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photometry

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(fō-tŏm′ĕ-trē) Measurement of light rays.

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photomicrograph

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(fō″tō-mī′krō-grăf) [″ + mikros, small, + graphein, to write] A photograph of an object under a microscope.

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photomultiplier tube

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(fō″tō-mŭl′tĭ-plī″ĕr) ABBR: PMT. In radiography, an electronic vacuum tube designed to convert light photons into electrical pulses. It is used to digitize incoming light photons prior to the creation of computerized images in nuclear medicine and other imaging modalities.

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photon

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(fō′tŏn) [Gr. photos, light] A light quantum or unit of energy of a light ray or other form of radiant energy. It is generally considered to be a discrete particle having zero mass, no electric charge, and indefinitely long life.

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photopenia

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(fōt″ō-pē′nē-ă) [photo- + -penia] In radiologic image analysis, an area of decreased energy or tracer uptake. photopenic (fōt″ō-pē′nik), adj.

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photoperceptive

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(fō″tō-pĕr-sĕp′tĭv) [″ + L. percipere, to receive] Capable of perceiving light.

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photoperiod

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(fōt″ō-pēr′ē-ŏd) [photo- + period] The daily duration of exposure to light of a living thing. photoperiodic (fōt″ō-pēr″ē-od′ik), adj. photoperiodically (fōt″ō-pēr″ē-od′i-k(ă-)lē), adv.

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photoperiodism

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(fōt″ō-pēr′ē-ŏ-dĭzm) [photoperiod + -ism] The periodic occurrence of biological phenomena in relationship to the presence or absence of light. In most animals, the sleep-wake cycle is a form of photoperiodism.

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photophobia

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(fō″tō-fō′bē-ă) [″ + phobos, fear] Unusual intolerance of light, occurring in measles, rubella, meningitis, and inflammation of the eyes. SYN: photodysphoria.

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photophoresis

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(fō-tō-fŏr-ē′sĭs) A technique used in treating cutaneous T-cell lymphoma. It incorporates exposure of a lymphocyte-enriched blood fraction, obtained by use of apheresis to ultraviolet A light after the patient has ingested the cytotoxic agent 8-methoxypsoralen. SYN: extracorporeal photochemotherapy.

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photopsia, photopsy

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(fō-top′sē-ă, fō-top′sē) [Gr. photos, light, + opsis, vision] The subjective sensation of sparks or flashes of light in retinal, optic, or brain diseases.

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photoreaction

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(fō″tō-rē-ăk′shŭn) [″ + LL. reactus, reacted] A chemical reaction produced or influenced by light.

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photoreactivation

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(fō″tō-rē-ăk″tĭ-vā′shŭn) Enzymatic repair of lesions such as can be produced in DNA by ultraviolet light.

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photoreception

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(fō″tō-rē-sĕp′shŭn) [″ + L. recipere, to receive] The perception of light rays in the visible light spectrum.

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photoreceptor

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(fō″tō-re-sep′tōr) [photo- + receptor] Any of the sensory nerve endings or cells that are capable of being stimulated by light. In humans, these include the rods and cones of the retina.

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photoscan

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(fō′tō-skăn″) A representation of ...

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