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Adapting Video for VI Learners

Effects of visual impairment

Difficulties with colour vision

What is meant by colour vision?

Normal colour vision is the ability of the visual system to perceive and discriminate between colours on the visible spectrum for instance, blue, green, yellow, red, purple, orange and to appreciate brightness and saturations of the colours.

Colour 'blindness' may range from difficulties in distinguishing red/green colours (the most common type which affects about 1 in 8 males in the population) whilst another type of colour perceptual problem presents problems in seeing colours on the blue/yellow part of the spectrum. Total colour blindness, where 'colours' are perceived as shades of grey, is very rare and is usually associated with a visual defect.

What effect does colour 'blindness' produce?

For the person with normal sight, difficulties with colour vision may just be an inconvenience. However, for someone with low vision it is yet another problem to contend with. It is recognised that because so many eye conditions are a result of problems with the central part of the retina, the macula, which consists of the cone cells responsible for colour vision, many low vision students may have difficulties in perceiving

  • specific pairs of colour hues e.g. shades of blue or distinguishing between hues from adjacent colours in the spectrum e.g. red and orange or indigo and violet
  • luminosity loss - some eye conditions such as rod-cone dystrophies and some forms of achromatopsia result in loss of sensitivity to longwave light, therefore red/brown colours appear much darker than they really are.

Complete colour blindness

Monochrome colour blindness(D)>Specific colours

Example: red / green colour blindness

If you can see the picture above as a movie, you can experiment with degrees of colour blindness by using the video controls. Set the slider to the far left for normal vision and to the far right for maximum red/green colour blindness. Many people will lie somewhere between the two.

What general situations make this worse?

Poor levels of lighting and glare from all sources plus reduced contrast of printed or video material make it difficult for the person with colour perceptual difficulties. Similarly, print or other materials made up of hues or close colours on the spectrum make it difficult to perceive any differences.

What general situations make this better?

Use the video controls to obtain optimum contrast and brightness.

When producing handouts to go with the course of study or study materials on screen, consider using these colour combinations

  • light colours on a black background
  • dark colours on a white background
  • light yellow on a dark blue background
  • dark red on a light green background

Use dark colours from both ends of the spectrum with high brightness mid-spectral colours eg;

  • light yellow on a dark blue background
  • dark red on a light green background  

Avoid using high brightness colours from both ends of the spectrum on dark mid-spectral colours eg;

  • dark green on bright red  

Avoid using white or grew on any colour of a similar brightness eg;

  • yellow on white of similar brightness  

Avoid mixing hues from adjacent parts of the spectrum eg;

  • blue on green of similar brightness  

Avoid using pastel shades together eg;

  • lavender on pink

adapted from Knoblauch, K and Arditi, A (1994)

How does this affect viewing video screens?

Poor contrast and brightness adjustment will diminish the impact of the video. It will be difficult to read any text on the screen if the contrast between the print and the background colour is poor.

What would improve screen access?

If colour is vital to the understanding of a scene on the video then supplementary information needs to be given to the student. This point perhaps emphasises the need to be totally aware of the content and implications for students with a visual impairment before showing the video so that additional materials etc. can be prepared well in advance.

What names of eye conditions should I watch out for?

All those with a central loss of vision for example:

  • achromatopsia
  • Best's disease
  • coloboma
  • cone dystrophies
  • macula degeneration
  • optic nerve disorders e.g. optic atrophy
  • optic dysplasia and hypoplasia
  • Stargadt's disease
  • strokes