A table listing the wavelengths and frequencies for the peak sensitivities of • human's three types of cone, • cat's three types of cone, • dog's two types of cone, and peak solar output.
| Color | Wavelength | Frequency | ||
| infrared | longer than 700 nm, (invisible to humans; may be felt as heat) | fewer than 430 THz | ||
| deep red: | 700 nanometers, (longest visible wavelength for humans) | 430 terahertz | ||
| red (human peak): | 575 nm | 520 terahertz | ||
| dog peak in yellow-green: | 555 nm | 540 terahertz | ||
| cat peak in yellow-green: | 550 nm | 540 terahertz | ||
| green (human peak): | 535 nm | 560 terahertz | ||
| cat peak in the light-blue-green: | 500 nm | 600 terahertz | ||
| solar peak: | 480 – 520 nm, (in the light blue to green range, depending on the temperature used to represent the surface of the sun, which is not clearly defined) | 620 – 580 THz | ||
| cat peak in blue: | 450 nm | 666 terahertz | ||
| human peak in blue: | 445 nm | 674 terahertz | ||
| dog peak in blue-violet | 429 nanometers, | 700 terahertz | ||
| violet | 400 nm, (shortest visible wavelength for humans, except for a few) | 750 terahertz | ||
| ultraviolet | less than 400 nm, (invisible to humans, although I read that in World War II the US Navy found that a few sailors could see further into the ultraviolet than most) | more than 750 THz |
Birds can see ultraviolet and have at least four types of color sensitive cone cells. Humans have only three types of cone. (`Rods' are for dimmer, grey, night vision.)
On the other hand, bees are like humans in that they have only three receptor types. But in contrast to humans, bees are sensitive to ultraviolet but not to red.
Ronn Blankenship, who started me on this, said that dogs have two types of color sensitive cones, not three like humans. They see like certain kinds of color blind humans. Blankenship added that to help humans with red-green color blindness, the lights in traffic signals are red-orange and blue-green. This way the lights do not look exactly the same.
Update: A site for veterinarians says that
Blankenship went on to say that cats have at least two types of cone, and possibly three, but that
Update: The veterinarians' site says that cats are weak trichromats. Feline cones peak at 450, 500 and 555 nm. They live in a world of fuzzy pastels.
He added that `some types of birds have five types of cone'. I find it impossible to imagine such a bird's color vision. They see many more shades than we.
Scientists already suspected birds' eyes contain molecules that are thought to sense Earth's magnetic field. In a new study, German researchers found that these molecules are linked to an area of the brain known to process visual information.
In that sense, "birds may see the magnetic field," said study lead author Dominik Heyers, a biologist at the University of Oldenburg.
The new research helps explain how this natural compass may work.
Heyers and his colleagues injected migratory garden warblers with a special dye that can be traced as it travels along nerve fibers.
The team put one type of tracer dye into the eyes and another in a region of the brain called Cluster N, which is most active when birds orient themselves.
When the birds got their bearings, both tracers traveled to and met in the thalamus, a region in the middle of the brain responsible for vision.
Blankenship provides more detail:
- rods — black & white vision in dim light
- cones — color vision in bright light
- double cones — color vision
Moreover, according to Blankenship's link,
Update: The veterinarians' site says that acuity is 30 cycles per degree (cpd) for humans, 18 cpd for horses, 12 cpd for dogs and 6 cpd for cats , which means a resolution of
- 1 arc-minute for humans
- 1.67 arc-minutes for horses
- 5 arc-minutes for cats
- 2.5 arc-minutes for dogs
Many birds see more acutely than humans,
If I understand this right, hawks' resolution is 12 arc-seconds!
The veterinarians' site also says
Blankenship goes on to say
You can create star charts to show what humans, cats, and owls see. Simply set differing limiting magnitudes:
- 4.5 for a human in a light-polluted area;
- 6.5 for a human with a dark sky;
- 8.3 for a cat with a dark sky, since a cat's eyes are 6 times as sensitive as a human;
- 11.5 for an owl with a dark sky, since an owl's eyes are 100 times as sensitive as a human.
Update: According to a site on British garden birds,
As for why color vision developed in the first place, Mickey Rowe writes,
