sources identically whatever their colour vision, and so a subject with defective colour vision can still perform an isomeric match. An isomeric match will not therefore work to detect colour vision deficiencies. With a metameric match, it is not a comparison of like with like, and so any colour vision defects should affect the perception of each light source differently, and therefore become apparent in the match the subject makes.

Thus, according to a first aspect of the present invention, there is provided an anomaloscope for the testing of colour vision, comprising: a comparison light source comprising a blue light source and a yellow light source arranged so as to provide a light source comprising a mixture of their light; a reference white light source which comprises a light source or sources which when viewed together presents a white light, and which does not provide an isomeric match with the comparison light source; an optical arrangement for allowing a subject to view the reference white light source and the comparison mixed blue and yellow light source simultaneously; and means for allowing a subject to vary the proportion 5 of blue and yellow light in the comparison light source, whereby the subject may attempt to match the comparison mixed blue and yellow light source to the reference white light source.

As well as the particular optical properties set out above, it is preferred for the light sources used in the anomaloscope of the present invention also to have particular mechanical and other properties. For example, the light sources are preferably relatively robust, and easy to control, and use relatively little power, as that would make them more suited to the anomaloscope use that is envisaged. The light sources are therefore preferably made up of solid state devices, such as LEDS, for this reason.

In a preferred embodiment, the comparison light source comprises a yellow LED and a blue LED. The Applicants have found available yellow and blue LEDs to be solid state light sources suitable for use as the separate blue and yellow light sources for the "mixing" comparison light source.

As discussed above, the reference white light source should provide a metameric match to the comparison, mixed blue and yellow light source, i. e. should have a different spectral composition to the combination of the blue and yellow lights making up the comparison light source. Thus, for example, where the blue and yellow light sources making up the comparison mixed light source are substantially saturated (i.e. monochromatic), it is preferred that the reference white light source does not display (strong) spectral peaks at (the same) blue and yellow wavelengths. It is preferred therefore for any light source component or components making up the reference white light source to not in themselves provide an isomeric match with the individual blue light source or with the individual yellow light source making up the comparison light source.

The reference white light source is preferably as - 6 spectrally different from the blue and yellow comparison light source as possible. Thus, the reference white light source most preferably comprises a broadband spectrum, preferably without substantial spectral peaks, and most preferably encompassing, so far as is possible, all (visible) wavelengths at equal intensities.

The Applicants have considered a number of potentially suitable white light sources for the anomaloscope. Incandescent light bulbs and fluorescent tubes, while providing a suitable metameric light source for the colour match, are not preferred for use in the anomaloscope due to their potentially unsuitable electrical characteristics and the heat that they produce.

The Applicants have also found electro-luminescent sheets to be unsuitable, as although they are purported to be "white", the Applicants have found that they in fact show two spectral peaks, at blue and yellow wavelengths, rather than a broadband distribution across all wavelengths. This renders them unsuitable for the colour match test, since the sheet is effectively an isomeric match to the blue and yellow components of the comparison light source, such that a test subject will simply match the blue comparison component to the blue peak of the electroluminescent sheet, and the yellow comparison component to the yellow peak of the electroluminescent sheet. Electroluminescent sheets also require an AC voltage which may be inconvenient.

There are now becoming available "white" LEDs, which would have been expected to be ideal for this application.

However, the Applicants have recognized that those white LEDs which are in fact tricolour LEDs combining red, green and blue elements to produce a white sensation when viewed are in fact unsuitable for colour vision testing. This is because the relative spectral power distribution of the LED in fact shows three - 7 - substantial peak wavelengths of red, green and blue and is not a continuous broadband spectrum. Furthermore, the blue spectral peak is typically very similar to the blue "spectrum" of blue LEDs that could be used to form the blue part of the blue and yellow comparison mixed light source.

The effect of this is that the ''blue'' part of the reference white light LED would tend to be isomerically matched by a test subject with the blue light of the comparison mixed light source, thereby preventing any distinction by subjects between those two light sources whatever the subject's colour vision. This would still leave a comparison to be made with the yellow component of the comparison mixed light source, but that comparison would effectively be being made with the red and green spectral peaks in the reference white light LED, i.e. the subject would be assessing red and green versus yellow. As discussed above, this is in fact the Rayleigh match and so in practice any results obtained would be affected by the red-green vision of the subject, and so the test could not give an accurate tritan colour vision assessment. This is undesirable.

Thus, it is also preferred for the spectral composition of the reference white light source not to include a metameric match for one or more of the components of the comparison mixed light source (such as the above red and green spectral peaks providing a metameric match for the yellow component of the comparison mixed light source), particularly where the spectral composition of the reference white light source already includes a component that is an isomeric match for another component of the comparison mixed light source.

Thus, the reference white light source is most preferably such that its "strong" spectral components (spectral peaks), if any, do not individually act as isomeric matches with the individual yellow and/or blue - 8 - light sources making up the mixing comparison light source, and/or do not act in combination as a metameric match with the individual yellow and/or blue light sources making up the mixing comparison light source.

There is another form of white LED available, which comprises a broadband blue LED which excites a yellow phosphor. However, the Applicants have found that this form of LED also does not in itself produce a suitable white light source for the colour match, as it in fact shows a blue spectral peak and a yellow spectral peak in its spectrum and therefore provides more of an isomeric match with the mixed blue and yellow light source, rather than a metameric match.

However, the Applicants have found that by adding a green light source, preferably in the form of a green LED, to the 'white'' LED comprising a blue LED which excites a yellow phosphor, then a suitable white light source that is a sufficiently metameric match to the mixed light of the blue and yellow comparison light source (particularly where it is made up of blue and yellow LEDs) can be produced. This is believed to be because the green light source effectively fills in or compensates for the gap (between the blue and yellow peaks) in the spectral composition of the light from the blue LED which excites a yellow phosphor, such that the combination of the light sources has a suitably continuous broadband spectrum that is perceived by a subject as being white and is a metameric match for the perceived white colour produced by the mixed light of the blue and yellow components (LEDS) of the comparison light source.

Thus in a particularly preferred embodiment, the reference white light source comprises a blue LED which is arranged to excite a yellow phosphor, and a separate green LED for providing light of wavelengths between the blue and yellow peaks of the spectrum of the light produced by the blue LED when it excites the yellow - 9 - phosphor, which when viewed together present a white light.

Thus, according to a second aspect of the present invention, there is provided an anomaloscope for the testing of colour vision, comprising: a reference white light source which comprises a blue LED which is arranged to excite a yellow phosphor, and a separate green LED for providing light of wavelengths between the blue and yellow peaks of the spectrum of the light produced by the blue LED when it excites the yellow phosphor, which when viewed together present a white light; a comparison light source comprising a blue LED and a yellow LED arranged so as to provide a light source comprising a mixture of their light; an optical arrangement for allowing a subject to view the reference white light source and the comparison mixed blue and yellow light source simultaneously; and means for allowing a subject to vary the proportion of blue and yellow light in the comparison light source, whereby the subject may attempt to match the comparison mixed blue and yellow light source to the reference white light source.

The present invention in this aspect and embodiment at least thus provides an anomaloscope that can test the Pickford-Lakowski match (i.e. tritan vision), but which uses solid-state devices as its sole light sources, and can therefore be relatively robust, inexpensive, and easy to construct and use, and reliable, as compared to existing anomaloscopes for testing tritan vision. The anomaloscope of the present invention can also be made, and preferably is, portable. This is achieved by presenting to a subject a white light reference made up of a green LED in combination with a "white" LED (which in fact comprises a broadband blue LED which excites a yellow phosphor), which is then matched to a mixture of light from blue and yellow LEDs. - 10

In a particularly preferred embodiment, one or more additional red and/or green LEDs are also included in the reference white light source, as this facilitates a broader and more evenly distributed spectrum, and may also help to make it easier to alter the position of the reference white light source on the CIE diagram (e.g. to provide a better match to the blue and yellow comparison light source). Preferably a range of coloured LEDs is used, such as blue, blue-green, green, yellow, orange, red, etc., as that will provide a broad spectrum white light source that is significantly different from the blue-yellow comparison light source.

The optical arrangement of the anomaloscope should present the white light reference and the comparison blue-yellow mixture to a subject simultaneously, preferably in a side-by-side arrangement, and preferably such that they can easily be viewed with one eye. This can be achieved as desired.

The optical arrangement preferably allows the subject to view the light sources without interference from external light. This can be achieved, for example, by the subject viewing the light sources down an enclosure, such as a tube. Where an enclosed viewing arrangement, such as a tube, is being used, the inside of the viewer, e.g. tube, is preferably arranged to be non-reflecting, such as by painting the inside of the viewer matt black, or by lining the inside of the viewer with a non-reflective material, so as to avoid reflections interfering with the test.

The optical arrangement is preferably arranged to position the subjects eye a set and fixed distance from the light sources. This can be convenient for designing the remainder of the optical arrangement. This can again, for example, be achieved by having the subject view the light sources down a fixed length tube.

The optical arrangement is preferably such that the light emitted from the light sources as seen by the - 11 subject is directed onto the fovea only. (As the anomaloscope is used to measure colour vision deficiencies, the only photoreceptors that need to be stimulated are the cones, which make up the entire fovea). In a particularly preferred embodiment, the optical arrangement is such as to cause the light emitted by the light sources as seen by the subject to fall within a half angle of two degrees, i.e. for there to be a viewing angle of two degrees. The viewing angle can be set by, for example, arranging the height of the object to be viewed (i.e. the light sources as seen by the subject) and the distance the eye is from that object (i.e. the light sources) appropriately.

The Applicants have found that a convenient size of anomaloscope to achieve these aims, which keeps the anomaloscope reasonably small and portable, is to use a viewing distance of 100 mm and a light source size of 3.5 mm diameter at that distance.

Although the light sources could be viewed directly, it is particularly preferred for the viewer to see their image effectively at infinity, as the eye is then relaxed and unaccommodated. Setting the image at infinity also avoids subjects who have hyperopia (long- sighted vision) having difficulty focussing the light sources due to their proximity to the eye. The image of the light sources can be set at infinity by, for example, using a suitable lens arrangement, such as a double convex lens (which will focus the diverging light from the light sources into parallel rays so that the image appears to be at infinity).

The lens arrangement can conveniently be mounted in or at the end of the viewing tube, where used, and where a double convex lens is used, should be mounted the focal length of the lens from the light sources so as to set the viewed image at infinity. (In such an arrangement, although the lens alters the image of the light sources such that it appears to be at infinity, - 12 the angle that the light enters the eye is unchanged from the angle without the lens. Thus the focal length of the lens should be the same as the distance required to give the 2 viewing angle, e.g. 100 mm in the example given above.) The LEDS used to provide the light sources should, in the case of the blue and yellow LEDS providing the mixed comparison light source, preferably be saturated (i.e. provide a narrow wavelength bandwidth, and ideally monochromatic light), although in practice it is possible for the blue LED to have a broader bandwidth as is often the case with such devices. A suitable wavelength for the blue LED would be around 450 nm and a suitable wavelength for the yellow LED would be around 565 nm. The wavelengths of the blue and yellow LEDS preferably plot onto, or as close as possible to, a tritan confusion line on the CIE diagram. In a preferred embodiment, a blue LED of around 430 to 450 nm wavelength is matched to a yellow LED of wavelength 565 nm, 585 nm or 610 nm.

For the preferred reference white light source discussed above, the 'white" LED component of that source should be in the form of a ''white.' light source which consists of a blue LED that excites a yellow phosphor inside the covering of the LED. The "green" compensating LED should be such as to provide light of a wavelength suitable to "fill in" the spectral dip in the wavelengths between the blue and yellow "peaks" of the "white" LED's spectrum. A suitable wavelength for the "green" LED has been found to be around 505 nm. It preferably provides a ''broader'' green wavelength spectrum (so far as this is achievable), i.e. is preferably not fully saturated.

The light from the blue and yellow LEDS, and the light from the "white" and green LEDS, needs to be mixed, respectively, to provide the comparison mixed blue and yellow light source, and the reference white - 13 light source, that are viewed by the subject. This mixing of the light from the LEDS making up each light source should be such that the subject can (and will) perceive the light sources as being 'white", and can otherwise be achieved in any suitable manner. It is preferably achieved by flashing the respective LEDS on and off alternately to produce a perceived colour mixture. The flashing should take place at a frequency above the critical fusion frequency of the human eye (which can be from around 50 Hz up to around 100 Hz depending on the circumstances), so that the subject does not perceive any flicker. Thus, flashing frequencies over 100 Hz are preferred. A flashing frequency of 312.5 Hz has been found to be suitable.

The relative amounts of the blue and yellow light in the comparison light source should be adjustable to allow the subject to vary their proportions to try to match the reference white light source. This adjustment is preferably provided by means of a control which a subject can use mounted on the anomaloscope.

The way that the relative amount of blue and yellow light in the mixed comparison light source is varied can be arranged as desired and will depend, for example, on how the light mixing is achieved. Where the light is mixed by alternately flashing the LEDS, the relative amount of each colour in the mixture as perceived by a subject is preferably varied by adjusting the percentage of the flashing time that each different coloured LED is on for. In this case, the anomaloscope preferably then includes control means whereby a subject can adjust the relative percentages of flashing "on" time for the blue and yellow LEDs. (Where the LEDs making up the white light reference source are Flashed to "mix" them, the "green" and "white'' LEDS are preferably flashed on for equal percentages of the flashing "on" time, so as to obtain an equal mixture of their light.) As well as it being possible to adjust the relative - 14 mixture of blue and yellow light in the comparison light source, it is also preferably possible for a subject to be able to adjust the brightness of the reference white light source, most preferably in a manner that is independent of any adjustment of the brightness or relative mixture of the blue and yellow LEDS forming the comparison light source. Such brightness adjustment can be achieved as desired and how it is done will again depend on how the light for the white light source is mixed. Thus, where the light is mixed by alternately flashing the LEDS, the brightness of the reference white light source is preferably adjusted by varying the percentage of time that the reference white light source LEDS are flashed on for (i.e. the ratio of the "on" to the "off" time for the LEDS). In this case, the anomaloscope preferably then includes control means whereby a subject can adjust the relative percentages of on time for the LEDS making up the reference white light source.

It would also be possible to allow the brightness of the mixed comparison light source to be adjustable in use, for example in a similar manner, if desired.

The subject could view the various LEDS directly.

However, they are preferably viewed through a window, e.g. a circular window, as that then allows the size of the object to be viewed (i.e. the light sources) to be more easily controlled.

The viewing window is preferably divided in two by a screen so as to separate the reference white light source and the mixed blue and yellow comparison light source. The screen preferably divides the window in half (and thus would provide two semicircular light sources side-by-side where the window is circular).

The LEDS and window are preferably arranged such that the light from the LEDS is dispersed across the whole of the appropriate part of the window. This can be helped by potting the LEDS in an appropriate - 15 translucent or transparent mixture that will help to even out any variations in brightness across the viewing window (while not attenuating the light too much).

Alternatively, the LEDS (or other light sources) could be arranged to face away from the viewer, with their light being reflected back towards the viewer in such a manner that an even spread of illumination is achieved.

This could be done, e.g., by using an appropriately shaped light reflecting chamber to reflect the light back towards the viewer.

It is also preferred that the LEDs themselves are mounted out of direct line of sight of the subject (e.g. through the window) when in use, so as to help to avoid any very bright spots of light occurring in the window.

This can conveniently be achieved by, for example, mounting the LEDS in a cylinder which tapers to a narrower viewing window at one end, with the LEDs arranged such that the walls of the cylinder then block direct lineof-sight viewing of the LEDS in use. The cylinder is, as discussed above, preferably divided in two by a screen to separate the two light sources.

However, this may not be necessary where, for example, the LEDs being used have a sufficiently broad angle of view such that their light can be adequately distributed across the viewing area without any apparent "hot-spots. This may be possible when using SMT LEDS,

for example.

The LEDS are preferably laid out and arranged within the anomaloscope such that their spatial distribution provides substantially uniform (so far as is possible) brightness and colour across the region viewed by a subject. This helps to avoid the presence of any "hot spots" that could affect the colour vision test.

As discussed above, it is preferable to provide a particular size of object to be viewed in the anomaloscope. The viewing window could be suitably sized to achieve this, or alternatively, an appropriately sizedaperture could be placed at an appropriate position in front of the viewing window (as seen by the subject).

The lighting of the LEDs, their adjustment by a subject, etc. can all be controlled by appropriate electronics. The control circuits can be analogue or digital as desired, although a digital arrangement is preferred as that is believed to provide greater reliability and ease-ofuse. Any suitable digital system could be used, such as discrete logic, microprocessor control or an application specific integrated circuit (ASIC).

The power source for the electronics could be mains or batteries, as desired. Batteries enhance portability, but have more limited useful life before needing replacing.

The anomaloscope is preferably able to display the results of the test, i. e. the relative proportions of yellow and blue light in the mixed comparison source and, preferably, where provided for, of the brightness of the reference white light source. This can be provided by, for example, appropriate scales, dials or markings around the subject-operated controls.

Preferably, however, a more sophisticated display is provided, such as a liquid crystal or LED display.

The overall anomaloscope can be constructed as desired. It is preferably portable and easy to handle, and most preferably the anomaloscope can be rested on a surface when in use, and can be both hand-held and deskmounted. Suitable basic constructions have been found to be a telescopestyle design, and a microscope-style design having a base which can be rested on a surface and which will support the anomaloscope in use.

Although the present invention has been described with particular reference to the Pickford-Lakowski match - 17 for testing tritan vision, the principles behind it can equally be applied to other colour matches used to assess tritan vision, such as the Moreland match. In each case, the reference and comparison light sources are preferably comprised of appropriate LEDs, and form metameric matches, with the colours used lying on appropriate tritan confusion lines on the CIE diagram.

Thus, according to another aspect of the present invention, there is provided an anomaloscope for the testing of colour vision, comprising: a comparison light source comprising two or more light sources arranged so as to provide a light source comprising a mixture of their light; a reference light source which comprises a light source or sources arranged so as to provide a light source comprising a mixture of their light, and which does not provide an isomeric match with the comparison light source; an optical arrangement for allowing a subject to view the reference light source and the comparison mixed light source simultaneously; and means for allowing a subject to vary the proportion light from each light source in the comparison light source, whereby the subject may attempt to match the comparison mixed light source to the reference light source; wherein the reference light source and the light sources making up the comparison light source are selected such that their colours lie on or substantially on a tritan confusion line on the CIE diagram.

This aspect of the invention can, as will be appreciated by those skilled in the art, include any one or more or all of the preferred and optional features of the invention discussed herein, as appropriate.

The methods in accordance with the present invention may be implemented at least partially using software e.g. computer programs. It will thus be seen that when viewed from further aspects the present invention provides computer software specifically adapted to carry out the methods hereinabove described when installed on data processing means, and a computer program element comprising computer software code portions for performing the methods hereinabove described when the program element is run on data processing means. The invention also extends to a computer software carrier comprising such software which when used to operate an anomaloscope system comprising data processing means causes in conjunction with said data processing means said system to carry out the steps of the method of the present invention. Such a computer software carrier could be a physical storage medium such as a ROM chip, CD ROM or disk, or could be a signal such as an electronic signal over wires, an optical signal or a radio signal such as to a satellite or the like.

It will further be appreciated that not all steps of the method of the invention need be carried out by computer software and thus from a further broad aspect the present invention provides computer software and such software installed on a computer software carrier for carrying out at least one of the steps of the methods set out hereinabove.

The present invention may accordingly suitably be embodied as a computer program product for use with a computer system. Such an implementation may comprise a series of computer readable instructions either fixed on a tangible medium, such as a computer readable medium, for example, diskette, CD-ROM, ROM, or hard disk, or transmittable to a computer system, via a modem or other interface device, over either a tangible medium, including but not limited to optical or analogue communications lines, or intangibly using wireless techniques, including but not limited to microwave, infrared or other transmission techniques. The series of computer readable instructions embodies all or part - 19 of the functionality previously described herein.

Those skilled in the art will appreciate that such computer readable instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Further, such instructions may be stored using any memory technology, present or future, including but not limited to, semiconductor, magnetic, or optical, or transmitted using any communications technology, present or future, including but not limited to optical, infrared, or microwave. It is contemplated that such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation, for example, shrink-wrapped software, pre-loaded with a computer system, for example, on a system ROM or fixed disk, or distributed from a server or electronic bulletin board over a network, for example, the Internet or World Wide Web.

A number of preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows schematically an anomaloscope arrangement in accordance with the present invention; Figure 2 shows the optical layout of the anomaloscope of Figure 1; Figures 3A and 3B show the mounting of the LEDs in the anomaloscope of Figure 1; Figure 4 shows schematically a digital control arrangement for the anomaloscope of Figure 1; Figures 5A and 5B show an alternative anomaloscope layout; Figure 6 shows the spectrum of a "white" LED that can be used in the anomaloscope of the present invention; and Figure 7 shows an alternative arrangement for the mounting of the LEDs in the anomaloscope. -

Figure 1 shows an anomaloscope in accordance with the present invention. It comprises a body 1 made from a plastic tube and having a silicone rubber eyepiece 2 at one end which supports a lens 3. An iris 4 is mounted inside the tube 1 to cut down glare and reflection from the inside of the tube. There is also a battery container 5.

The anomaloscope includes two light sources. The first is a reference white light source 6 which is made up of a White LED (in fact comprising a blue LED which excites a yellow phosphor on its cover) and a compensating green LED, in accordance with the present invention.

A suitable "white'' LED for the reference white light source 6 is the NSPW 510 BS LED from Nichia Chemical Industries, Ltd. Figure 6 shows the spectrum of this LED. As can be seen, its spectrum in fact comprises a "blue" peak 40 at around 450 nm wavelength and a "yellow't peak 41 at around 560 nm wavelength. The spectrum also shows a gap or dip 42 at around 500 nm wavelength. The Applicants have found and recognised that this spectrum means that this "white" LED is unsuitable for use on its own as the reference white light source in the anomaloscope.

However, the Applicants believe that by filling in or compensating for the gap 42 in the 'white" LED's spectrum, a suitable reference white light source is obtained. In the present embodiment this compensation is achieved by adding a green LED which provides light of wavelengths in the gap 42 of the "white" LED's spectrum. A suitable such "green" LED has been found to be LEDtronics' BP280 CWB2K LED, having a wavelength of 505 nm. (Other LEDs can, of course, be used, the choice of which is governed by the criteria discussed above.) The second light source is a comparison light source 7 which comprises a blue LED and a yellow LED, with the relative amount of blue and yellow light as - 21 perceived by a subject in that light source being adjustable. A suitable yellow LED for this light source is LEDtronics' L200 GWGlK LED having a wavelength of 565 nm and a suitable blue LED is LEDtronics' BP 280 CWBlK LED having a wavelength of 450 nm. (Again, other LEDs can, of course, be used, the choice of which is governed by the criteria discussed above.) As can be seen from Figure 1, the two light sources are viewed side-by-side. Figures 3A and 3B show the mounting of the LEDs in more detail and will be discussed further below.

The anomaloscope provides subject-adjustable non-interacting adjustments of the brightness of the white reference light source 6 and of the proportions of yellow and blue light in the comparison mixed light source 7 (whose luminance is constant). The subject adjustments are achieved by means of a subject-adjustable mixture control 8 and a subject-adjustable brightness control 9. Those controls can, for example, comprise rotary dials.

The lighting of the LEDs that provide the light sources and the subject controlled adjustment is all controlled via a suitable electronic controller 10.

Suitable control arrangements are described in more detail below with reference to Figure 4.

Figure 2 shows the optical layout of the anomaloscope. There is a viewing tube 22 having a total length of 140 mm down which the test subject views the LED light sources so as to prevent external light from interfering with the light from the LEDs and confusing the subject. The tube also conveniently positions the subject's eye a set distance away from the light sources.

The viewing tube 22 includes at one end an LED cylinder 11 (which has a length of 40 mm) in which the LEDs are mounted. The LED cylinder 11 has a window 16 having a diameter of 7 mm through which the light from the LEDS iS viewed. A lens 3 through which the subject views the light sources is mounted at the other end of the viewing tube. The viewing tube 22 also includes an aperture 21 having a length of 10 mm and a diameter at its end distal from the LEDS of 6.3 mm, positioned in front of the LED cylinder 11, and between the LED cylinder 11 and the lens 3.

The optics of the anomaloscope is designed so that the light emitted from the LEDS is directed onto the fovea only. (As the anomaloscope is used to measure colour vision deficiencies, the only photoreceptors that need to be stimulated are the cones, which make up the entire fovea.) In this embodiment, the optics of the anomaloscope are designed such that the light emitted by the LEDS falls within a half angle of 2 .

The viewing angle of 2 is set by an appropriate choice of the height of the object to be viewed (i.e. the window 16 in the LED cylinder), and the distance the eye is from that object to be viewed.

As the distance from the eye to the object also determines the length of the viewing tube 22 required, that distance is preferably kept to a reasonable size to keep the anomaloscope relatively small and portable.

That distance will also determine the focal length of the lens 3, and so should be a standard length.

A distance of 100 mm from the window 16 of the LED cylinder 11 to the viewing lens 3 has been found to

fulfil these specifications. At that distance, the

height of the object (i.e. the radius of the window 16 in the LED cylinder 11) should be 3.5 mm to achieve a viewing angle of 2 .

As the light rays from the window 16 will initially radiate out and reflect off the walls of the viewing tube 22, an appropriately sized aperture 21 is placed in front of the window 16 to ensure that the light emitted from the window 16 initially follows the desired 2 viewing angle path. In an anomaloscope having the - 23 preferred dimensions discussed above, an aperture 21 situated 10 mm in front of the window and 6.3 mm in diameter will allow light at an angle of 2 or less only to pass through. (Light at an angle greater than 2 is absorbed by the aperture walls).

The 2 viewing angle of the light sources can be undermined by stray reflections off the inside wall of the viewing tube 22. It is preferred therefore that the inside of the viewing tube 22 is non-reflective. For example, the inside of the tube could be sprayed with matt black paint. In a more preferred arrangement, a non-reflective material is used to line the inside of the viewing tube, such as black felt, black cotton or black velvet.

In one preferred arrangement, the inside of the tube is lined by using a thin flexible foam material that can be rolled up into a tube and inserted into the viewing tube 22, but will naturally try to unfurl, thus pushing itself against the inside walls of the tube 22.

That avoids the need to glue a non-reflective lining inside the tube 22. The foam lining can be kept in place by the lens 3 at the top of the tube and the LED cylinder 11 at the bottom of the tube. The foam can either carry the non-reflective material on what will be its surface facing the inside of the tube, or have that surface sprayed matt black.

The light sources are viewed via the lens 3. This lens is arranged to set the image as seen by the subject at infinity. This is done because focussing the image of the window 16 may prove difficult for subjects who have hyperopia, long-sighted vision, due to the proximity of the window 16. If the image is out of focus and blurred it will make it difficult for the test subject to distinguish between the two halves of the window and therefore produce an accurate colour match.

By setting the image of the light sources to appear to be infinitely far away, the problem of the image being - 24 too close for hyperopic subjects is eliminated and the subject's eye will also be relaxed and unaccommodated.

The lens 3 used is a double convex lens. By setting that lens its focal length away from the window 16 of the LED cylinder 11 (i.e. the object being viewed), the light from the window appears to be parallel (i.e. from an object an infinite distance away). Thus in the preferred arrangement discussed above, the lens used to focus the image at infinity is a double convex lens with a focal length of 100 mm. (Of course, different lens arrangements could be used to set the image at infinity, if desired).

A double convex lens is used because the light rays emitted from the LEDS Will diverge, and therefore a converging lens is required to focus the rays into parallel rays. Although the lens alters the image of the window 16 so that the light rays from it are parallel, the angle that the light enters the eye is unchanged from the angle without the lens. Thus the focal length of the lens has to be the same as the distance required to give the 2 viewing angle, i.e. mm.

Figures 3A and 3B show in more detail the mounting of the LEDS in the anomaloscope. The LEDS providing the light sources are housed in a cylinder 11 which has a circular window 16 at it front (as viewed by the subject) for the light to emit from. Figure 3A shows a side sectional view of the LED cylinder 11 and Figure 3B a rear view.

The LED cylinder 11 is divided into two by a thin piece of aluminium 12, which allows the separation of the reference white light source 6 comprising the "white" LED 13 and green LED 17, from the comparison blue/yellow mixed light source 7 comprising the blue LED 14 and the yellow LED 15. As shown in Figures 3A and 3B, both light sources are allocated an equal half of the viewing window, although that is not necessary.

The LEDS are secured into place by using a potting mixture of a translucent resin, such as a mixture of Araldite and white paint, so as to even out variations of brightness while not attenuating the light too much.

The potting mixture helps to disperse the light from the LEDS across the whole of the window. As can be seen from Figure 3B, the LEDS are also positioned out of direct line with the viewing window 16, so as to avoid any "hot spots" or very bright spots of light occurring in the window.

The size of the LED cylinder 11 should be appropriate for the size of the viewing tube. Thus its diameter should be the same as the inside diameter of the viewing tube, such as 32.2 mm. The length of the cylinder is around 40 mm so as to allow sufficient space to house the LEDS. The window 16 of the LED cylinder is the object being viewed, and so for an anomaloscope of the dimensions discussed above, its radius should be 3.5 mm (i.e. a diameter of 7 mm).

Figure 7 shows an alternative arrangement for the mounting of the LEDS. In this arrangement, the LEDS 13, 14 are mounted facing away from the viewer, and a light reflecting chamber 70 is provided to reflect the light back towards the viewer through a viewing window 16.

The arrangement also includes a stimulant screen 71, a light source divider 12, and an iris aperture 72. In this arrangement, the viewing window 16 is 100 mm from the lens 3, and has a diameter of 6 mm. The iris aperture 72 has a diameter of 8 mm and is 57 mm from the lens 3. This arrangement also helps to provide more even illumination as seen by the viewer and to avoid the presence of "hotshots".

The illumination of the light sources and their adjustment by the subject is controlled and driven by an appropriate electrical circuit in the anomaloscope. The anomaloscope is powered by batteries, such as two standard AA cells which are mounted in a conventional battery holder 5 (Figure 1) that allows the batteries to be changed easily when necessary. It could alternatively or additionally be mains powered.

The anomaloscope's circuit is required, as discussed above, to provide non-interacting adjustments of the brightness of the reference white light source and the proportions of blue and yellow in the constant- luminance mixed blue and yellow comparison light source.

The mixture of the blue and yellow light is achieved by flashing the blue and yellow LEDS on and off alternately to produce a perceived white. The LEDS are flashed at a frequency above the fusion frequency of the human eye, so that the eye cannot distinguish the flicker. The overall frequency of oscillation is preferably set to about 312.5 Hz (although lower frequencies could be used) so that there is no chance of any flicker being perceived. The mixed colour perceived by the subject is adjusted by adjusting the percentage of the flashing time that the blue or yellow LED is on for. Thus the colour ranges from blue, when the flashing time of the yellow LED is zero, through perceived white, to yellow when the blue LED flash time is zero.

The reference white light source is formed from the mixture of the "white" LED and the compensating green LED. Again, the perceived colour mixture is achieved by flashing the two LEDS on and off alternately (but for the same overall amount of time) to produce a perceived mixed colour. The brightness of the white light source is adjusted by varying the percentage of time that the LEDS are flashed on for. When the LEDs are off there is no light, i.e. black, and as the LEDS are on for progressively more time, the perceived brightness increases. Again, the flashing should be over the critical fusion frequency of the human eye.

The flashing of the LEDs is controlled by a suitable electric circuit, which can receive instructions from the test subject via the anomaloscopets controls to vary the mixture and brightness of the light sources. It should be able to flash the LEDs in antiphase with a variable mark-space ratio.

The anomaloscope could be controlled using a suitable analogue control circuit or by using a suitable digital circuit. A digital arrangement is preferred, as digital circuits tend to consume less power, thus leading to a longer battery life, and to be more reliable. A digital circuit may also be relatively low cost, allow the results to be presented more clearly, and easier to use.

Figure 4 shows a block diagram of a suitable digital circuit for controlling the anomaloscope. The main component is a microcontroller 50, which receives inputs from a colour control 51 and brightness control 52 which can be adjusted by a subject, and controls the operation of the LEDs 53 accordingly, and provides an appropriate output display 54 of the results of the test. The overall circuit is driven by a power supply unit 55, which can be batteries, mains or a combination thereof.

In the embodiment of Figure 4, a microcontroller is used for the main digital control. However, as well as such a microprocessor and software controlled circuit, other suitable forms of digital control, such as discrete logic or an application specific integrated circuit could be used, if desired.

The colour control 51 and brightness control 52 can be any suitable forms of controls, such as up and down buttons, or rotary controls. Rotary controls are preferred as they may be easier to use. Suitable rotary controls would include a resistive potentiometer which supplies a voltage dependent on position. That voltage could then be converted via an analogue-to-digital l - 28 converter to generate a digital control input into the microcontroller 50. Another suitable form of control would be a digital contacting or optical encoder which produces digital pulses whose repetition is dependent on the rotational speed and whose phase is dependent on the direction of rotation.

The microcontroller 50 has to control the mixing of the LEDs and their relative brightness in response to the inputs from the subject operated controls. This could be done by, for example, using a digital-to-analogue converter to control the current flowing through the LEDs, or by pulse width modulating the voltage across the LEDs to control the average current.

The circuit also includes a suitable display 54 for displaying the results of the test, i.e. at least the brightness setting set by the subject and the mixture chosen by the subject to match the reference white light source, so that an assessment of any colour disorders can be made. Suitable displays would be, for example, a seven segment LED display, a liquid crystal display, or an alphanumeric liquid crystal display module.

An LED type display would probably be less expensive, but would tend to use more current than the other displays - thereby leading to a shorter battery life where batteries are used. An alphanumeric display would be more expensive, but has greater flexibility in terms of the information that can be displayed. The use of a more complex display, such as an alphanumerical dot matrix LCD display would allow more sophisticated results to be displayed. For example, two rows of text can be provided, the first comprising the percentage of the colour mixture and the second the percentage of brightness.

A suitable set of digital components has been found to be a PIC16F76 or PIC18F252 microcontroller from Microchip, Bourns ECWOJ Digital Contacting Encoders, and an Hitachi LM016XMBL Display.

The physical construction of the anomaloscope can be arranged as desired. It should accommodate the various components of the anomaloscope and the appropriate optical setup together with being relatively easy to use. The anomaloscope is preferably capable of being both desk-mounted or handheld. It is preferably readily portable. The subject operable controls are preferably arranged to be situated on the outside of the anomaloscope and on the side corresponding to the respective side of the viewing window which the control varies for ease of use.

One suitable arrangement for the anomaloscope has been found to be a telescope-style design, as illustrated in Figure 1. This houses the optics, circuits and batteries in a straight tube, with the subject operable controls on either side of the tube.

An alternative design, having a microscope-style, is shown in Figures 5A and 5B. Figure 5A is a side cross-sectional view and Figure 5B is a front view of the anomaloscope. The microscope design shown in Figures 5A and 5B uses a tube for the optical arrangement, but the electronics and batteries are now housed in a base box 61. Thus, there is still a viewing tube 22 mounting a lens 3 and having an eyepiece 2, with the subject operated controls 8 and 9 mounted on either side of the tube and the LEDs mounted in their cylinder 11 at the end of the tube. However, a larger, flat box 61 which can be stood on a surface is provided at the base of the anomaloscope for containing the control circuit 10, batteries 5 and output display 62. The base box has a flat base to allow the anomaloscope to be mounted on a desk for ease of use.

In this microscope design, the viewing tube is angled at 70 from the horizontal to allow for a more comfortable viewing position for the subject when the anomaloscope is rested on a desk.

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The subject operated controls are mounted on either side of the viewing tube at right angles to the viewing tube. The controls are mounted on the respective sides of the viewing tube to the half of the LED viewing window that they control. For example, the right-hand control controls the colour mixture, which is varied on the right-hand side of the LED window as seen by the subject. This allows the test subject a simpler understanding of how to operate the anomaloscope thus making it more user- friendly.

The display is mounted on the side of the base box furthest from the subject when looking into the eyepiece. This prevents the subject from seeing the results of the test as they are using the anomaloscope.

In use of the anomaloscope, the subject would adjust the mixture control to try to match the reference white light source. The relative proportions of blue and yellow light in the mixture as matched by the subject is displayed and can be recorded and used to assess the subject's tritan colour vision.

The anomaloscope of the present invention provides a relatively inexpensive, reliable and easy-to-use, robust and portable anomaloscope for testing tritan vision that has been found to provide satisfactory test result. Particularly when using a digital control circuit, the anomaloscope is reliable as well as more ergonomic and user-friendly.

Where digital control circuits in particular are used, it would be possible if desired to include in the anomaloscope a means for compensating for tritan vision variations that may be due to factors other than the factor being tested for. For example, tritan vision tends to vary with age, so if desired the anomaloscope could be arranged to compensate the results it displays for the known age of the subject (for example using previously tested results showing the age variation).

For example, it is envisaged that the anomaloscope - 31 of the present invention may have particular application to the detection of sightthreatening diabetic retinopathy. Deficiencies in tritan vision are a symptom of sight-threatening diabetic retinopathy.

One difficulty with testing for diabetic retinopathy in this way is that the human eye also deteriorates with age in a manner which affects tritan vision. (In effect, the lens of the eye yellows with age, thereby attenuating blue light so that less of it is seen.) Furthermore, one effect of diabetes is to further accelerate this ageing effect. It istherefore important when testing for sight-threatening diabetic retinopathy using a tritan colour vision test to be able to distinguish between tritan colour vision deficiencies caused by diabetic retinopathy, and such deficiencies simply due to "ageing" effects.

Equations have therefore been developed which can derive an effective lens yellowing "age", and the effective tritan deficiency that is caused by such a lens yellowing "age", based on the actual age of the test subject and the duration of the test subject's diabetes (if any).

For example, Moreland in 1993 derived two equations which relate Lutze and Bresnick's (1991) linear lens yellowing model with Pokorny et al's (1987) linear model of normal lens "ageing": E = A + 2.54T - 3.3 (1) E = 0.30A + 0.76T + 40.9 (2) where: E = Age of a normal control group having the same lens absorption characteristics as diabetics, A = Age of the diabetic, and T = Diabetic duration (T must be,1.5 years). - 32

Equation (1) is used when E is greater than 20 but less than 60. Equation (2) is used when E derived from equation (1) is greater than 60. These "lens equated" formulae can be used to derive the lens-equated age of a diabetic subject, which can then be used to select the appropriate ''lensequated'' control data from an appropriate control database to give the effective lens yellowing caused by normal lens "ageing" for the subject. Any remaining tritan deficit can then be attributed to diabetic retinopathy.

By taking account of the tritan deficiencies caused by ageing in this manner, the remaining deficiencies (which can be assumed to be due to diabetic retinopathy) can be determined and therefore a measure of the extent of sight-threatening diabetic retinopathy in the tested subject can be obtained.

Thus, where the present invention is to be used for an assessment of diabetic retinopathy, it is preferably arranged to display the effective tritan colour deficiency caused by diabetic retinopathy alone, by incorporating into the anomaloscope suitable means, such as software, to derive the effective lens yellowing "age" of a test subject given their actual age and the duration of their diabetes, and to then effectively remove the tritan deficiency that would be caused by that effective lens yellowing "age", so as to leave a measure of the remaining tritan colour vision deficiency that is likely to be caused by diabetic retinopathy alone (which is then displayed). . - 33

Claims

1. An anomaloscope for the testing of colour vision, comprising: a comparison light source comprising a blue light source and a yellow light source arranged so as to provide a light source comprising a mixture of their light; a reference white light source which comprises a light source or sources which when viewed together presents a white light, and which does not provide an isomeric match with the comparison light source; an optical arrangement for allowing a subject to view the reference white light source and the comparison mixed blue and yellow light source simultaneously; and means for allowing a subject to vary the proportion of blue and yellow light in the comparison light source, whereby the subject may attempt to match the comparison mixed blue and yellow light source to the reference white light source.

2. The anomaloscope of claim 1, wherein the optical arrangement is arranged to position the subject's eye a fixed distance from the light sources.

3. The anomaloscope of claim 1 or 2, wherein the optical arrangement is arranged such that the light emitted from the light sources as seen by the subject is directed onto the subject's fovea only.

4. The anomaloscope of claim 1, 2 or 3, wherein the comparison light source comprises a yellow LED and a blue LED.

5. The anomaloscope of claim 1, 2, 3 or 4, wherein the reference white light source comprises a blue LED which is arranged to excite a yellow phosphor, and a separate - 34 green LED for providing light of wavelengths between the blue and yellow peaks of the spectrum of the light produced by the blue LED when it excites the yellow phosphor, which when viewed together present a white light.

6. The anomaloscope of claim 4 or 5, wherein the light from the LEDs making up each light source is mixed together by flashing the respective LEDs on and off alternately to produce a perceived colour mixture.

7. The anomaloscope of claim 6, wherein the relative amount of blue and yellow light in the mixed comparison light source is varied by adjusting the percentage of the flashing time that each different coloured LED is on for.

8. The anomaloscope of any one of the preceding claims, further comprising means for allowing a subject to adjust the brightness of the reference white light source.

9. The anomaloscope of any one of the preceding claims, further comprising: means for displaying the results of the test.

10. An anomaloscope for the testing of colour vision, comprising: a comparison light source comprising two or more light sources arranged so as to provide a light source comprising a mixture of their light; a reference light source which comprises a light source or sources arranged so as to provide a light source comprising a mixture of their light, and which does not provide an isomeric match with the comparison light source; an optical arrangement for allowing a subject to - 35 view the reference light source and the comparison mixed light source simultaneously; and means for allowing a subject to vary the proportion light from each light source in the comparison light source, whereby the subject may attempt to match the comparison mixed light source to the reference light source; wherein the reference light source and the light sources making up the comparison light source are selected such that their colours lie on or substantially on a tritan confusion line on the CIE diagram.