Visual Adaptation

Visual adaptation occurs so rapidly that we are usually not aware of it when we pick something from under the desk or from a closet. In a few occupations also the speed of visual adaptation needs to be tested. There are expensive instruments in clinical medicine for the measurement of the adaptation curve. In occupational health services we are rarely interested in that type of information but want to know the speed of cone adaptation or visual acuity at low luminance levels.

The speed of cone adaptation, i.e how soon a person starts to discern colours at low luminance levels, can be measured with the paediatric CONE Adaptation test. The luminance level is chosen to be the level that is used in the task, usually a deep mesopic luminance.

The person is instructed to sort the caps with three different colours each colour in its pile, first in the usual room light and when this is done the test itself is done at the low luminance level. The person starts to sort the caps immediately when the lights are turned off. Since the white chips glow in dim light, they are picked first. Since they are five it takes about 4-5 seconds to sort them into a pile at which time the other colours become visible.

Measurement of visual acuity at low luminance level requires a suitable filter to be placed in front of the visual acuity test that is on the usual small or large (EDTRS) lightbox. Such filters can be made using several layers of neutral densitity filters and measuring the luminance of the test surface with a luminance meter.