What is the definition of photosensitivity?

Although there is not a definition accepted by all, most dermatologists define photosensitivity as the development or exacerbation of a skin eruption and/or symptoms (including pruritus or pain) following exposure to sunlight. In some instances, a patient may not specifically relate the eruption to sun exposure, usually due to a delay in the onset of signs or symptoms following sun exposure. Thus, if a skin eruption is photodistributed, even without a definite history of exacerbation following sun exposure, many dermatologists classify it as a photosensitive dermatosis. Some photosensitivity reactions (e.g., phototoxic drug reactions) are very similar to sunburn but occur with less intense sun exposure than would normally be required to induce sunburn in that individual.

Millard TP, Hawk JL: Photosensitivity disorders: cause, effect and management, Am J Clin Dermatol 3:239–246, 2002.