Photometer
SCIENTIFIC INSTRUMENT
Luminometer
An apparatus for measuring the intensity of light emitted by a given
lamp or other source of illuminating power. They may be classified into
several types.
Calorimetric or Heat Photometers act by measuring relatively the heat
produced by the ether waves (so-called radiant heat) emitted by the
source. The accuracy of the instrument is increased by passing the rays
through an alum solution. A thermopile, or an air thermometer, may be
used to receive the rays.
Chemical Photometers. In these the light falls upon sensitized
photographic paper. The depth of coloration is used as the index of
illuminating power.
Direct Visual Photometers. These include Rumford's Shadow Photometer,
Bunsen's Bar Photometer, and Wheatstone's Bead Photometer, in which the
light is estimated by direct visual comparison of its effects.
Optical Photometers. These include Polarization Photometers, in which
the light is polarized; Dispersion Photometers, in which a diverging
lens is placed in the path of the rays of light so as to reduce the
illuminating power in more rapid ratio than that of the square of the
distance.
Selenium Photometers, in which the variations in resistance of selenium
as light of varying intensity falls upon it is used as the indicator of
the intensity of the light.
Jet Photometers, for gas only, in which the height of a flame under
given conditions, or the conditions requisite to maintain a flame of
given height, is used to indicate the illuminating power.
The subject of photometers has acquired more importance than ever in
view of the extensive introduction of the electric light. (See Candle,
Standard--Carcel--Violé's Standard--and Photometers of various
kinds.)