People with low vision sometimes have problems with adaptation. It takes their visual system an inconvenient length of time to adapt to the higher or lower level of light in the environment they have moved into compared to the environment they were in.
Dark adaptation should take place when moving from light to a dark environment. One way to help patients with this is to use a neutral grey drop-in filter (neutral grey) before going outside. This can be placed between any current prescription glasses and the patient’s eyes. If the patient does not wear prescription glasses for distance vision a plano pair with a neutral grey tint can be used. See the grey filters in the middle of the image below for an example of the depth of tint required. Photochromic lenses do not react fast enough to be of use in this situation. The neutral grey filter will lower the adaptive state of the retina, making it easier for the patient to cope when re-entering a dark environment after having been in a light environment.
Some patients will have problems with light adaptation. This should take place when moving from a dark to a light environment. This is a problem caused by conditions involving cone dysfunction, such as cone dystrophy. In this condition, only the rod photoreceptors are functioning. The aim is to minimise rod bleaching in bright conditions. Specialist dark red lenses are required. See the filter in the lower left-hand corner of the image below.