Colour Testing Apparatus

1. Colour Testing Apparatus perceptually accurate high dynamic range colour compared to many primaries at fixed locations on spectrum monochromatic light

2. Projector Array Multiple projectors lined up and with external color filters • Spectrum of projector modified to operate without colour wheel • for comparison normal RGB levels are also shown • the light source is UHP Mercury Arc This level is about half of natural sunlight. Panasonic PT-D Series of projectors were used for initial trials but poor optics and insufficient lens shift meant they could not be used for a projector array. The RGB levels below show the light levels normally seen from this projector. This level is current HDR Television

3. Ideal Light SourceSunlight vs Xenon Short Arc Sony makes the only consumer projector with a xenon lamp Ideal illuminant is simply equal energy across visible spectrum. Natural sunlight is close to ideal, but xenon is not far off. Natural Sunlight can vary between 5 and 500, but most common is about 100, which is roughly the maximum level that is comfortable for humans. ideal light source Light projected by Sony VPL-VW100 at 1 m2 about 1/100th this level.

4. Optical Filter Design The filters shown are very precise. Each of these filters has been sourced and can be ordered. Data is from spectrography accurate to less than 1nm. The white balance is very important. Ideally it should be completely flat. Colours are always with reference to the white balance. This graph shows the filters used with an ideal light light source, summed together.

5. Filters applied to conventional UHP light Each coloured line represents actual output for each projector This graph shows what the actual light source from a projector array will look like. The filters are applied to each projector and the intensities are adjusted to make the output of each primary approximate an overall equal energy spectrum. The result is still a bit spiky but is acceptable. The more narrow each primary the less unevenness matters. There is a gap between 490-500nm. Many people cannot see colour correctly in this area. Gap may be filled with additional projector. It is important to display a reference white to set the colour balance for the viewer. The best white is a flat spectrum equal energy light. Humans are inherently Red and Purple weak. If these are boosted above white levels then they will appear stronger (appearing lumiescent, like ‘flame’ red for example). Empirical colour matching tests will show how well the boosted purple & red will work.

6. current display technology The Apple 5K red primary is made by adding the light from two fairly monochromatic LEDs together. One at 625nm and the other at 645nm. This shows older flourescent backlighting vs more recent LED backlighting. Visible is the tradeoff between yellow and green/red within products from the same manufacturer. The Apple MacBook Pro has opted to optimize for yellow, whereas the Apple 5K display has opted for improved reds. The Apple 5K is also more than 2x as bright.

7. high dynamic range displays New generation displays that advertise high dynamic range. The variety within the primaries shows there is no general consensus on where on the spectrum the primaries should be. These blue primaries are getting dangerously close to being purple. For most people blue is at about 475nm. Sony goes down to about 465nm, which in isolation most people see as slightly purple.

8. Reproducing Colour: taking inspiration from nature There are 4 conclusions that are immediately apparent when studying organisms that have evolved colour displays: • Absorption and reflection are extremely precise and in the areas of importance almost completely flat – almost to the accuracy of the instrument. This reflects the inner workings of the visual systems which are the intended targets of the colour display.As can be seen from the Primula flower, white is almost completely flat from 450nm to 700nm.

9. colour is produced by shifting wavelength • Colour is changed by shifting the wavelength point between absorption and reflection. This can be coded simply as wavelength.

10. the importance of yellow Almost all mammals are dichromats and can see only blue or yellow. This is reflected in organisms that display colour. Most colour takes yellow as a starting point and then shifts toward the red. Even black is often a variation of this basic principle. • Yellow starts at approximately 525nm, and represents a hard boundary, with any light below that boundary activating the blue sensor which is subtracts from yellow and adds white. The transition point squared ... to show exactly where it is located on the spectrum.

11. colours ranging from yellow to red Natural colour shifts by wavelength, but not dynamically. It is very difficult to shift wavelength for a display that can change colour. But a set of narrow primaries can approximate this shift. • Yellow is the base for most colours. Shifting it to the right produces orange and red, and reversing that shift produces goes from yellow to green.

12. Colour Space Traditional colour models are defined by their primaries and at best can display half of the colours we can see. By using many primaries, we can come very close to being able to display all the colours human perception can see. More primaries can be added for greater fidelity, or fewer can be used if necessary. We can measure how close by use of the CIE colour chart. Monochromatic (pure) colours are the outer border of the colour space. Each point is the mid-point of the primary (which is (more or less) monochromatic light) Space enclosed by the colour model defines the colours that can be displayed.

13. Flexible Number of Primaries It may be observed from the CIE colour diagram that Green is the most difficult primary. A hard coded blue barrier that varies significantly between individuals means a tuneable green primary is essential. Other primaries are soft coded and more flexible. The minimum number of primaries is therefore 5: Red, Yellow, Green High, Green Low, and Blue. with all primaries tuneable: 8 + purple: 9 with tunable purple and magenta + boost: 12 With extra precision on cyan: 15 20 primaries will reproduce almost all colours that can be perceived, and would be expected to be a hard upper bound. This is analogous to new high fidelity 3-dimensional audio standards, where the number of speakers is variable. Upwards of 6 speakers are mandated, even though humans have only 2 ears. The number of primaries can be varied according to requirement for colour accuracy. Monochromatic (pure) colours are the outer border of the colour space. Space enclosed by the colour model defines the colours that can be displayed.

14. Testing Colour Perception By using tuneable primaries, it is possible we can display 100% of the colours that human perception can see. That would mean the CIE colour model is flawed. The hypothesis that primaries tuned to the individual will allow all colours to be reproduced (without the problem of negative primaries) will be tested empirically. The primary focus, however, will be on reproducing colour with greater accuracy, and it is clear from the CIE model of colour that a greater number of primaries allows for more colours to be reproduced. This is analogous to audio; where 2 speakers should suffice to reproduce any sound. But human audio perception is dedicated to locating sound in a 3-dimensional space and in practice sound produced by 2 speakers is not sufficiently 3-dimensional. Current ‘surround sound’ audio uses 6 or 8 speakers. But even this is insufficient. The next generation of ‘surround sound’ will use 64 speakers in a commercial cinema and 12-18 speakers for home cinema. Surround sound does not work if we code only for stereo, and in the same way if we code only for RGB this stops any attempt to reproduce better colour. No matter how many primaries, there will always be small gaps for colour reproduction. But colour coding can be much better than human perception. A Red-Green, Blue-Yellow camera can accurately record any colour any human (or bird, reptile or insect) can see. Due to fact that perceptual product of multiple levels of processing by the brain, it is likely that colour reproduction will never be 100%. The most important part is to code colour so that we can get arbitrarily close to 100%.

15. Circular Colour Model used for coding colour • Colours are coded according to underlying principles of colour perception:opponent pairs of Red-Green and Blue-Yellow. • Colour reproduction is done by tuning the display to each individual, according to their ability to see colour. • Colours are coded as ratios of Red-Green and Blue-Yellow – with the assumption these are fundamental and universal. • Colour is represented abstractly as the intended perception, not as brightness values for individual primaries. Producing full colour images: • Ordinary digital camera with colour filter removed. B&W conversion or dedicated B&W camera. • 4 special colour filters:Red, Yellow, Green and Blue. • Take same picture 4 times, once with each filter.

16. Guaranteed Color Many people see colour in dramatically different ways, but the fundamentals are the same for everyone. Coding colour in a standard way according to the fundamentals allows colour to be reproduced to the ability of the individual to perceive it, rather than the lowest common denominator. Testing the individual’s colour ability and then tuning the display specifically to their colour vision allows colour to be coded and reproduced with perceptual accuracy. The color model can therefore guarantee that a colour will be perceived as intended.(or as close as intended according to the viewer’s ability to perceive and the monitor’s ability to produce it). The Colour model will simply specify the intended colour: Green for example. The display must reproduce that colour for the specific individual (or group). Green for person A Green for person B Green for person C Space enclosed by the colour model defines the colours that can be displayed.

17. in practice Many projectors separate light into 3 parts, and produce 3 images internally which are then combined to form a single image. With current technology: two projectors can produce 6 primaries. three projectors can produce 9 primaries. four projectors can produce 12 primaries.

18. an example Person A wants to create a Green with just a hint of Blue. Lets assume Person A sees colour very accurately and his green is right at 525nm. B sees colours slightly differently and the best green for her is at 535nm. C is a bit colour blind and he sees green at 550nm and he doesn’t see any blue until 500nm. Person A sits in front of his monitor which has been tuned to his specific colour vision and he wants to create the colour green with just a hint of turquoise, which for him would be 515nm. His monitor can’t display this, so it gives him the closest colour to what he wants, which is a hint of turquoise with a touch of white (which is an equal mix of the closest primaries at 525 and 510. A sees this small amount of white and is not entirely happy, but it’s the closest to what he wants to achieve. He checks his colour selector and it shows a perfect 515, so he knows the problem is with his monitor. So even though his monitor cannot display the colour he wants A can still choose the correct colour. A knows that anyone who will see his colour will be able to see it as he intends, or as close as possible depending on the limitations of their display and their vision. He now sends his colour to B and C to look at. B sees green at 535 and for her a slightly turquoise green is at 525. The 515 looks like a perfect turquoise to B. Her monitor can display both colours perfectly because B just bought a high end cinemaphile monitor with extra primaries. Because her monitor knows her colour vision it maps A’s colour to 525. She therefore sees the colour that A sent just as A intended. C is a bit colour blind and sees green at 550nm. Between 510 and 525 things lose their colour and look greyish. At 500nm things start to look blue-green. C’s colour vision is very different from the average colour vision that the CIE colour diagram shows, so this is not much help in getting C to see colours as well as possible. He would need a colour chart tailor made for people who see colour like C does. The results from a colour vision test on C show there is a gap in his colour vision, and this allows C’s monitor to know it cannot display colour in this range, and as a result it just ignores the primaries in this range and goes straight from 550 to 480. However, C has a special high end monitor with an extra primary at 495 to help him see colours more accurately, so the monitor can go from 550 to 495 before going to 480. With this extra primary, C can see A’s colour just as well as B can. Now if B wants to devise a visual test to detect C’s aberant colour vision she will create an image with a gray background and turquoise dots that form numbers or letters. On a normal monitor C would see just gray and could not read the numbers or letters because his vision does not see light in that range as coloured. But because B used the new universal colour model the colours are coded as they are expected to be seen. When the test is displayed on C’s personal monitor it translates the colour that B sees at 515 to 495 for C and as a result C can read the letters and numbers on C’s test easily. Because C’s colour vision is not very good he often takes his new mobile phone (with a screen coded with his colour prescription) with him to help him see colours better. Now C can see flowers that appear to him unaided as all pretty much the same black or very dark red as bright magenta on his screen. C can now clearly see the different shades of magenta or purple, and can discriminate between these coloured flowers in the same way that birds or bees can. C can see magenta and purple just fine but only as a mixture of red and blue, not spectrally as birds can. When C goes for walks in the countryside with A they no now longer disagree about colour. A has very good colour vision but nevertheless its not quite as good as that of a bird. As a result A now sometimes asks C to help him to identify colours that are at the extreme ends of the visible spectrum. When C edits his photos he also doesn’t make the mistake anymore of making his photos look cyan or turquoise. That’s because his monitor no longer shows him those colours which he cannot see correctly. This is a great help but no colour model can improve the resolution of C’s colour vision and as a result C’s edits are never quite as color accurate as those of A. Green for person B Green for person C Best that can be achieved for Person A for turquoise-green.

19. Testing Colour In principle this is a very simple test. We just go through all the colours of the spectrum and at each colour we see if we can find a match with the primaries of our colour model. This is empirical testing with human subjects. A subject will be presented with a colour and will have a simple dial to find a match with. The center will be the spectral colour and the surround the colour produced by the primaries, which are at fixed positions on the spectrum. Produced by primaries from projector array Produced by pure monochromatic light that can be varied to anywhere on the visible spectrum

20. using conventional projectors to test colour This is a conventional projector where the light is reflected with a single digital micro-mirror device (DMD). Remove the colour wheel and you have access to all the light. The problem is that the UHP lamps produce poor quality light that is unsuitable for continuous filtering. It is possible to modify the lamps to replace them with xenon light. This is difficult and expensive, but can be done Ideally, we want a single DLP projector which comes with a xenon lamp. ProjectorCentral (which has a comprehensive database of projectors) does not list a single projector with this specification.

21. Linear Variable Filter A linear variable filter filters the light to a narrow band of wavelengths and this can be varied through the whole spectral range. When we place the filter on a computer controlled linear stage and we can select any arbitrary wavelength in the range. We replace the colour wheel (which is discrete spectral filter) with the linear variable filter. This gives us a projector with a light source we can vary continuously across the visible spectrum.

22. Christie RPMS-500Xe 500w Xenon Lamp Single DMD with a colour wheel. Lens is short throw – a long throw lens will be needed 500w is insufficient for full screen – with long throw lens the full image can be reduced to the colour matching rectangle Large, but modular which makes it easy to work on and modify.

23. Sharp XG-PH50/70 • 620mm vertical shift520mm horizontal shift • 50% flip advantage • 120mm to middle of lens • 190x400mm Stack: 2x4 array Shift is based on 1000x750mm image.

24. Mitsubishi XD2000 • 520mm vertical shift245mm horizontal shift • 50% flip advantage • 215mm to middle of lens • 170x430mm Stack: 1x4

25. Projection Design F3 • 825mm vertical shift900mm horizontal shift • 110mm to middle of lens • 200x500mm Stack: 3x4 or 4x4 Illustrative only, one column will simply be upside down to bring lenses as close as possible.

26. NECPX800X • 400w x2 • 500mm vertical100mm horizontal • 190x500mm Projector in the middle behind front row of projectors

27. Colour Diagnostic Testing • To display colour with perceptual accuracy the spectral location of the primaries needs to be determined – Red, Yellow, Green and Blue. • Spectral regions with colour anomalies can be managedeg. deuteranomaly leads to lack of colour between 490 and 510nm where cyan is normally perceived. These regions are dealt with, leaving only a perceptually linear shift between green and blue. • Perceptual mid-points between primaries are established. For greater accuracy the quartile midpoints can also be determined. • Sensitivity to colour brightness is determined.

28. locate the perceptual primaries primaries • Red, Yellow, Green, Blue primary midpoints • Magenta, Orange, Cyan, PurpleMany mid-point colours are treated by most people as perceptually distinct (almost as if they were primaries) and therefore subjects can be asked to located the ‘best’ Orange. About half of people can see spectral Purple and about one in ten can see a spectral Magenta. Cyan recognition is poor, with most people reporting blue-green.

29. boundaries of primaries • some individuals have very broad primaries, in which case the transition starting points needs to be determined • some individuals have primaries that are very precisely located. • other individuals are ‘hypersensitive’ and have overlapping primaries (that is a ‘pure’ primary can’t be found). In this case the best position is used.

30. deal with any anomalies • transition between primaries should be lineareg. green should transition to blue step by step, starting with a green that has a hint of blue, to green-blue which has equal proportions of blue and then with a steady decline in the proportion of green until the blue is pure. • some people have colour anomalies, eg. individuals with deuteranomaly see the spectral region between 490-510nm (normally perceived as cyan or blue-green) as uncoloured (grey or ‘whitish’). This is dealt with, giving an even distribution of blue-green colours, with cyan at the midpoint.

31. locate midpoints & quarter midpoints • a gradient between 2 adjacent primaries will be used to determine midpoints accuratelyeg. midpoint between green and yellow has no commonly used name, whereas a gradient shows the perceptual distribution. • quarter points can also be determined for additional perceptual accuracy

32. Colour Resolution • perception of banding used to determine Just Noticeable Difference of spectral colour JNDs are a measure of perceptual accuracy • Colours should be coded roughly at 4x JND. There are 2 types of JND • Brightness difference – DICOM • Spectral difference – shift in wavelength

33. Correcting Anomalous Colour Vision Standard sRGB – cyan is not in the centre as it should be cyan corrected so that it is the centre of the gradient cyan as seen by someone with deuteranomaly (red-green colour blindness, most common at about 5% of male population) the blue-green gradient as seen by someone with large areas of invariancy at the primaries

34. Recording Colour one universal standard for perceptually accurate colour monochrome camera with 4 custom filters