Introduction: Photosensitivity Basics
Photosensitivity is an increased sensitivity or abnormal response of the skin to sunlight or artificial light. In particular, both UVA radiation (longer wavelengths) and UVB radiation (shorter wavelengths) have been observed to trigger unusual reactions of the skin in people with certain disorders or those who are taking particular medications.
The most common manifestation of an increased photosensitivity is the appearance of lesions of various shapes and sizes on areas of the skin that have been exposed to sunlight. The time required for such a response to occur can be anywhere from under 30 minutes of exposure to sunlight to hours spent in it.
When a person suffers from an exaggerated sensitivity to sunlight, he or she most often will exhibit some form of dermatitis (a rash caused by an allergy or physical contact with a particular substance) on the part of the body that was exposed to light. Thus, naturally hidden body parts such as the skin of the upper eyelid or areas covered in hair such as the scalp are better protected and do not produce such rashes.
The most effective way to protect your skin and prevent damage to it, regardless of whether or not you have a photosensitivity disorder, is to minimize exposure to direct sunlight. Application of a high SPF sunscreen to all exposed areas and wearing protective clothing are also important practices in defending your skin from the sun's harmful UV rays.
Willis, Isaac. "Photosensitivity and Phototherapy." Dermatology in General Medicine. Second ed. 1 vols. New York: McGraw-Hill, Inc., 1979. (http://www.dermnetnz.org/reactions/photosensitivity.html)