Category Archives: knowledge

colour lighting



Very excited with the temporary installation of our new spectral lighting system at Leeds University. Whereas most coloured lights are based on RGB, we have a system that has a lot of spectral control (it works by having 11 different coloured LED primaries). We have several PhD students who are using these lights with their research. Nic and Yiting are looking at the effect of light and colour on alertness and also on impulsivity. Meanwhile, Soojin (pictured) is looking at the effect of colour on creativity (though in her study we won’t be using really saturated colours like those shown in the pictures). Hoping for some great publications on this soon. However, if you are interested in whether coloured lighting can affect heart rate and blood pressure take a look at our AIC publication (pdf) that we presented in Tokyo in 2015.

RGB displays are more complicated than you think

Nexus_one_screen_microscope

Most people assume that display screens are based on RGB – that is the amount of red, green and blue light emitted is controlled in three signals. We tend to think that there is an RGB ‘value’ at each pixel. However, the reality is a bit more complicated. The picture above is a close up of the sort of display on the Samsung Galaxy S phones, as well as the Nexus One. It is called an RGBG pentile layout. This layout was introduced because our eyes are more sensitive to green light (so green pixels don’t need to be as physically large to appear just as bright to our eyes). However, it means that the ‘pixel’ in a standard AMOLED display consists of 8 colours: RGBG on top of BGRG. Some people claim this leads to less sharp images compared to the standard RGB displays of LCD displays (see below) that are sometimes referred to as real-stripe displays.

LCD-rgb-subpixel-matrix

Some of the AMOLED displays have an RGBW layout, which adds a white subpixel next to the standard RGB subpixels. This allows the display to have an edge in brightness due to a dedicated white subpixel. With that advantage the backlight doesn’t need to be as bright, which saves battery since the backlight is a major user of battery in a mobile device. There is also Samsung’s latest Super AMOLED display technology that has a new subpixel arrangement called the Diamond Pixel. The first phone to use this pentile type was the Galaxy S4. There there are twice as many green subpixels as there are blue and red ones, and the green subpixels are oval and small while the red and blue ones are diamond-shaped and larger (the blue subpixel is slightly larger than the red one).

Displays are much more complicated and varied than you might think. One consequence is that it is not so easy to compare the resolution of different displays technologies beacause they vary in what they call a pixel.

non-visual effects of light

Most people know that the ear system has two functions: hearing and balance. It is less well known that the visual system also has two functions. The first is seeing. The second is a set of non-visual functions including circadian rhythm. Mechanisms are being discovered that are particularly sensitive to blue light. So short-wavelength, or blue, light inhibits melatonin which is a chemical that makes you drowsy. So looking at bright lights late at night, especially blue ones, can contribute to a poor night’s sleep. So put your smart tablet away now and go to sleep!

In all seriousness though, I knew there was a reason why I do not like watching Chelsea on Match of the Day.

colour

Studying these functional effects of colour and how they can be used in design is a major theme of the research I lead at the University of Leeds in the School of Design. If you have interest in these areas please contact me.

Looking for colour blind people

colour blind example

Most colour blindness is hereditary. The faulty ‘gene’ for colour blindness is found only on the X chromosome. You have two X chromosomes if you are female or an X and a Y chromosome if you are male. It is because females have two copies of the X chromosome that they are far less likely to be colour blind. A male inherits his X chromosome from his mother and his Y chromosome from his father. So men do not inherit colour blindness from their fathers but from their mothers who can be carriers if they have one faulty X chromosome. Snoooooooooze. Probably you are bored reading this. The real point of this post is to say that Bradford University in the UK are studying colour blindness and are seeking females who are not colour blind but who have a child or a sibling who is. If this sounds like you please get involved in the study, help someone get their PhD, and maybe find out something interesting and useful. For more details see here.

colour helps you sleep

cat-686827_640

Light in our natural environment tends to be bluer first thing in the morning and redder at dusk.

Researchers from the University of Manchester looked at the change in light around dawn and dusk to analyse whether colour could be used to determine time of day. They constructed an artificial sky beneath which they placed mice and they then measured the body temperature of the mice for several days and their body temperature was recorded. The highest body temperatures occurred just after night fell when the sky turned a darker blue – indicating that their body clock was working optimally. When just the brightness of the sky was changed, with no change in the colour, the mice became more active before dusk, demonstrating that their body clock wasn’t properly aligned to the day night cycle.

According to Dr Timothy Brown: “This is the first time that we’ve been able to test the theory that colour affects the body clock in mammals. It has always been very hard to separate the change in colour to the change in brightness but using new experimental tools and a psychophysics approach we were successful. What’s exciting about our research is that the same findings can be applied to humans. So in theory colour could be used to manipulate our clock, which could be useful for shift workers or travellers wanting to minimise jet lag.”

colour physics 101

kindle_colourphysicsfaq

Download my colour physics FAQ e-book for the Kindle here.

Also available as a physical book from Amazon.

  • What is colour?
  • How does colour vision work?
  • Why is the sky blue?
  • What is the colour spectrum?

The answers to these and many other related questions about colour physics are each provided in a short and easy-to-understand form. Will delight and entertain colour professionals and curious members of the public.

accurate colour on a smartphone or tablet

Electronic displays can vary in their characteristics. Although almost all are based on RGB, in fact the RGB primaries in the display can vary greatly from one manufacturer to another. Colour management is the process of making adjustments to an image so that colour fidelity will be preserved. In conventional displays – desktops and laptops – the way this is achieved is through ICC colour profiles. Colour profiles store information about the colours on a particular device that are produced by RGB values on that device. So to make a display profile you normally need to display some colours on the screen and measure the CIE XYZ values of those colours; you then have the RGB values you used and the XYZ values that resulted. The profiling software can use these corresponding RGB and XYZ values to build a colour profile so that the colour management engine knows how to adjust the RGB values of an image so that the colours are displayed properly. Building a profile often requires specialist colour measurement equipment – though this can often be quite inexpensive now. If you are using your desktop or laptop display and you have never built a profile then you are probably using the default profile that was provided when your display was shipped. The default profile will ensure some level of colour fidelity but particular settings (such as the colour temperature or the gamma) may not be adequately accounted for. If you want accurate colour then you should learn about colour profiling.

It all sounds simple except for the fact that ICC colour profiles are not supported by iOS or Android operating systems on mobile devices. I find this really surprising but that’s how it is for now. Maybe it will be different in the future.

This means that ensuring colour fidelity on a smartphone or tablet is not so straight forward. So what can you do?

Well, there are two commercial solutions to this problem that I am aware of. They are X-rite’s ColorTrue and Datacolor’s SpyderGallery. ColorTrue and SpyderGallery are apps that will use a colour profile and provide good colour fidelity. These are great solutions. Perhaps the only drawback is that the colour correction only applies to images that are viewed from within the app. Having said that, they allow your standard photo album photos to be accessed – but the correction would not apply, for example, to images viewed using your web browser. This is why a proper system implemented at the level of the operating system would be better, in my opinion.

There are two alternatives. The first would be to implement your own colour correction and modify the images offline before sending them to the device. This would not suit everyone – the average consumer who just wanted to look at their photos for example. But it is what I typically do here in the lab if I want to display some accurate colour images on a tablet. But if you were a company and you wanted to display images of some products for example – it might be a reasonable approach. It has the advantage that the colour correction will work when viewed in any app on the device because the colour correction has been applied at the image level rather than the app level. But it does mean you need to do this separately for each device and keep track of which images are paired to each device. This is ok if you have one or a small number of devices but maybe not so good if you have hundreds of devices.

The second alternative would be to build your own app. If you want to do things with your images that you cannot do in ColorTrue or SpyderGallery or if you have lots of devices and you can’t be bothered to manually convert the images for each device, then you could install your own app that implements a colour profile and then does whatever else you want it to do.

how colour vision works

yellow

Really super article by Ana Swanson in the Washington Post about colour vision and how it works. As she explains, it is not really correct to think of the long wavelength visible light as being red. It is better, as Newton knew of course, to say that the long-wavelength light has the ability to cause the sensation of redness in us. She gives a nice visual example of how the spectrum looks to a dog, something (by coincidence) that I was only talking about in a lecture last week. As she says:

Is what I see as “blue” really the same thing as what you see as “blue”? Or have we both learned the same name for something that looks different to each of us?

Her article is really worth reading.

There is just one thing I take issue with. It may be ‘nit picking’. But she says “A green leaf, for example, reflects green wavelengths of light and absorbs everything else.”

My image, at the top of this post, shows the reflectance of a typical yellow object. At each wavelength the reflectance is between 0 and 100 per cent. But notice that it is not zero at any wavelength in the range shown (400-700nm). That means that the object reflects light at every wavelength. And it is not 100 at any wavelength meaning that it also absorbs to some extent at every wavelength. It’s just it absorbs more at the shorter wavelengths than at the longer wavelengths and it reflects more at the longer wavelengths than at the shorter ones. But notice one other remarkable thing – the yellow object reflects more light at 700nm (a wavelength we would normally associate with red) than it does at 580nm (a wavelength we might normally associate with yellow).

Yes, the reflected light does look yellow. But, the notion that a “A yellows object reflects yellow wavelengths of light” is misleading. It suggests that the yellow object only reflects, for example, the wavelengths in the spectrum we would normally think of as yellow (around 580nm) and absorbs the rest. This is just not how things are.

On CIE colour-matching functions

In 1931 the CIE used colour-matching experiments by Wright and Guild to recommend the CIE Standard Observer which is a set of colour-matching functions. These are shown below for standard red, green and blue primaries. These show the amounts – known as tristimulus values – of the three primaries (RGB) that on average an observer would use to match one unit of light at each wavelength in the spectrum. Why are these so important? Because they allow the calculation of tristimulus values for any stimulus (that is, any object viewed under any light as long as we know the spectral reflectance factors of the surface and the spectral power of the light).

650px-CIE1931_RGBCMF.svg

I gave a lecture this week about these and so they are fresh on my mind. I wanted to use this blog post to explain two things about the colour-matching functions that may be puzzling you. The first was stimulated after the lecture when one of the students came up to me with a question. You will note that for some of the shorter wavelengths the red tristimulus value is negative. Hopefully you are aware that no matter how carefully we choose the three primaries we cannot match all colours using mixtures of those three in the normal sense. What we have to do is to add one of the primaries to the thing we are trying to match and then match that with an additive mixture of the other two primaries. The question from the student was, wouldn’t that change the colour of the thing that is being matched? The answer is that it would of course. But it’s ok.

We normally represent this matching with an equation:

S ≡ R[R] + G[G] + B[B]

which simply means that the stimulus S is matched by (that is the symbol ≡) R amounts of the R primary, G amounts of the G primary, and B amounts of the B primary. The values R, G and B are the tristimulus values. I put square brackets around the primaries themselves to distinguish them from the amounts or tristimulus values of the primaries being used in the match.

Now when we add one of the primaries to the stimulus (the thing we are matching) itself, we can write this equation:

S + R[R] ≡ G[G] + B[B]

The new colour, S + R[R], can now be matched by an additive mixture of the other two. Hmmmmmm? You may ask. How does that work? Well, we can rearrange this equation to make:

S ≡ -R[R] + G[G] + B[B]

In other words, matching the additive mixture of the original stimulus S and some red with some green and blue, means that – if it were possible – we could match the original stimulus S with the same amount of green and blue and a negative amount of the red. I appreciate that this is mathematical but I hope that it is maths that anyone could understand. It’s not rocket science. Just simple adding and subtracting. This is how we arrive at the colour-matching functions above. No matter what RGB primaries we use one of them will have to be used in negative amounts to match some of the wavelengths. In practice, this is done by adding it to the stimulus as described above. Of course, you may also know that the RGB colour-matching functions were transformed to XYZ colour-matching functions. These are the XYZ values everyone is familiar with. But that is another story I will devote another post to one day.

The second question though, is isn’t this just arbitrary? If we used a different set of RGB primaries wouldn’t we get a different set of colour-matching functions? Again, the answer is yes, but again it doesn’t matter. The whole point about the CIE system was to work out when two different stimuli would match. If two stimuli are matched by using the same amounts of RGB then by definition those two stimuli must themselves match. If we used different RGB primaries the amounts of those tristimulus values would change, of course, but the matching condition would not. Two stimuli that match would also require the same RGB values as each other to match them, not matter what the primaries were (as long as they were fixed of course). So the key achievement of the CIE system was to define when two stimuli would match. However, it was also useful for colour specification or communication but that does indeed depend upon the choice of primaries and requries standardisation.

I hope people find this post useful. Post any questions or comments below.