see also:

• ophthalmology
• eye color phenotypes and disease associations
• https://enchroma.com/blogs/beyond-color/how-color-blind-see
• https://enchroma.com/pages/types-of-color-blindness
• Enchroma online color blindness test and link to corrective glasses - make sure you do this without any color correction settings applied to your computer display (check color filter in your settings!
• there is also considerable variation amongst people in speed of perception of visual input changes (“temporal resolution” as tested by critical flicker fusion threshold) range is from 35 to 60Hz - this may impact eye-hand coordination such as ball sports ¹⁾

• humans see with rods for night time vision and 3 types of cones for general color vision and are thus trichromatic
  • each of the three types of cones contains a different type of photosensitive pigment, which is composed of a transmembrane protein called opsin and a light-sensitive molecule called 11-cis retinal.
• trichromatic people with typical color vision can potentially “see” about 1 to 7 million distinct colors although ability to readily distinguish them is partly due to whether they have a language for those colors
• light may activate all cones at the same time depending on wavelength and its intensity an the brain must use the inputs from at least two types of cones to ascertain the color by comparing the different signal levels from each
• their peak sensitivities lie in the blue (short-wavelength S cones with S-opsin peak 420nm), green (medium-wavelength M cones peak 530nm) and yellow-green (long-wavelength L cones, peak 560nm) regions of the color spectrum.
  • S cones make up 5–10% of the cones and form a regular mosaic.
    • all primates with the exception of Aotus exhibit an S-opsin which is encoded by an autosomal gene on chromosome 7
  • L cones and M cones are randomly distributed and are in equal numbers. L and M opsins are adjacent opsin genes on the X chromosome
    • M-opsin (middle wave sensitive, encoded by OPN1MW gene), the cone most sensitive to green light
    • L-opsin (long wave sensitive, encoded by OPN1LW gene), the cone most sensitive to red light
• human rod cell (night and mesopic twilight vision) sensitivity is greatest at 500 nm (bluish-green) wavelength
  • during twilight and most urban night light levels, normal humans may have some added rod vision to give tetrachromacy
• color blind people have a mutant version of the genes that produce these opsins
  • in humans, two cone cell pigment genes are present on the X chromosome: the classical type 2 opsin gene OPN1MW
    • if this gene is a mutant gene, the female will be a carrier with trichromatic vision (unless both their genes are mutant or X-inactivation inactivates the good gene) and on average 50% of their sons will be have the mutant gene but as they only have one X chromosome, the will then be “red-greeen color blind”
    • female carriers of this mild color blindness (15% of women) can theoretically have 3 normal cones plus the mutant “blind” cone which may give them physical capacity for tetrachromacy however perhaps only a minority have functional tetrachromacy as this relies upon neural circuits to interpret the extra color channel.
• those with deutan or protan color blindness may see only about 10% of those colors, however, those with mild forms can improve their color vision by either:
  • buy glasses (such as those from Enchroma) which selectively filter out a narrow range of light wavelengths which reduce the overlap between red and green cones
  • use computer software to adjust the computer displays
    • Apple iOS has a colour filter setting under Settings: Accessibility: Display Accommodations which may help
    • Windows, Settings, Ease of Access, Color Filters
    • Daltonize for Google Chrome

• the common ancestor of all vertebrates was a tetrachromat with 4 photopsins in their complement (SWS1, SWS2, Rh2, LWS), but mammals evolved dichromacy, due to the nocturnal bottleneck, losing two of their four cones and retaining only the SWS1 (UV–sensitive) and LWS (red–sensitive) opsins
  • placental mammals were mainly or even exclusively nocturnal through most of their evolutionary story, starting with their origin 225 million years ago, and only ending with the demise of the non-avian dinosaurs 66 million years ago, and most mammals are still nocturnal.
  • catarrhine primates (including apes, humans and Old World monkeys) as well as heterozygous female New World monkeys are the only known placental mammalian trichromats
  • trichromatic platyrrhine primates (New World monkeys) in addition addition to the S-opsin, generally have only a single opsin gene locus, but it is polymorphic, with different alleles encoding opsins of different peak wavelength.
  • catarrhine primates (including apes, humans and Old World monkeys) eyes have three different kinds of cones
    • 35 million years ago, the LWS class of opsins in catarrhine ancestors split into OPN1MW and OPN1LW, while the SWS opsin gradually shifted from its ancestral UV–sensitivity form 80mya to a violet-sensitive form with a peak wavelength of ~420nm 30mya presumably to allow a more diurnal lifestyle with better twilight sensitivity and higher acuity thanks to the larger lens which incidentally blocked UV light and would make UV cones less useful
      • UV light is inevitably scattered and absorbed on its path through the eye (lens proteins strongly absorb UV light below 310nm in particular), UV ocular media transmittance (OMT) depends mainly on the axial length of the lens
      • the reduction in UV transmittance results in other benefits - less chromatic aberration (multi-focal lenses combined with slit-shaped pupils address this issue in other vertebrates), less Raleigh light scattering, and less UV damage to the retina which is particularly a benefit for long-living animals
      • seven genetic mutations are linked to losing UV vision and gaining the blue light vision that most humans have today over the course of millions of years (F46T, F49L, T52F, F86L, T93P, A114G and S118T, include 5040 potential pathways for the amino acid changes required to create genetic changes in the short wavelength sensitive, or blue opsin)
  • humans cannot see ultraviolet light directly because the lens of the eye blocks most light in the wavelength range of 300–400 nm and shorter wavelengths are blocked by the cornea
    • the photoreceptor cells of the retina are sensitive to near ultraviolet light, and people lacking a lens (a condition known as aphakia) see near ultraviolet light (down to 300 nm) as whitish blue, or for some wavelengths, whitish violet, probably because all three types of cones are roughly equally sensitive to ultraviolet light (with blue cone cells slightly more sensitive).

• a form of red-green color blindness characterized by the shifting of green light-sensitive M cone cells closer to red-sensitive cells than is normal, and are too sensitive to yellows, oranges, and reds
• This causes “green-deficient” color blindness
• greens, yellows, oranges, reds, and browns may appear similar, especially in low light.
• it can also be difficult to tell the difference between blues and purples, or pinks and grays and they miss seeing all the range of pinks and purples in sunsets
• 6% of males, 0.4% of females have this milder form Deuteranomaly
• in addition, 1% of males have no green receptors Deuteranopia so greens look like dark purples

• a form of red-green color blindness characterized by the shifting of red light-sensitive L cone cells closer to green-sensitive cells than is normal.
• the red cones are not absent but do not detect enough red and are too sensitive to greens, yellows, and oranges
• This causes “red-deficient” color blindness
• greens, yellows, oranges, reds, and browns may appear similar, especially in low light. Red and black might be hard to tell apart, especially when red text is against a black background.
• 1% of males, 0.01% of females have this milder form - Protanomaly
• in addition, ~1% of males have no red receptors Protanopia so reds look like dark greens

• most commonly acquired later in life due to aging of the eye or medical complications.
• It is characterized by a reduction in the sensitivity of the blue light-sensitive S cones such that blue shades seem darker and less vibrant.
• In extremely rare cases tritanopia can be inherited in AR manner
• can cause confusion between blue versus green and red from purple.

• also known as “complete color blindness”
• it is the only type that fully lives up to the term “color blind”
• extremely rare
• those who have achromatopsia only see the world in shades of grey, black and white.
• in some cases low vision disorders such as progressive cone dystrophy can cause a gradual deterioration of color vision that eventually turns into complete achromatopsia.

• most animals have cones with SWS1 (UVA–sensitive) opsin (apes and humans have moved the light sensitivity of this away from UV as highlighted above)
• the ability to actually see UV light depends on the size and structure of the eye in these animals
  • UV light is inevitably scattered and absorbed on its path through the eye (lens proteins strongly absorb UV light below 310nm in particular), UV ocular media transmittance (OMT) depends mainly on the axial length of the lens
• animals with high UVA transmittance through the lens often have slit shaped pupils to allow multi-focal lenses to reduce the increased chromatic aberration caused by UV light

• UVA (315-400 nm)
• UVB (280-315 nm)
• UVC (100-280 nm)

• better twilight vision as a substantive proportion of downwelling illumination from the twilight sky is in the violet-UVA range (esp. 320-550nm, but almost none at night when most of the illumination is above 550nm) ²⁾
• better under-water vision - water absorbs less violet/UVA than it does red wavelengths, so having better vision at the violet end allows better vision at ocean depths
  • the long wavelengths of the light spectrum—red, yellow, and orange—can penetrate to approximately 15, 30, and 50 meters respectively, while the short wavelengths of the light spectrum—violet, blue and green — can penetrate further, to the lower limits of the euphotic zone.
  • the least water attenuation is for 418nm light which is blue-violet and happens to relatively coincide with the peak spectral output of the sun (6000degK black body radiation) and is able to penetrate down to around 200m depths
• bees and other insects use UV to see colors or patterns on plants that can direct them to nectar
• birds use UV for feeding as well as mating activities
• reindeer may use ultraviolet light to see polar bears, which, in visible light, blend in with the snow
• rodents use it to follow urine trails
• some animals use their UV photoreceptors to control simple, innate behaviors, guiding navigation and orientation behavior, mate assessment and intraspecific communication

• decreased visual acuity:
  • increased Raeigh scattering from small particles in the eye
  • increased chromatic aberration - all animals with slit-shaped pupils have multi-focal lenses to address this
• potential increased retinal damage from UV light if lens is highly transmitting for UV light