Control of Light Intensity

We need to control the amount of light entering our eye because
a) under too strong light intensity, our retina may be damaged;
b) under low light intensity, we need more light to enter our eye to form a brighter image.

Our antagonistic muscles of the iris are responsible for controlling the size of the pupil to regulate the amount of light entering. This is done by a reflex mechanism, i.e. an involuntary, automatic response.

Reflex action of the pupil

Stimuli	strong light	weak light
Receptors	retinal photoreceptors
Response
	circular / sphincter muscle contracts radial / dilator muscle relaxes	radial / dilator muscle contracts circular / sphincter muscle relaxes
	pupil diameter decreases	pupil diameter increases
	less light enters	more light enters

In very strong light, our eyelids will also close.

Do you know that when strong light is shone onto one eye only, the pupil of the other eye will also become smaller? The former phenomenon is called pupillary light reflex, the latter is called consensual light reflex.

The drug Atropine can prevent the circular iris muscles from contracting. It is sometimes used by opthalmologists when they want to examine our eye.

Back to main Physiology page