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A Brief on Colour Blindness

Look at the image below. Do you see the number 21, 74, or nothing?

If you answered 74, you are not colourblind. If you see the number 21, you have red-green colour blindness, and if you see nothing, you have monochromacy. Keep reading to find out what each of these means!

What is colour blindness?

Our eyes contain millions of cones and rods. Cones are responsible for detecting different wavelengths of light and send signals to the brain relaying this information. Depending on the combination of signals, different colours can be perceived by the brain. Colour blindness is the characteristic used to describe the inability to detect certain colours or wavelengths due to the loss of function of certain cones.

Anatomy of colour

Colour is created by mixing red, blue, and green wavelengths that bounce off of objects. These reflected wavelengths hit the back of our eyes on a sheet called the retina. The retina is what contains photoreceptors: the cones that detect colour and rods that detect lightness. Rods are only active at night to allow you to see in the dark yet are the most abundant on the retina. There are three types of cones: short, medium and long-wavelength cones. Cones detect different wavelengths of light. S-wavelengths cones detect blue coloured light, M-wavelength detects green light and L-wavelength cones detect red light. Below is what the layout of cones looks like. Once the cone detects the wavelength, it sends a biochemical and electrical signal to the visual cortex at the back of the brain through a set of cells called thin-stripes blobs. The brain processes the info, and colour is perceived!

Mosaic of cones on retina (Arizona State University).

What are the main types of colour blindness?

Those who have normal vision have trichromacy, where they possess all three cones. Trichromacy permits one to see all colours.

Colourblindness begins when one loses one or more cones. Dichromacy occurs when only two cones work and can result in the following types of colour blindness:

  • Protanopia: This is a type of red-green colour blindness that occurs when one has lost the L-wavelength cone (red cone).

  • Deuteranopia: This is another type of red-green blindness resulting from the loss of the M-wavelength cone (green cone).

  • Tritanopia: Tritanopia is a type of blue-yellow colour blindness where the individual cannot distinguish between blue and yellow colours. This results from the loss of the short-wavelength cone (blue cone).

If an individual has lost two or more cones, then they have monochromacy. There are two types of monochromacy:

  • Rod monochromacy: This type of colour blindness happens when the individual has lost function of all three cones. This type of colourblindness is extremely rare and will present itself at birth. Individuals with rod monochromacy see the world in greyscale.

  • Cone monochromacy: This occurs when the individual only has function in one cone. Individuals that have cone monochromacy are unable to distinguish one colour from another because the brain relies on comparisons between cones to see colour.

Different types of colour blindness (Sayu).

The prevalence of colour blindness

Red-green colour blindness is most common. Around 8% of males are red-green colour blind or see the two colours differently in some ways. Less than 0.5% of females have red-green colour blindness. This discrepancy occurs due to the distribution of chromosomes: the genes for the red and green colour receptors are located on the X chromosome. Since men have one X chromosome while females have two, females are more likely to be unaffected.

Around 0.01% of the population has tritanopia and monochromacy even less—around 0.00001%.

Tests for colour blindness

Ishihara charts are a very common test for colour blindness. Most times, they will have you look at an Ishihara chart, which shows circular diagrams with a mix of spots of different colours, called a colour plate. There will be a number or line hidden inside the diagram and depending on the colour blindness of a person, the lines can be seen differently or not at all.

Another test is the anamaloscope test, where you must match two colours. Individuals look through an eyepiece and must turn knobs that change the light level of red and green lights to create the desired colour.

The final common test is a hue test. The hue test is relatively simple—individuals look at different blocks of colour and arrange them into a rainbow.

If you suspect that you have colour blindness, you can set up an appointment with your eye doctor. Otherwise, you can try taking some common online colour blindness tests, although keep in mind that these might not be 100% accurate.


Color Blindness. (2019, July 3). Retrieved February 06, 2021, from https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/color-blindness

Color vision deficiency. (n.d.). Retrieved February 06, 2021, from https://www.aoa.org/healthy-eyes/eye-and-vision-conditions/color-vision-deficiency?sso=y

Rods and cones. (2010, January 06). Retrieved February 06, 2021, from https://askabiologist.asu.edu/rods-and-cones

Featured image courtesy of Subin from Pexels.

Article Authors: Vanessa Wong, Olivia Ye

Article Editor: Valerie Shirobokov