SSC logo Scottish Sensory Centre
university of edinburgh

Adapting Video for VI Learners

Optimising colour

Sometimes colour is important in your chosen material, either because colour cues are used as learning organisers, or because colour is intrinsic to the subject. Electrical engineers are bedevilled with colour coded components; false colour satellite photographs are used to bring out geological and agricultural features; and food safety depends on recognising tainted produce.

Where colour matters, so does helping VI students who have colour vision problems. Sometimes (but rarely) all colour is lost, to the extent of excluding that person from certain jobs. Hopefully, proper guidance will avoid the situation in which you find that your material is inappropriate because the course as a whole is unsuitable.

The more normal situation is that you jointly need to find a coping strategy for a local difficulty in the course - and maybe beyond it.

Complete colour blindness

If your student is one of the very few people who have no colour vision at all (achromatopsia), things will be perceived in shades of black/white/grey and so adjusting contrast and brightness will be of interest to you. For these students you will also need supplementary material. Consider providing still frame pictures which have been marked in monochrome to highlight the areas of interest, and the use of spoken and text support in the form of scripts. Watch out for the kinds of problems shown below, where most of the information is badly designed to be overly colour dependent, and is lost in monochrome.

>Restricted colour vision

However, like many other visual impairments, colour blindness is not an all or nothing problem. Some people simply can't differentiate between certain colours with the intensity of others: red/cobalt green, and violet-red/green are examples. Such problems aren't rare: 8% of the male population and 0.5% of the female population have trouble with red/green discrimination. Again, loss of ability to differentiate is a matter of degree. You can imagine the effect as being like reducing, say, bright red to pinky-brown, and bright green to light moss.

Colour monitors: compensations and limitations

Depending on the power of the colour controls your monitor or television has (or which you have added through a separate Audio-Visual control box), you may be able to intensify overall colour, or even a specific band. TV sets allow a degree of overall control, whilst AV controllers let you operate on red, green, and blue channels individually.

On the other hand, some people have trouble in seeing certain small areas of colour because of localised damage to their central visual field. Small areas can be crucial: the results of a chemical reaction may only occupy a small area of screen. A decent sized monitor can help to maximise usage of unaffected areas.

By the time people with visual impairments reach 'student' status, they will know how near they need to be to a TV or monitor to see what they need to see. For some that might be very close: down to 15 or 20 cm. Although this will help seeing, it can cause eye discomfort through dust attracted by screen static, and it can be very tiring.

Teachers: if there are alternatives, don't encourage students to view the screen this closely. For computer displays, try other means of increasing image size such as increasing the size of text and using screen doublers. Consider offering an LCD display - they are a lot less problematic than conventional cathode ray tubes. Students: review your viewing habits, and discuss them with your teacher and eye doctor if you have to stray within normal monitor viewing distances of about 30 cm for extended periods.

There are other traps for the unwary. One is that colour TV is a compromise between accuracy and bandwidth, since there is only so much information which can be packed into the signal. The compromise leans in favour of the black and white signal: colour information in video is a fairly crude wash over a detailed monochrome signal. You can easily see this by turning up colour on a normal TV, and noticing how the colour tends to bleed from red objects into the local image. For those reading these notes on video-enabled multimedia we've simulated this below: use the controller to experiment with different colour levels.

So although cranking up the intensity of a colour might help to distinguish it from others, there is the risk that in doing so you lose the finer underlying spatial features.

Another trap comes from the fact that a video image contains much less colour complexity than the real world. There is a restricted range of intensities which can be displayed, and any adjustments you may will worsen this restriction. Increasing, say, the green channel brightness takes all the signals in it closer to the upper green limit (called 'saturation'). Differences between shades are compressed out, the result being like a child's painting, in which 'all trees are green', and leafy variation is lost. Again, some care is needed to balance between improving distinguishability, and losing, in this case, colour detail.