Smith & Walker Optometrists

The Eye and Colours

Colour blindness is a misleading term, since practically everbody sees colours but some individuals are poor at discriminating between certain shades. About 1 in 12 men inherit a color deficiency (via the maternal grandfather) whereas only 1 in 250 women are affected.

Optometrists generally use colour plates to screen for colour deficiencies. These are a sensitive test for identifying individuals with affected colour vision but occasionally a question mark remains over whether or not the colour vision is normal. Very often it is impossible for the test to categorise the defect (other than as a 'Red-Green' anomaly) or to determine how severe the problem is. We can arrange for further tests to be made at a specialist centre if detailed classification is required.

Broadly speaking the red-green colour deficiencies fall into two categories. One group is insensitive to deep red and may confuse some shades of red, brown and green. The other group confuse shades of red, green and yellow. Shades of red, magenta and grey may be confused. However so called 'colour blind' individuals can still correctly identify most instances of red and green etc. Confusion only arises with certain hues and saturations.

Although a colour vision weakness can bar entry into certain careers, on the whole it poses very few problems in day to day life. The following list identifies some careers which require normal colour vision. However since there are often various specialisations within each vocation it is wise to check entry requirements individually as low-grade colour deficits may be acceptable.

Armed forces: Various disciplines are ruled out

• aircrew
• bridge watchkeepers
• seaman officers

Check with forces career offices as many sub specialisations have lower colour vision requirements.

Civil Aviation:

• Flying personnel

If they fail the standard colour plate test they may still qualify with a less stringent occupational test. Otherwise licence exclusions and limitations may be imposed.

• Electronic/Electrical engineering
• Fire Officers

Merchant Navy:

• Deck officers and ratings
• Engineer officers: may be a handicap

Police:

• Categories depend...

Each force is responsible for its own standards - those unable to identify the principal colours are usually excluded.

• Railway

Defective colour vision may be a handicap at times in the following occupations:

• anaesthetist
• art work
• beautician
• some catering careers
• cartography
• design
• some engineering disciplines
• florist
• forensic scientist
• hairdresser
• horticulture
• interior decorator or designer
• jeweller
• laboratory technician
• paintmaker or sprayer
• radio and TV servicing
• textiles

There is no cure for inherited colour vision defects, although it is sometimes possible to fit contact lenses that aid colour discrimination. These do not restore normal colour vision and do not lift the exclusions from occupational requirements. Although the lists ablove may seem daunting it is important to remember that weak colour vision presents very few problems in most careers and everyday life.

It might be said that the eye works rather like a camera (although the eye came before the camera!). Light is focused onto the retina at the back of the eye. The retina is a transparent lining which contains light receptors. Some receptors are cones, which convey colour, whereas the receptors called rods deal with low light situations. Normal colour vision relies on the presence of three different photopigments within the cones. If one pigment is either absent or slightly defective an individual's colour discrimaination will fall.

The artist uses what he calls primary colours - blue, red and yellow, which in equal chemical measure combine in paint to form secondary colours of green, purple and orange, and these subtractive primary colours combine to form black. Additive primary colours are based on the physics of light, where red, green and blue are called the primary colours, producing yellow, cyan and magenta as secondary colours and white as the product of all three primary colours. These three primary colours in different relationship form any colour. In fact the colours are wavelengths of the electromagnetic vibrations of light.

Artists note that the use of blue in a painting gives distance and red brings objects closer. Yellow is bright and sharp. Given that white light is made of different wavelengths, then actual different colours have slightly different properties of focussing. Short wavelenths are focused closer to the lens than longer wavelengths which come to a focus further back. This phenomenon of longitudinal chromatic abberation is used by optometrists to fine-tune their prescriptions.

When colour coded signals reach the brain they are perceived i.e. colour is appreciated. It is a psychological reaction.