This is not yellow

A few people have asked me about this interesting and entertainig youtube clip – This is not yellow.

It’s worth looking at. It makes the point that when you look at colours on the screen (whether it is your computer screen, your TV or your mobile phone) although you see a full range of colours, all that is there is mixtures of red, green and blue light. In principle this is true – in practice it’s a bit more complicated because the screen doe snot emit just three wavelengths. For practical reasons the RGB primaries on a display are more broad band. Nevertheless, the essence of what is being said is true; when you look at yellow on the screen it is not a single wavelength that you would associate with yellow that is being emitted. Hence, the “This is not yellow”.

However, the clip doesn’t go far enough. It suggests that this is a problem with displays and that when you see a real lemon, for example, you are seeing real yellow because the lemon absorbs all the wavelengths of light except yellow (which is reflected). Sadly this is not true either. Let’s look at the reflectance profile of a typical yellow object. I can’t promise it is a lemon but a lemon would be pretty similar.

yellow

What this graph shows are the wavelengths of light along the x-axis and, along the y-axis, the per cent of each wavelength that the yellow object reflects. Notice that it does not absorb all wavelengths excecpt the ones that would be seen as yellow in the spectrum (essentially about 580 nm). Rather, the physical yellow object reflects all wavelengths in the spectrum because the reflectance is greater than zero at all wavelengths. The physical yellow object also absorbs all wavelengths in the spectrum to some extent because the reflectance is less that 100% at all wavelengths. Obviously some wavelengths are reflected more than others. But it isn’t even the wavelengths at about 580 nm that are maximally reflected. The yellow object reflects more red wavelengths than it does yellow wavelengths. So why does the lemon look yellow? For the same reasons that the lemon looks yellow on the screen; because the light being reflected activates the cones in the human visual system in a certain way. So I am not knocking this video – rather, I want to say that it makes a good point about displays but that this point also relates to colours in the subtractive world. It raises the issue of what we mean when we say something is yellow either on a screen or in the physical world.

2 thoughts on “This is not yellow

  1. Perhaps the author of the movie meant that in the case of the movie there are three color stimuli (which mix additively), while in the case of lemon there is a single color stimulus.

    1. Yes, he could have meant that. I doubt it though. At one point he specifically says that the real lemon reflects only yellow light. Even if he did mean that for the yellow lemon there is only a single stimulus I am not even sure that is true. The colour of the lemon probably results from the subtractive mix of several natural colorants in the lemon skin. In the case of the physical lemon and the computer-displayed lemon, both result in a physical stimulus that has light at most, if not all, wavelengths. (This is because the RGB on the screen is not just three wavelengths of course.) In both cases, the physiological process for why the stimulus looks yellow is pretty much the same in each case and is very much connected with the principle of univariance of cone response.

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