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Colour Vision
Colour vision begins with the detection of different wavelengths of electromagnetic radiation by cones cells in the retina. Typically, there are three types of cone cells, one is most sensitive to short (S) wavelengths, one to medium (M) wavelengths, and the third to medium-to-long (L) wavelengths.
Colour Vision Deficiencies
There are many types of colour vision deficiencies.
- Deuteranomaly and deuteranopia are the most common forms of red-green colour blindness. The M-cone cells are malfunctioning or missing, making differentiating some colour hues difficult.
- Protanomaly and protanopia are less common forms of red-green colour blindness. The L-cone cells are malfunctioning of missing.
- Tritanomaly and tritanopia are forms of blue-yellow colour blindness. The S-cone cells are malfunctioning of missing.
- Finally, achromatopsia, or total colour blindness, is inability to see colour.
Super Colour Vision
Instead of colour vision deficiencies, there are tetrachromats who can see millions more colours that typical people can’t see. They have four types of cone cells rather than three. The extra cone cells allow a tetrachromat to see more detail or variety within the visible spectrum. Tetrachromats are rare. The first person to be proven by science to have tetrachromacy is known as cDa29 in a paper titled “The dimensionality of colour vision in carriers of anomalous trichromacy”, published in 2010 by Gabriele Jordan et al. In 2014, artist Concetta Antico shares her experience with the media. See BBC’s Future article titled “The women with superhuman vision”.
Ultraviolet Vision
While the visible spectrum is considered limited to the colours in a rainbow, ie, red, orange, yellow, green, blue, indigo, violet, people with a condition called aphakia can see ultraviolet light. The S-cone cells in the human retina are sensitive to the UV spectrum, however, the UV light is not easily seen because it is normally filtered out by the lens of the eye. When the lens is removed or missing due to congenital defects, people are able to see shorter wavelengths beyond the visible spectrum.
Infrared Vision
Infrared light, the longer wavelengths beyond the visible spectrum, are visible to people as well. Initially scientists reported seeing occasional flashes of green light while working with an infrared laser that was thought to be invisible to the human eye. Studies have confirmed that rapidly pulsing infrared light is detectable by the human eye and perceived as visible light. Quantum mechanics modeling indicates that it is feasible for two photons (instead of one) to be absorbed at the same time, their combined energy making them activate photoreceptors.
Impossible Colours
The next step in colour vision after detection is visual processing. There are ways the brain can be tricked into seeing “impossible” colours. There are three types of impossible colours:
- Chimerical colours cannot be seen directly. But they can be generated as an optical illusion by looking at two colours in succession. The process involves staring at a saturated colour for 60 seconds to induce fatigue in the cone cells, and then looking at a different colour.
By Zowie at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=37817204
- Forbidden colours are composed of pairs of hues (red-green and yellow-blue) whose light frequencies automatically cancel each other out in the human eye, they're supposed to be impossible to see simultaneously. Try unfocus or cross your eyes to stack the two “plus” (+) signs on top of each other. What colour do you see?
By JZSIX - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=71507791
By JZSIX - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=71507876
- Imaginary colours - (also referred to as non-physical or unrealisable colours) Just like imaginary numbers, an imaginary colour is a mathematically concept. It is a colour that cannot be seen but it can be described mathematically as a point in a colour space.
Do you see what I see?
