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densitometer(den-să-tom -ĕ-ter) An instrument for measuring the density of exposure of a photographic emulsion. Densitometers have a light source and photocell to measure the transmission (or reflection) of the sample. They are used in obtaining quantitative measurements from photographic records of spectra, diffraction experiments, etc.
a device for measuring the optical density of developed photographic materials. They are classified according to the method of measurement (direct readout and the comparison principle), the type of light receiver (the eye, a photocell, or a photomultiplier), the type of output data (nonrecording devices and automatic recording devices), and the magnitude of the field being measured (densitometers proper, as compared with microdensitometers, which are sometimes referred to as microphotometers).
In devices with direct readout, a single light beam whose initial strength Φ0 is compared with the strength Φ of the beam after passage through a photosensitive layer is usually used. Devices whose operation is based on the comparison method have two light beams originating from the same source, one of which is the measuring beam and the other is the reference beam. In the most widely used type of photoelectric densitometer, the two beams are directed at two photoelectric receivers that are in a difference-reading circuit (that is, if the strength of the two beams is equal, the signal of the two receivers is zero) or are alternately connected to the same receiver. The difference signal, which is caused by unequal strength of the two beams, is brought to a zero reading with the aid of a variable light attenuator (for instance, a gray photometer wedge) inserted into one of the beams being compared and calibrated in units of optical density D = log Φ0/Φ.
The accuracy of densitometer measurements averages ±0.02 optical density units over the entire measurement range. For the best models the range is 5-6 density units; for relatively simple devices it is 2.5-3.0 density units (transmission coefficients of 10-5-10-6 and 0.003-0.001, respectively). The most typical densitometer models made in the USSR are the DFE-10 nonrecording wedge densitometer with selenium photocells (Dmax = 3.0) and the IFO-451 recording wedge microdensitometer with a photomultiplier, which is capable of measuring optical densities up to 2.5 units.
REFERENCEGorokhovskii, Iu. N., and T. M. Levenberg. Obshchaia sensitometriia: Teoriia i praktika. Moscow, 1963.
IU. N. GOROKHOVSKII
ii. An instrument for measuring the optical density (photographic transmission, photographic reflection, visual transmission, etc.) of a material, generally of a photographic image.