Tag Archives: vision

Colour Mixing

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or Why do screens and monitors use red, green and blue pixels whereas printers use cyan, magenta and yellow ink?

The human eye contains three types of cones, light-sensitive cells that are sensitive to specific wavelengths (rather than rods, which are sensitive only to brightness/darkness).

The sensitivity of the three types of cones peak at wavelengths of 564-580 nanometres, 534-545 nm and 420-440 nm. These correspond to red, green and blue light, and by comparing the signals from the three types of cones the brain creates colours along red-green and blue-yellow axes (hence why there is no such colour as “greenish-red” or “yellowish-blue”). White light is created by the brain when all three cones are stimulated by the same amount.

For a colour to be perceived light must enter the eye. The big difference between the pixels on a computer screen and the inks used in printing is that light from screens enters the eye directly whereas light from printed materials must be reflected first. Screens use additive colour mixing, and printing inks use subtractive colour mixing.

test-motorbike

Source: Kodak test image library

To create the test image shown above on a screen, which emits light, the image is split into red, green and blue components. The darker areas are where less light is emitted.

test-motorbike-Rtest-motorbike-Gtest-motorbike-B

To create this image in print, the image is split into cyan, magenta, yellow and black components (because the mix of cyan, magenta and yellow alone does not produce a perfect “strong” black, and would require the images to be lined-up perfectly). The lighter areas are where less ink is printed.

test-motorbike-Ctest-motorbike-Mtest-motorbike-Ytest-motorbike-K

If both red and green light enter the eye in equal amounts, the brain creates the colour yellow.* To create the colour yellow on the printed page, both red and green light must be reflected from the page, and the only way to do this is to absorb blue. If yellow text is viewed under a blue light it will appear black, as all the light incident on the ink will be absorbed.

Colour Additive (Light) Subtractive (Inks)
Red Emit red Absorb green and blue
Green Emit green Absorb red and blue
Blue Emit blue Absorb red and green
Cyan Emit green and blue Absorb red
Magenta Emit red and blue Absorb green
Yellow Emit red and green Absorb blue

If cyan and magenta inks are printed on top of each other and illuminated by white light, the cyan will absorb red and the magenta will absorb green. The net effect of this is that only blue light is reflected, so printing cyan and magenta on top of each other creates the perceived colour blue.

If you tried to do this with, for example, red and green ink, the result would be black, as the red would absorb the blue and green and the green would absorb the red and blue. The reason you cannot print using red, green and blue inks is that red, green and blue ink absorbs more than one colour, whereas cyan, magenta and yellow do not. The only way to produce light colours is to start with light inks.

Ink Absorbed Reflected Colour Perceived
Cyan Red Green and blue Cyan
Magenta Green Red and blue Magenta
Yellow Blue Green and red Yellow
Cyan + Magenta Cyan absorbs red Blue Blue
Magenta absorbs green
Magenta + Yellow Magenta absorbs green Red Red
Yellow absorbs blue
Cyan + Yellow Cyan absorbs red Green Green
Yellow absorbs blue

If you’d like proof of the table above, try printing out squares of red, green and blue and looking at them under a microscope.

magnifier-colours

blue-ink

It is easy to see, in the image of blue ink above, the cyan and magenta “pixels” (drops of ink) that create the blue colour. It’s more difficult, but possible, in the image of red ink below to see the magenta and yellow “pixels”.

red-ink

* There is no way to guarantee that both you and I perceive the colour that we call yellow in the same way. We might both call it yellow, but we could be seeing vastly different things.

Eigengrau

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Close your eyes. What colour do you see? Black? Look again.

eigengrau

Eigengrau (“intrinsic grey”), also called “dark light” or “brain grey” is the dark grey colour “seen” in the absence of light. Eigengrau appears lighter than a black object viewed in normal light because the brain prioritises contrast over true colour representation. For example, in the diagram below the two circular dots are the same colour, but appear to be different because of the way that they contrast with their backgrounds.

contrast-illusion

Eigengrau is produced when rhodopsin molecules undergoing a process of spontaneous isomerisation, indistinguishable from the process that would occur if the rhodopsin molecule had been struck by an incoming photon of light. In individual rod cells these events occur only about once every 100 seconds, but as there are about 125 million rod cells in each human eye this level of background signal is enough to produce eigengrau.

What is a ‘Retina’ display?

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Apple describes some of its products as featuring a “Retina” display. But what does that actually mean?

The individual pixels (each one made up of three red, green and blue subpixels) that make up my laptop’s display, viewed through a magnifier.

The main claim that Apple makes of its Retina display is that the pixels used are so small that they are too small to be seen individually by the human eye. In physics terms, this means that these pixels are below the resolving power of the human eye.

The resolving power of the human eye is about 60 arcseconds, or 0.0167 degrees. This means that any two objects separated by an angle smaller than this will appear as one object to the eye. The minimum vertical or horizontal spacing between two items which are visible as separate items is therefore given by dtan(θ) where d is the distance to the items and θ is the resolving power of the eye.

Assuming that the display in question is held or viewed at a distance of 30 cm from the eye, this distance is found to be 0.0873 millimetres. This means that a person with normal vision will be able to discern individual pixels on any display with fewer than 11.5 pixels per millimetre.

As can be seen from the graph above, the screen of the iPhone 4 does possess a greater density of pixels than the human eye can perceive; but the iPad 3 and the just-released 2012 MacBook Pro do not. (None of this matters of course, because “Retina” is just a trademark that Apple uses as a marketing term.)

An argument could be made, in the case of the MacBook Pro, that the distance between the screen and the eye would usually be larger than 30 cm. If the distance was 50 cm that would make the resolution of the eye 6.88 pixels per millimetre and therefore give the 2012 MacBook Pro a “true” retina display.

Night vision

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The vision of human beings is well-adapted to daylight; the human eye has evolved to see in the range of wavelengths that are brightest in the spectrum of light that the Sun emits.

The intensity of the light the Sun emits by wavelength, with the visible region highlighted.

But humans don’t see particularly well in the dark. The cones that are responsible for colour vision don’t function well at low light intensities, which is why night vision is almost entirely monochromatic – in the dark humans see in black and white.

When moving from bright light into darkness the first thing that happens to the eye is that the pupil dilates to allow in more light. The iris dilator muscle causes the pupil to increase in diameter by a factor of five (from 2 mm to 10 mm), increasing the amount of light entering the eye by about twenty-five (52) times, but this isn’t enough for true night vision.

The chemical rhodopsin that is present in the rod (brightness-sensing) cells is responsible for night vision. When exposed to light, rhodopsin immediately (within 200 femtoseconds*) splits to form a chemical called photorhodopsin, and then soon afterwards (within a few picoseconds) another chemical called bathorhodopsin.

The splitting of rhodopsin is accompanied by the formation of other chemicals called retinals, and during this splitting process a signal is sent down the optic nerve to the brain, registering the detection of light. (Retinal is created from vitamin A, and so people with a diet lacking in vitamin A frequently suffer from night blindness.)

A molecule of rhodopsin (rainbow-coloured) embedded in a lipid bilayer.
A (black) retinal molecule is bound within the rhodopsin.

Over time, and at a consistent rate, the opsins and retinals recombine to form rhodopsin. If the eye is exposed to bright light all the rhodopsin splits at once (a process called photobleaching). When subsequently exposed to darkness there is therefore no rhodopsin to split and the eye cannot detect light properly. The person in question must wait for the rhodopsin to naturally recombine over time before proper vision can return, a process that takes between ten and thirty minutes to occur. When fully accustomed to the dark, the eye is between ten thousand and a million times more sensitive to light than previously.

The rhodopsin in human eyes is less-sensitive to red light than to other colours and therefore night vision is not particularly effected by red light. This is why red light is used in darkrooms and in aircraft before night-time parachute jumps.

Human eyes, unlike the eyes of many animals, do not have the tapetum lucidum which gives those animals superior night vision. The tapetum lucidum sits behind the retina and acts like a mirror, reflecting back photons of light that were not initially absorbed by the retina, giving the retina a “second chance” to detect the light. This improves their night vision and is what gives rise to the phenomenon of “eyeshine” often seen when taking photographs of animals.

The tapetum lucidum seen in a dissected calf’s eye.

“Eyeshine” is very obvious in this photograph of a raccoon.

* Interestingly, the splitting of rhodopsin into photorhodopsin and retinal seems to be the fastest chemical reaction that has been directly studied.

Colour blindness

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Following on from an earlier post about the human eye’s inability to see the colour blue in detail, I’m taking a look at colour blindness.

I’ll be using this test image of a forest and rainbow throughout.

The two most common types of colour blindness both occur in the red-yellow-green part of the colour spectrum and are commonly referred to as red-green colourblindness, because sufferers cannot distinguish between the two colours. In both cases red and green appear yellow (i.e. as a combination of red + green = yellow).

An inability to perceive the colour red is called protanopia and occurs in some form in about 2% of males and 0.01% of females.

An inability to perceive the colour green is called deuteranopia and is the most common, occurring in some form in about 7% of males and 0.4% of females.

Protanopia and deuteranopia are very similar, but there is a subtle difference between the two if you look very carefully. The difference is easier to see in the animation below that flicks back and forth between the two.

The reason that protanopia and deuteranopia are more common in males than females is that colour blindness is most commonly inherited from a gene on the X chromosome. Men (XY) have a much higher risk than women (XX) because the colour-blindness gene is recessive: with two X chromosomes there is a chance that one of the Xs has the normal colour vision gene and that will dominate.

The third form of colour blindness, tritanopia, is much rarer and not sex-linked, because the gene that controls it is carried by chromosome seven which is present in both sexes. It occurs in about 0.01% of the population and results in short wavelength blue light being shifted towards longer, greener wavelengths. If protanopia and deuteranopia are red-green colourblindness then tritanopia could be described as blue-yellow colourblindness.