What is colour blindness?

May 1, 2021

Not everyone sees the world the same way you do. Literally. People with colour blindness can’t distinguish certain colours, for example, red and green. Colour blind people aren’t blind people. They just perceive colours differently. So what exactly is colour blindness?

To understand colour blindness, we need to understand colour vision first

When light from the environment enters the eye, it is focused onto the layer at the back of the eye called the retina, which contains millions of tiny light-sensitive cells called photoreceptors. When incoming light interacts with these specialised photoreceptors, they send information in the form of an electrical signal along the optic nerve to the brain.

The human eye has two main types of photoreceptors: rods and cones. Rods are what we use for peripheral vision and for seeing at night or in low light conditions, and they are not capable of distinguishing colour. On the other hand, cones are responsible for colour vision and for allowing us to see crisp detail in the central part of our vision. They are found mostly in the centre of the retina.

Humans have three types of cones for colour vision; blue, green and red, corresponding to the colour of light that stimulates them most strongly. The different types of cones respond differently to different colours of light based on the type of pigment they contain. The combination of different wavelengths of light activating the different types of cones allows most people to see and distinguish between many hues and many different colours.

So, now that we know how humans perceive colour, what is colour blindness?

When cones do not have one or more of the usual pigments or there is a genetic mutation in a pigment that impairs its function, the cones will be unable to be stimulated by certain wavelengths of light. This leads to a condition called colour blindness or colour vision deficiency, which is the inability to distinguish between or perceive certain colours.

Do all colour-blind people perceive colour the same way?

No, there are different forms or types of colour blindness, some with genetic causes and others that are caused by disease such macular degeneration or diabetes or damage from the environment such as certain types of drugs.

For example, a relatively common hereditary form of colour blindness occurs when the pigment that is supposed to be sensitive to green light in green cones is mutated and becomes more sensitive to red rather than green light, meaning that people born with this condition have problems perceiving the colour green, but are still good at distinguishing between other colours. One of the rarer types of colour blindness involves the complete absence of red cones, which results in an inability to distinguish between yellow red and green, and results in reduced brightness perception of some colours.

Most people with colour blindness can see colours, just not all of them. They have a harder time differentiating between colours, and this can depend on how dark or light a colour is. Although it is very rare, complete colour blindness also exists. Called achromatopsia, people who suffer from this condition can only see things as black and white or in shades of grey.

How does someone know if they are colour blind?

You can do a colour blindness test, which is what optometrists use to detect if someone is colour blind.

Have you ever seen circles like the ones above when going for your annual eye exam? This is the Ishihara Colour Vision Test, the most widely used screening test for colour vision and colour deficiency today. If you can see all the numbers in each circle, your colour vision is probably normal, and all your cones are doing their job. If some or all circles do not appear to have numbers in them, this may indicate that you have a colour deficiency in your vision.

So how does this test work? Well, the seemingly randomly coloured dots are actually intentionally arranged in such a way that a person with normal colour vision will be able to see a number within the dots, while a person with colour vision deficiency will either be unable to see any number or will see a different number to what they should be observing. For example, in the image shown above, a person with red-green colour blindness will not be able to see the 8 in the image on the right. The image they see will look more like the one on the left, devoid of any number.

Causes and risk factors – who is prone to colour blindness?

Different forms of colour blindness affect different groups of people. For example, the most common form of inherited colour blindness, red-green colour blindness affects males much more frequently than females. This is because the genes that are responsible for this condition are passed down on the X chromosome. Males have 1 X chromosome from their mother and 1 Y chromosome from their father, whereas females have 2 X chromosomes; one from each parent.

If a male is born with a defective gene on their X chromosome, there is no second X chromosome to help with colour vision.

On the other hand, if a girl has a defective gene on 1 X chromosome but the other is normal, that will be enough to have normally functioning colour vision. Only if both X chromosomes are defective will a female suffer from red-green colour blindness, but this is rare.

Other hereditary types of colour blindness such as blue-yellow colour blindness that affects the blue light and yellow light photoreceptors are inherited through other chromosomes that are found equally in males and females and therefore affect both sexes equally.

Some diseases that can cause colour blindness are diabetes, glaucoma, macular degeneration or Alzheimer’s disease.
Other causes of colour blindness can include medication – such as drugs used to treat heart problems, high blood pressure and infections – as well as ageing, or chemical exposure.

Are there treatments or a cure for colour blindness?

Unfortunately, there is no cure for inherited colour blindness, but certain types of glasses, contact lenses and other visual aids may help to distinguish between colours.

For colour blindness caused by reasons other than genetics, treating the underlying disease or injury, or stopping a medication that is causing the problem may help.

Although having colour vision deficiency may be annoying and may inhibit certain career aspirations, in most cases, it is thankfully not a serious threat to vision.