Technically, contrast vision is how good we are at telling different shades of grey from each other. It is measured by finding out how big a difference in grey is needed for two shades of grey next to one another to be told apart.
Reduced contrast sensitivity has an impact on how the individual sees and can cause an image to appear faded or washed out, like this example. It is possible to have perfect visual acuity but to have low contrast sensitivity.
The picture below has been simulated by a beta version of SightSim to show approximately how the image would look to someone with reduced levels of contrast sensitivity.
Contrast sensitivity needs to be measured and acted upon; being told that high levels of contrast are needed or 'responds to high contrast' provides little useful information.
In real life contrast is mixed with colour. Dark blue is blue with black added, light blue is blue with white added. The ability to tell them apart depends mainly on contrast sensitivity, not the ability to see colour.
The diagram below has a prime red in the middle, with white to the left and black to the right. The bottom stripe is the same prime red as the middle. The level of colour contrast sensitivity is the point where the individual is unable to differentiate between the bottom stripe and the vertical stripe above it.
A child with low colour contrast sensitivity, owing to reduced contrast sensitivity may for example only be able to tell the difference between every fourth colour in the above diagram.
It is essential to know this.
Faces for example have very subtle shades of the same skin tone that children with reduced contrast sensitivity may only be aware of as a single shade. For the same reason it may not be possible to see a green frog on grass despite apparently sufficient visual acuity. So it is very important to check what children can see in colour pictures in their story books and educational material, because children with low vision can never let us know what they do not see.
We can never use our vision to decide what can be seen by children with low vision. We can only use materials we've checked they can see.
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