Light perception

Rods state: . Duplicate the perceived image


Visual perception depends on the stimulated area of the retina. At low (scotopic) light levels only the rod system is active.
  • Set the Luminance to 'Scotopic'. Click on the retina and observe the perceived text. It may help to dark-adapt the retina!
    The fovea is the central gray region at the intersection of the dotted lines. What happens when the stimulation is focused on the fovea? Why?
    Can you find the blind spot?
  • Change the wavelength. What colors are perceived best?
    What light wavelengths would you recommend for a night-vision display? How about an environment that doesn't interfere with dark adaptation?

  • Set the Luminance to 'Photopic'. Only cones are active at these light levels. Click on the retina and observe the perceived text. Where can you read it?
    Does the perceived color matches the 'rainbow' above the stimulation wavelength? Why?

  • Set the Luminance to 'Mesopic'. Compare the color perception over the fovea, and over adjacent regions. Can you explain this result?

    Color blindness: Click on the cone symbols on the right to simulate color perception in the absence of some of the cone types. Red-green is the most common color blindness. Simulate it by inactivating the green cone. Now try to match color perception of 520 and 560nm wavelength light.
    Hint: start with one wavelength and click on Duplicate to save the perceived image. Now shift the wavelength and the intensity of the light till the new output is similar to the original image.
    Repeat the exercise with trichromatic cone setup. What is the difference between the simulations?


    Central retina is cone-dominated retina whereas peripheral retina is rod-dominated. The center of the fovea is known as the fovea. The fovea lies in the middle of the macula area of the retina to the temporal side of the optic nerve head. It is an area where cone photoreceptors are concentrated at maximum density, with exclusion of the rods, and arranged at their most efficient packing density which is in a hexagonal mosaic.

    Color vision: As we have seen before, Rods are very sensitive but slow and their response saturates at light levels where cones function optimally. Cones are less sensitive but are fast and can adapt to the brightest lights, being almost impossible to saturate. A photoreceptor responds only to the energy it absorbs. All wavelength of light can produce identical responses from a photoreceptor if the energy absorbed by the cone is the same for these wavelengths. Cones are therefore color blind producing a univariant response reflecting only the amount of energy they absorb. Detecting objects by the energy reflected from their surfaces, however, can fail when objects reflect a similar amount of energy as their background. Here is where color vision becomes important. Wavelength contrast can detect objects when energy contrast is absent or minimal. In order to detect objects by differences in spectral reflectance, two or more different types of cones are needed. This is an important concept for understanding color vision. For divariant color vision, two cone types must exist and be sensitive to different parts of the visible spectrum, preferably as different as possible. The range of the visible spectrum depends on the ability of light to penetrate the eye and be absorbed by the photoreceptors. Ultra-violet light is absorbed by the anterior segment of our eyes and seldom reaches the photoreceptors. Infra-red light penetrates our eye readily but its quantal energy may be too small to activate opsins.

    Color blindness

    The most common cause of color blindness is an inherited problem in the development of one or more of the three sets of color sensing cones in the eye. Males are more likely to be color blind than females, as the genes responsible for the most common forms of color blindness are on the X chromosome. Red-green color blindness is the most common form, followed by blue-yellow color blindness and total color blindness. Red-green color blindness affects up to 8% of males and 0.5% of females of Northern European descent.

    Composed by Alon Poleg-Polsky, 2019