The problem can be the result of a range of eye conditions.
Conjunctivitis (pink eye) - Conjunctivitis is an inflammation of the thin, clear covering of the white of the eye, which is sometimes linked to allergies. Allergic conjunctivitis can cause light sensitivity.
Corneal abrasion - Damage to the surface layer of the cornea can lead to photophobia. This can be the result of a minor injury, a problem with a contact lens, or the presence of dirt or grit in the eye.
Detached retina - If the retina becomes detached from its support tissue, it can no longer function properly, so is unable to control the amount of light entering the eye.