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an instrument for the measurement of optical densities on small portions of photographic images, such as spectrograms and X-ray, astronomical, and aerial photographs. It is a variant of another optical measuring instrument, the densitometer, and differs from it in that it has microscopic optics, with a magnification of 25–40 X. Microphotometers are divided into the single-beam type, which operates by the directreading method, and the much more common two-beam type, in which the intensities of two light beams are equalized in a manner similar to the procedure for two-beam densitometers. They are also divided into nonrecording types (requiring a separate measurement for each portion of an image) and recording types. In the latter the results of measurements along a given line (straight line, circle, and so on) are recorded continuously.
The maximum optical density Dmax that can be measured by a given microphotometer is related to the area cr of the region of an image being measured by the formula lODmax/σ = const. Here the constant characterizes the sensitivity of the microphotometer; for various types it may range from several hundredths to several tenths of a micron (µ). This means, for example, that the most sensitive instruments can measure optical densities close to 3.0 for areas of about 100 μ2. This sensitivity is better than that of ordinary densitometers by a factor of tens of thousands to hundreds of thousands.
Multistage photoelectric multipliers are most often used as the light detectors in modern microphotometers (in older models, selenium photocells were used). Their measurement accuracy is usually 0.01–0.03 units of optical density.
The main types of microphotometers are the isophotometric (equidensitometric) variety, which can determine the geometric locations of points on a photographic image of equal optical density and describe them in the form of an isophot, or equidensit; and microspectrophotometers, which use monochromatic light to measure the spectral absorption curves of thin-film objects that change color sharply along their surface (for example, chromatograms).
REFERENCESGorokhovskii, lu. N., and T. M. Levenberg. Obshchaia sensitometriia: Teoriia i praktika. Moscow, 1963.
IU. N. GOROKHOVSKII