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( Photosensitivity)
Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light. Sensitivity of the skin to a light source can take various forms. People with particular skin types are more sensitive to sunburn. Particular medications make the skin more sensitive to sunlight; these include most of the tetracycline antibiotics and the heart drug amiodarone. Particular conditions lead to increased light sensitivity. Patients with systemic lupus erythematosus experience skin symptoms after sunlight exposure; some types of porphyria are aggravated by sunlight. All types of skin cancer are linked to excessive sunlight exposure, and the rare hereditary condition xeroderma pigmentosum (a defect in DNA repair) accelerates this risk manifold. While the term "photoallergy" or "sunlight allergy" is commonly employed in many of the conditions mentioned, there is in fact no actual allergy in the technical sense of the word if the reaction is a purely photoirritant type. Skin photoallergies occur when light is required to trigger the interaction between a chemical material in the skin and host tissues with the resultant formation of an antigen that is recognized by the host immune system. Typically, these types of reaction are similar to a hapten-carrier reaction in contact sensitization reactions with the exception that light is needed to trigger the binding of the hapten to the host carrier molecule. Certain electronic devices, such as photodiodes and charge-coupled devices, are designed to be sensitive to light. They are constructed to take advantage of the photoelectric effect, the emission of electrons from matter upon the absorption of electromagnetic radiation. When light (one form of electromagnetic radiation) impinges on the active surface of such a device, electrical current flowing through or electrical charge stored in the device will increase or decrease in proportion to the intensity and wavelength of the light. This trait allows the device to perform regulating and sensing functions of many kinds. For example, a photoresistor circuit may sense ambient light to turn on a street lamp at dusk. Digital cameras use charge-coupled devices whose extreme sensitivity to light allows them to convert incoming photons into varying electrical charges with great accuracy. The varying charges are then encoded in a binary file which can be stored and later viewed on a computer screen or other medium.
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