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Adapting Video for VI Learners

Effects of visual impairment

Contrast sensitivity

What is meant by contrast sensitivity function?

Contrast sensitivity function (CSF) is a subjective measurement of a person's ability to detect a low contrast pattern stimuli, usually vertical stripes of decreasing shades of black to grey. The resulting measurement is said to give a more accurate representation of the eyes' visual performance. CSF has been likened to an audiogram where a persons highest detectable pitch is measured and well as the ability to hear all lower pitches.

What effects does a low contrast sensitivity function produce?

A low CSF can reduce the amount of detail that can be seen especially in lower levels of lighting or at a distance. Difficulties can be experienced with making sense of poor contrasting print materials, for instance worksheets and with moving around safely in dim light.

Similarly, text on videos which is poorly contrasted against the background colour will be difficult to read. For example, black print on a yellow background provides an excellent contrast but light blue print on a grey background is very difficult to read.

Image shows the differences in legibility of black/yellow and blue/gray text and backgrounds(D)>What general situations make CSF worse?

Low levels of lighting, glare and poor colour contrast within the environment, on the screen or in print.

Graphic showing poor contrast situation(D)

View video for sighted pupil (1 M) or VI pupil (1 M)>What general situations make CSF better?

Paying attention to positioning of light sources, avoiding glare of any kind e.g. from coming through windows or being reflected off shiny surfaces, and being aware of the need for good quality handouts.

How does this affect viewing video screens?

Distance vision may be blurred so seating near the monitor is important. Controls need to be set so that the contrast and brightness of the picture is high.

What would improve screen access?

Paying attention to the colour of text and the background colours so that a good contrast will be achieved. Where filming requires dark scenes, make sure that the commentary reflects what is being seen.

What names of eye conditions should I watch out for?

All those with a combined central or peripheral field loss of vision for example:

central field loss -

  • macula degeneration
  • Best's disease
  • Stargadt's disease
  • achromatopsia
  • cone dystrophies

peripheral field loss -

  • retinitis pigmentosa
  • hemianopia
  • chorioretinitis
  • glaucoma
  • aniridia
  • Marfan's syndrome
  • retinal detachment
  • Leber's amaurosis

combined loss -

  • coloboma
  • optic nerve disorders e.g. optic atrophy
  • optic dysplasia and hypoplasia
  • strokes

in addition -

  • albinism
  • high myopia and amblyopia
  • sensitivity to glare