Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light. In medicine, the term is principally used for abnormal reactions of the skin, and two types are distinguished, photoallergy and phototoxicity. The photosensitive ganglion cells in the mammalian eye are a separate class of light-detecting cells from the photoreceptor cells that function in vision.
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, heart drugs amiodarone, and sulfonamides. 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. A rare hereditary condition xeroderma pigmentosum (a defect in DNA repair) is thought to increase the risk of UV-light-exposure-related cancer by increasing photosensitivity.
Photosensitizations are classified as primary if an ingested plant contains a photosensitive substance, like hypericin in St John's wort poisoning in sheep, or buckwheat plants (green or dried) in horses.
In hepatogenous photosensitization, the photosensitzing substance is phylloerythrin, a normal end-product of chlorophyll metabolism.  It accumulates in the body because of liver damage, reacts with UV light on the skin, and leads to free radical formation. These free radicals damage the skin, leading to ulceration, necrosis, and sloughing. Non-pigmented skin is most commonly affected.
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, electric current flowing through or electrical charge stored in the device will increase or decrease in proportion to the intensity and wavelength of the light, although there is an upper limit to the amount of electrons released vs the increased intensity of the light, this comes out of quantum mechanics. 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 an array of photodiodes 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.
Interpretation in chemistry
Chemicals that are photosensitive may undergo chemical reactions when exposed to light. These chemicals, such as hydrogen peroxide and many prescription drugs, are stored in tinted or opaque containers until they are needed to prevent photodegradation. Devices that are photosensitive include the human retina and photographic film (see film speed and sensitometry); their photosensitive materials undergo a chemical reaction when struck by light.
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- FDA article - Chemical Photosensitivity: Another Reason to Be Careful in the Sun
- Toxic, Phototoxic and Sensitizing Essential Oils
- Sensor sensitivity (ISO) in digital cameras
- How to avoid overxposure to sunlight for the photosensitive
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