Photometer.
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.)
Photometer, Actinic.
A photometer whose registrations are produced by the action of the light
being tested upon sensitized paper or plates, such as used in
photography. Some efforts at self-registering photometers have been
based on actinic registration of the height of a flame of the gas to be
tested.
Photometer, Bar.
A photometer in which the two lights to be compared are fixed at or
opposite to the ends of a bar or scale of known length, generally 60 or
100 inches. The bar is divided by the rule of the inverse square of the
distances, so that if a screen is placed on any part of the bar where it
receives an equal amount of light from both sources, the figure on the
bar will indicate the relative illuminating power of the larger lamp or
light in terms of the smaller. The divisions of the bar are laid out on
the principle that the illuminating power of the two sources of light
will vary inversely with the square of their distance from the screen.