Developed by Lea Hyvärinen, M.D.
We can see in bright daylight and in twilight. In daylight vision (photopic vision) we use cone cells. Activity in the cone cell pathway inhibits the rod cell pathway. When the luminance level becomes lower, input from cone cells decreases and input from rod cells increases (mesopic vision). When the luminance level decreases further, the cone cells’ contribution to vision stops, colours disappear. The image is in different shades of grey because rod cells do not convey colour differences (scotopic vision). When we enter a darker place it takes a few seconds before we start to see colours at the lower luminance level. This is called cone adaptation time.
In retinal degenerations cone cell adaptation time may become longer than normal quite early. Therefore the CONE Adaptation test can be used for screening of retinitis pigmentosa and follow-up of retinal function.
The Cone Adaptation Test (#252900) consists of fifteen 5 x 5 cm (2 x 2 in) red, blue and white plastic chips designed to help parents, teachers and doctors become aware of a child's visual difficulties in twilight. The test situation is not a formal test of night vision, but gives useful information on visual adaptation to lighting changes.
When we enter a room that is darker than the room where we were, during a few seconds, the room looks dark and then we start to see comfortably. This rapid adaptation to a lower luminance level is possible because cone cells adapt quickly within the range of their adaptation capability. Rod cell adaptation to very dim light is much slower.
In every day life we need to change our adaptation in the adaptation range of cone cells more often than in the range of rod cells. Actually, in cities children and many adult persons rarely are in pure scotopic conditions, i.e. where only rod cells function. There is a wide range of luminance levels where both cone and rod cells function. In that range cone adaptation is important because it is faster than the rod adaptation. If colours can be perceived we are within the range of cone cell function. Therefore perception of colours can be used to assess the speed of cone cell adaptation. This was first suggested by Thornton.
You may use normal twilight situations like coming in from an evening walk and not turning on the room lights, but starting with the play situation immediately at that low luminance level.
Remember, changes in night and twilight vision occur so slowly that the child does not notice them. This simple play situation helps to keep you aware of the child's present visual adaptation level to lighting changes. This includes both speed of adaptation and the final adaptation level reached after a few seconds or longer adaptation to the dark. Remember to report your observations to the child's eye doctor.
This simple test situation can be made to a more formal test by standardizing the luminance levels. In the beginning of the test ask the child/person to look at a white surface for one minute. Use always the same white surface and the same lamp at the same distance so the adaptation to high luminance level is equal in all measurements. Define the mesopic luminance level in similar way, i.e. use a small light in a corner of the room behind the child and the same dark cloth each time.
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