COLOR VISION

COLOR VISION

The intricacies of how we see—and perceive—colors

BY JENEAN CARLTON, ABOC, NCLC

Have you ever spoken with someone about the color of an object and, to your surprise, the person disagreed with you about the color?

This situation happens frequently, especially when colors aren’t true colors but rather a blending of hues. It has been proven that the human eye can distinguish up to a few hundred hues of color, but when pure spectral colors are combined with white light the number of distinguishable colors shifts into the thousands.

Why don’t we all see the same colors?

Individuals perceive colors differently because of the uniqueness of our photoreceptor cells in the retina as well as the wavelengths of light in the visible spectrum.

Here, we delve into the details of how our color vision works.

The Visible Spectrum

The electromagnetic spectrum (EM) represents all known radiant energy in the world. The visible spectrum is a tiny portion of the EM.

It is made up of long to short wavelengths of light that correspond to low and high frequencies of energy. The visible spectrum consists of red, orange, yellow, green, blue, indigo, and violet light.

A range of wavelengths of light corresponds with each color. Wavelengths of 400-450nm are violet, 450-500nm are blue, 500-570nm are green, yellow is from 570-590nm, orange is from 590-610nm, and red is from wavelengths of light from 610-700nm.

The order of the visible spectrum is constant, and the mnemonic ROY-G-BIV is commonly used as a means for remembering the order.

How We Perceive Color

Color vision is achieved by distinguishing objects based on the wavelengths of the light they reflect, transmit, or emit. Incoming wavelengths of light strike the photoreceptor cells in the retina (rods and cones) and set off a chain of events that result in vision.

Chemical messages from the rods and cones are carried to the bipolar cells and then to the ganglion cells in the retina. Messenger points between the cells are called synaptic zones.

The ganglion cells deliver information through the optic nerves, the visual chiasm, and then to the lateral geniculate nucleus of the thalamus. Impulses from the thalamus then travel to the occipital cortex and are processed into what we perceive as vision. Phototransduction is the term used to describe this entire process of light transforming into vision.

Trichromatic Vision

A small portion of the electromagnetic spectrum, the visible spectrum consists of red, orange, yellow, green, blue, indigo, and violet light

Rod cells and cone cells are the photoreceptive cells in the retina. Humans are said to have “trichromatic” color vision because we perceive colors based on three different types of cone cells. Each cone cell is sensitive to different ranges of light—green, red, and blue. These cone cells are called Deuteran (green), Protan (red), and Tritan (blue). Each cone cell is sensitive to a range of wavelengths, not just the wavelength for which they are called.

Protan cone cells are primarily sensitive to red wavelengths of light but also sensitive, to a smaller degree, to wavelengths of orange, yellow, and even some green light wavelengths.

Likewise, Deuteran cells are most sensitive to wavelengths of light associated with the color green but can also be activated by yellow and blue light wavelengths. The same is true for Tritan cone cells—they are mostly sensitive to blue wavelengths of light but also activate to some green and violet wavelengths.

The Bottom Line

We see colors because of the specialized cone cells in the retina. Color perception is subjective and is determined by how an individual’s brain responds to signals produced when wavelengths of light react with the cone cells.

People experience color uniquely because of the dissimilarities in how each person’s cone cells respond to light.

Color Vision Testing

A comprehensive eye exam includes a screening to check for color vision deficiencies. The most commonly used screening tool is the Ishihara Color Vision Test.

Developed by Dr. Shinobu Ishihara in 1917, the test is used to determine red-green color vision abnormalities. The Ishihara Color Vision Test consists of “plates” containing colored dots that are randomized in size and color.

Numbers within the dots are visible to patients with normal vision. People with red-green color deficiencies may find it difficult or impossible to see the numbers on the test plates.

If a color vision problem is detected, practitioners may administer the more detailed Farnsworth-Munsell color test to determine a more specific color vision diagnosis.

Color Vision Deficiencies

A color deficiency occurs when an individual’s cone cells don’t respond normally to light. Depending on the type of color deficiency, a person’s cone cells may be genetically defective or the individual could have been born without certain cone cells.

Patients with color vision problems may experience a slight shift in color perception or a significant problem with seeing colors.

The most common color vision problems are seeing green and red shades of color. This is why it’s not uncommon to meet people with a red-green color vision deficiency. Deuteranomaly is the condition caused by malfunctioning green cone cells. Deuteranopia is the condition caused by missing green cone cells, which is very rare.

Likewise, Protanomaly is the condition of malfunctioning red cone cells, and Protanopia indicates missing red cone cells.

Living With a Color Deficiency

Color perception is subjective and is determined by how an individual’s brain responds to signals produced when wavelengths of light react with the cone cells

Human color vision is a fascinating and complex topic. Color vision deficiencies are usually present at birth, and such individuals have never experienced normal vision. Because of this, they may not consider their vision to be abnormal at all.

Patients with color vision deficiencies use clues in their environment to overcome their vision problem. As an example, consider a child learning the color red. To the child with a red color deficiency, the color sample will appear to him as a shade of tan. The child then learns that whenever he sees a tannish color it is the color “red.”

Another way individuals learn to adapt to color vision deficiencies is to use clues to identify objects that are routinely seen. For example, seeing that the bottom circle on the traffic light is illuminated, a driver with a color vision deficiency knows this is the signal for “drive” just as they know that if the top circle is lit it means to “stop.”

However, people with true colorblindness—an extremely rare condition—pay close attention to shadows, luminosity, patterns, and textures to navigate their world.

Want More?

Read more about the color roles of the eye’s rods and cones, plus read about the “Island of the Colorblind” —exclusive features on our website.

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