Shawna Lee

Background to Color Vision

In order for the human brain to process color, there are three requirements:

1. Light contains multiple wavelengths in the visible spectrum.

2. Objects must absorb some wavelengths while reflecting others.

3. The visual system must have more than one receptor time active

at the same time.

Color Perception

Within the electromagnetic spectrum, people can perceive only a small portion of the wavelengths. (We can't see X-rays or Radio waves or UV waves.) Wavelengths are measured in nanometers (nm), and humans can see the wavelengths between ~400 nm and ~700 nm (the visible spectrum).

These light wavelengths are not colored, but our brains convert them to perceptions of color. Our brain uses the response of cone cells in our eyes to process color information. Cones only work in bright light. We also have rod cells in our eyes, and they are used in dim lighting. Therefore, color is something that our brain perceives, and not everyone or every animal sees the same color view of the world.

If our brains were not able to process these wavelengths into color perceptions, we would only be able to see in shades of gray.

*Interesting Fact: The rat is the only mammal that perceives the world only in shades of gray.


Color Spectrum

Red colors are assumed to be about the upper range of visible color (around 700 nm). Blue shades are on the lower range (around 400 nm). The other colors fall inbetween on the color spectrum, going in order from blue, green, yellow, orange, and then red. If we add all the wavelengths of light together, we would have white light.


Why are Objects Certain Colors?

Our brains interpret different wavelengths as different colors. The wavelength that reflects from an object is the one we perceive.

For example: If you were to look at a red apple, you would be seeing the wavelength that was reflected from the apple (650-700 nm) and not the wavelengths that were absorbed by the apple's surface (all other wavelengths). Green objects reflect the middle wavelengths (~450 nm), and absorb the other wavelengths.


Three Cone Types

Normal visual systems use three cone types and are able to distinguish millions of colors.

The three cone types have different peaks within the color spectrum:

1. Short wavelength peak - ~420

2. Middle wavelength peak - ~ 530

3. Long wavelength peak - ~560


Color "Blindness" (in most cases, really just a serious color deficiency)

Dichromats: A dichromatic person only has two cone types as opposed to the normal three of a trichromatic person. Dichromatic people can only see a few colors, which can make some aspects of life difficult (such as picking out clothes for the day, or even seeing necessary warning colors used for driving).

Different types

1. Tritanopia - A person with tritanopia is missing the short wavelength cones and would be considered blue/yellow colorblind. This is the rarest form of colorblindness which usually only results from damage on chromosome 7.

2. Deuteranopia - A person with this type is missing the middle wavelength cone and would be considered red/green colorblind.

3. Protonopia - Someone with this type of colorblindness is missing the long wavelength cone and would also be considered red/green colorblind.

*Interesting facts: Both Deuteranopia and Protonopia are a genetic form of colorblindness that affect 2% of males and only affect .001% of females. Even though the risk is less for females, the X chromosome is the carrier of color blindness.

Complete Colorblindness is very rare in humans. Such individuals only have one cone type (monochromats) and so can only see in shades of gray (like we do at night or in dim lighting).