| photomultiplier tube | A detector which amplifies a signal (by as much as 106) of electromagnetic radiation by an acceleration of electrons released from a photocathode through a series of dynodes; as each electron strikes a dynode stage, 3 to 4 electrons are liberated and accelerated to the subsequent dynode. (05 Mar 2000) |
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| photomyoclonus | Clonic spasms of muscles in response to visual stimuli. Origin: photo-+ G. Mys, muscle, + klonos, confused motion Hereditary photomyoclonus, photomyoclonus associated with diabetes mellitus, deafness, nephropathy, and cerebral dyfunction. (05 Mar 2000) |
| photon | <physics> A quantum unit of light energy. (09 Oct 1997) |
| photon density | The number of counted events recorded in scintigraphy per square centimeter or per square inch of imaged area. Synonym: count density. (05 Mar 2000) |
| photoncia | Any swelling resulting from the intense action of light. Origin: photo-+ G. Onkos, a mass (tumour) (05 Mar 2000) |
| photonosus | Any disease caused by excessive exposure to or unusual intensity of light, or resulting from phototoxicity or photoallergy. Synonym: photopathy. Origin: photo-+ G. Nosos, disease (05 Mar 2000) |
| photons | Discrete concentrations of energy, apparently massless elementary particles, that move at the speed of light. They are the unit or quantum of electromagnetic radiation. Photons are emitted when electrons move from one energy state to another. (12 Dec 1998) |
| photooxidase | <enzyme> Rhodoquinone accepts electrons from ubiquinone during chromatophore photooxidase activity Registry number: EC 1.6.99.- Synonym: rhodoquinone photooxidase (26 Jun 1999) |
| photopathy | Synonym: photonosus. Origin: photo-+ G. Pathos, suffering (05 Mar 2000) |
| photopeak | The characteristic energies of photons emitted by a radionuclide, used to set scanning parameters. (05 Mar 2000) |
| photoperceptive | Capable of both receiving and perceiving light. (05 Mar 2000) |
| photoperiod | <ecology> Recurring cycle of light and dark periods. The natural photoperiod is approximately 24 hours, and the ratio of light to dark hours slowly changes over the course of a year. In controlled experiments, the photoperiod is usually (but not necessarily) retained at 24 hours, and the light:dark ratio is typically constant. (09 Oct 1997) |
| photoperiodism | <biology> Events triggered by duration of illumination or pattern of light/dark cycles: often the wavelength of the illuminating light is important as for example in control of circadian rhythm in plants. See: phytochromes. (18 Nov 1997) |
| photopheresis | A process in which peripheral blood is exposed in an extracorporeal flow system to photoactivated 8-methoxypsoralen (methoxsalen) and ultraviolet light - a procedure known as puva therapy. Photopheresis is at present a standard therapy for advanced cutaneous T-cell lymphoma; it shows promise in the treatment of autoimmune diseases. (12 Dec 1998) |
| photophobia | <psychology, symptom> An abnormal visual intolerance of light. Origin: Gr. Phobos = fear (16 Mar 1998) |