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phosphorescence • photoelectric efficiency 529
phosphorescence The property of some materials
that ordinarily fluoresce to continue to glow after
the stimulus (light or an electron beam) has been
removed. Compare FLUORESCENCE.
phosphorescent screen A viewing screen coated
with a phosphor (e.g., oscilloscope screen).
phosphorous Exhibiting the properties of phosphor
(e.g., glowing after stimulation with light).
Not to be confused with PHOSPHORUS.
phosphorus Symbol, P. A nonmetallic element of
the nitrogen family. Atomic number, 15. Atomic
weight, 30.974. It is used as a dopant in semiconductor
processing.
phot The cgs unit of illumination: The direct illumination
produced upon a one-centimeterdistant
surface by a uniform point source of one
international foot-candle. Equivalent to one lumen
per square centimeter.
photocathode 1. The photomosaic of a video camera
tube. 2. The light-sensitive cathode in a phototube.
PhotoCD Trade name for an image-recording system
developed by Kodak, in which photographs
can be stored on compact discs. Viewing is accomplished
using personal computers.
photocell See PHOTOELECTRIC CELL.
photocell amplifier An amplifier used to boost the
output of a photocell. With respect to the nature
of the input signal, it can be an alternatingcurrent
(ac) or direct-current (dc) amplifier, depending
on whether the output of the photocell is
modulated dc or pure dc.
photochemical effect The phenomenon whereby
certain substances undergo chemical change when
exposed to light or other radiant energy. An example
of such a substance is the silver bromide, silver
chloride, or silver iodide on photographic film.
photoconductive cell A photoelectric cell, such as
the cadmium-sulfide type, whose resistance is
proportional to the intensity of light impinging
upon it. The photoconductive cell acts as a lightsensitive
variable resistor in a current path. Also
see PHOTOCONDUCTIVE MATERIAL.
photoconductive effect The tendency for the electrical
resistance of a substance to change when
infrared radiation, visible light, or ultraviolet radiation
strikes it. Different substances exhibit different
degrees of this effect.
photoconductive material A substance that exhibits
decreased electrical resistance when exposed
to infrared rays, visible light, or ultraviolet.
Some photoconductive substances are cadmium
selenide, cadmium sulfide, germanium, lead sulfide,
selenium, silicon, and thallous sulfide. Also
see ACTINOELECTRIC EFFECT.
photoconductivity The phenomenon whereby the
electrical resistance of certain materials (such as
cadmium sulfide, cadmium selenide, germanium,
selenium, and silicon) is lowered upon exposure
to infrared rays, visible light, or ultraviolet. Also
see PHOTOCONDUCTIVE MATERIAL.
photoconductor 1. See PHOTOCONDUCTIVE MA-
TERIAL. 2. See PHOTOCONDUCTIVE CELL.
photocurrent See PHOTOELECTRIC CURRENT.
photo-Darlington Also, photodarlington. 1. A phototransistor
fabricated as a Darlington amplifier
for high output current. 2. A combination of photodiode
(see LIGHT-SENSITIVE DIODE) and Darlington
amplifier.
photodecomposition Chemical breakdown by the
action of radiant energy. Also called photolysis.
photodetachment The removal of an electron from
an atom or ion, resulting from the impact of a
PHOTON.
photodetector 1. An illumination meter that uses
a PHOTOCELL. 2. See OPTOELECTRONIC COU-
PLER.
photodielectric effect The tendency for the dielectric
constant of a substance to change when
infrared radiation, visible light, or ultraviolet
radiation strikes it. Different substances exhibit
different degrees of this effect.
photodiffusion effect See DEMBER EFFECT.
photodiode See LIGHT-SENSITIVE DIODE.
photodisintegration In the nucleus of an atom,
disintegration resulting from PHOTON bombardment.
photoelasticity The tendency for the lighttransmission
characteristics of a substance to
change with externally applied forces.
photoelectric alarm An alarm actuated when a
light beam impinging on a photocell is interrupted.
photoelectric amplifier 1. An amplifier for boosting
the output of a photosensitive device. 2. An
OPTOELECTRONIC COUPLER possessing gain.
photoelectric cell A device that converts infrared,
visible-light, or ultraviolet energy into electricity
or electrical effects. It can function by producing
a voltage (see PHOTOVOLTAIC CELL, SELENIUM
CELL, SILICON CELL, SOLAR CELL, and SUN
BATTERY) or by acting as a light-sensitive resistor
(see LIGHT-SENSITIVE DIODE, PHOTOCON-
DUCTIVE CELL, and SELENIUM CELL).
photoelectric constant The quantity h/e, where h
is Planck’s constant and e is the unit electron
charge.
photoelectric counter A counting device (electromechanical
or fully electronic) that counts objects
as they interrupt a light beam impinging
upon a photocell.
photoelectric disintegration See PHOTODISIN-
TEGRATION.
photoelectric effect The phenomenon whereby
temporary changes occur in the atoms of certain
substances under the influence of infrared, visible
light, or ultraviolet radiation. Some of these
materials undergo a change in their electrical resistance,
whereas others generate electric current
(see, for comparison, PHOTOCONDUCTIVE MA-
TERIAL and PHOTOVOLTAIC MATERIAL).
photoelectric efficiency See QUANTUM YIELD.