Colour Vision Testing

Colour vision deficiencies are the inability for an individual to distinguish various shades of colour. They can come in multiple types, but red-green colour vision defects are the most common. Our optometrists at the Adelaide and Henley Beach branches of Innovative Eye Care use innovative equipment to detect various types of colour vision deficiencies.

Physiology of Light

Visible light is the part of the electromagnetic spectrum that human eyes are able to detect. Other commonly known examples of rays on the electromagnetic spectrum are seen in the picture below and include X-rays, microwaves, radio waves and UV (ultraviolet) rays. This spectrum is made up of various wavelengths and each wavelength in the visible spectrum of light corresponds to a particular colour. For example, redlight has a longer wavelength when compared to blue light.

Why do objects appear as different colours?

Objects can either absorb or reflect specific wavelengths of light. The colour of an object depends on which wavelengths of light are reflected by the object and enter our eyes. Let’s use a red apple as an example. When white light (which is composed of all wavelengths of visible light) shines onto an apple, the molecules within the apple absorb blue and violet wavelengths of light and reflect red wavelengths of light. This results in the apple having a red colour. If we were to shine only blue light onto the apple, all the blue light would be absorbed meaning that the apple would appear black (as seen in the picture).

Human Colour Vision

The cells involved in detecting light within the eye are called photoreceptors. These cells are located at the back of the eye which is known as the retina. These cells are activated by light and then transmit electrical signals through the optic nerve to then be interpreted by the occipital lobe of the brain located at the back of the skull.

There are two main types of photoreceptors, rods and cones. The human retina contains approximately 6 million cones and 110million rods.^1-2 Rods are responsible for vision at low light levels (also known as scotopic vision). Cones are responsible for vision at high light levels (also known as photopic vision), high acuity vision and colour vision.

There are three types of cones and similar to how a radio station is tuned to a specific frequency, each cone is tuned toa specific wavelength of light (or colour of light):

1. Short-wavelength or S-cones which detect red light.
2. Middle wavelength or M-cones which detect green light.
3. Long-wavelength or L-cones which detected blue light.

When used in combination, these three types of cones allow for humans to see all hues of colour in the visible spectrum of light which is approximately 10 million colours.

Colour Vision Deficiencies

Colour vision deficiencies occur when one or more of the cone cells are absent, not working or detect a different colour than normal. There are three broad types of colour vision deficiencies:

1. Rod monochromacy - this is true colour blindness where all types of cone cells are absent from the retina and only rod cells are present. Individuals with this deficiency have no perception of colour and have overall poor vision.
2. Dichromacy - where only two cone receptors are functional, and one cone receptor is absent from the retina.
3. Anomalous trichromacy - where all three cones' cells are present in the retina, but one type of cone detects a different colour than normal.

Within those broad categories exist further classification which classifies based on the type of cone cell which is missing or abnormal:

1. Protan - where the red cone is affected.
2. Deutan - where the green coneis affected.
3. Tritan - where the blue cone is affected.

Therefore, the possible colour vision deficiencies are as follows:

Rod monochromacy

Red-green colour vision deficiencies

• Protanope
• Deuteranope
• Protanomalous trichromat
• Deuteranomalous trichromat

Blue-yellow colour vision deficiencies

• Tritanope
• Tritanomalous trichromat

For example, if someone has a missing red cone but normal green and blue cones, they would be described as a protanope. If someone has a green cone which detected a different colour than normal but functional red and blue cones, they would be described as a deuteranomalous trichromat.

Frequency and Genetics

The most common forms of colour vision deficiencies are red-green defects. These defects are found in approximately 8%of males and 0.4% of females.³ The gene responsible for red-green colour deficiencies is located on the X chromosome which is why males are more likely to be affected than women. Women are able to be carriers of the defective gene meaning they are able to pass the gene onto their offspring but are unaffected themselves.

The gene responsible for blue-yellow deficiencies is located on chromosome 7 meaning that both males and females are equally affected and that these defects are much less common (less than 1% of the population).³

Testing

At Innovative Eye Care, we have a range of tools which are used to test an individual’s colour vision.

Ishihara Pseudoisochromatic Plates

This is the most commonly performed colour vision test and involves the identification of various numbers embedded in dots of colour. Individuals with colour vision deficiencies will have difficulties distinguishing any numbers. This test is a very efficient screening tool for red-green deficiencies but is unable to detect blue-yellow colour problems. Itis a required entrance test for several professions including the armed forces, aviation and jobs in the railway industry.

Farnsworth D15

This test consists of 16 coloured caps each containing a piece of paper of different colour. The patient is asked to place one of the caps that is closest in colour to the reference cap in the box next to it. This is continued until all caps are placed in the box. This test, unlike the Ishihara test, is able to detect red-green and blue-yellow defects.

Medmont C100 Colour Vision Test

This test requires the patient to look into small electronic box and adjust a knob located at the top of the box to a point where the yellow light flicker disappears or is at a minimum. This test is able to distinguish protan and deutan but interestingly, it can also diagnose women who may have normal colour vision but are carriers of the abnormal gene for protan or deutan colour defect.^4-6

Management and Treatment

Currently, there is no cure for any types of colour blindness however most people find that they will have few limitations throughout life.

Importantly, some colour vision deficiencies can be acquired later in life due to various ocular conditions and therefore colour vision testing in these individuals can aid in the diagnoses of specific diseases.^7-8

Occupational and Daily Living Counselling

An important aspect of the management of colour vision deficiencies includes occupational counselling. Normal colour vision is required (with some exceptions) for the following professions:

• Commercial aircraft pilots
• Commercial marine license
• Metropolitan fire services
• Police forces
• Electricians
• Train drivers

Colour vision defects can also affect other professions including those centred around art, textiles and painting, horticulture, cartography, histopathology and pharmacy. Other counselling revolves around adaptations in daily living tasks such as cooking (determining if meat is cooked), clothing choices, driving (identification of traffic lights) and general wellbeing (identification of rashes and sunburns).

Colour Blind Glasses - Do they work?

You may have seen viral and emotionalvideos on the internet of people with colour vision deficiencies trying oncolour blind glasses for the first time. The way these glasses work is byabsorbing and filtering out wavelengths of light between green and red thatnormally lead to colour confusion in the brain’s of people with colour visiondefects. This ultimately enhances the contrast between red and green colourshowever the results for most people can vary greatly. As stated above, there isno cure for colour vision abnormalities and in no way, do these spectaclesmodify a person’s cones, optic nerve or brain in order to fix a colour visionanomaly. It is important to understand that these glasses reduce the totalamount of light entering the eye and therefore it is not recommended to wearthese spectacles at night time. Furthermore, they can be quite costly and areusually not covered by private health insurance.

Future Research

There is current research looking into gene therapy for colour vision deficiencies. Researchers were able to restore colour vision in two adult monkeys which had red-green colour deficiencies however itis important to understand that more research is required in this field before human testing is even considered.^9-10

References

1) Remington LA, Remington LA. Clinical anatomy and physiology of the visual system. 3rd ed. St. Louis, Mo:Elsevier/Butterworth Heinemann; 2012. 292 p.

2) OpenStax College. Anatomy and physiology[Internet]. Houston, Texas: Rice University; 2013 [cited 2020 Apr 1]. Available from: https://openstaxcollege.org/files/textbook_version/hi_res_pdf/13/col11496-op.pdf

3) Gegenfurtner KR, Sharpe LT, editors.Color vision: from genes to perception. Cambridge ; New York: CambridgeUniversity Press; 1999. 492 p.

4) Harris RW, Cole BL. Diagnosing protan heterozygosity using the Medmont C-100 colour vision test. Clin Exp Optom. 2005Jul;88(4):240–7.

5) Alotaibi AZ, Ikpotokin EA, Oriowo OM. Assessment of the Medmont C100 test for colour vision screening of male SaudiArabians. Afr Vis Eye Health. 2011 Dec 10;70(1):14–20.

6) Dees EW, Baraas RC. Performance of normal females and carriers of color-vision deficiencies on standard color-vision tests. J Opt Soc Am A Opt Image Sci Vis. 2014 Apr 1;31(4):A401-409.

7) Hasrod N, Rubin A. Defects of colour vision: A review of congenital and acquired colour vision deficiencies. Afr VisEye Health [Internet]. 2016 Mar 24 [cited 2018 Mar 28];75(1). Available from: http://www.avehjournal.org/index.php/aveh/article/view/365

8) Simunovic MP. Acquired color vision deficiency. Surv Ophthalmol. 2016 Mar;61(2):132–55.

9) Cideciyan AV, Hauswirth WW, Aleman TS,Kaushal S, Schwartz SB, Boye SL, et al. Vision 1 Year after Gene Therapy for Leber’s Congenital Amaurosis. N Engl J Med. 2009 Aug 13;361(7):725–7.

10) Mancuso K, Hauswirth WW, Li Q, ConnorTB, Kuchenbecker JA, Mauck MC, et al. Gene therapy for red-green colour blindness in adult primates. Nature. 2009 Oct 8;461(7265):784–7.