DENSITOMETRY (Latin densitas density, density + grech, metreo to measure, measure) — measurement of the optical density (opacity) of a photographic plate or film, layer of gel, paper or other material where structures of the studied fabric, body (roentgenogram), distribution of a radioisotope (radioavtogramm) or the studied process (chromatographic or electrophoretic fractionation of substrate) are recorded.
According to D. it is possible to receive quantitative assessment of optical density in the roentgenogram, an elektroforegramma (see. Electrophoresis ). On the basis of these data the doctor can judge more objectively about patol, the process recorded on the roentgenogram and also structure of protein fractions — on an elektroforegramma, a ratio of amino acids — on a hromatogramma (see. Chromatography ), etc.
Principle (Fig.) it is based that the light flux, passing, e.g., through the roentgenogram or an elektroforegramma on which there are sites, various on optical density (opacity), is partially absorbed and, getting on the photodetector, it will be transformed to electric current, inversely proportional to amount of the absorbed light energy. If a lot of light was absorbed, i.e. there was big optical density of the site of the roentgenogram or an elektroforegramma through which there passed the light flux, then in the photodetector there will be small current; and, on the contrary, at the insignificant optical density of the explored site not enough light will be absorbed and last light flux will cause big current in the photodetector. Moving the roentgenogram or an elektroforegramma under a continuous light stream, the photodetector it is possible to register the curve of change of current corresponding to change of optical density of the studied roentgenogram or an elektroforegramma in the area of raying. If to carry out such raying postrochno, i.e. to scan the roentgenogram or an elektroforegramma, then it is possible to receive an overall picture of distribution of optical density on the big area of the studied roentgenogram, etc. D. make by means of devices — densitometers.
Densitometer consists of a source of light radiation, the optical system providing formation of a light flow from a light source, the detector of the light flow which passed through the studied object. In the elementary densitometers an object (the roentgenogram, a gel layer from the painted elektroforegrammy) is moved manually, and readings of optical density are taken visually on a galvanometer. On a scale of a galvanometer divisions in terms of optical density are put or determine optical density by the standard curve expressing dependence of current of the photodetector on optical density. In the universal densitometers providing densitometry and a densitografiya (graphic data recording of D.), an object moves automatically. As the photodetector in such densitometers photo cells, photo multipliers, photoresistance, pi photo diodes other devices transforming a light flux to an electric signal are applied. D.'s results are fixed on a paper strip through automatic registrar which writes down values of optical density on ordinate axis, and linear movements of an object on abscissa axis.
In densitometers with use of computer facilities processing of the signal received from the device is carried out with the help electronic computer (see). At the same time the zones of a densitogramma (roentgenogram) having the close value of optical density which is keeping within the limits of their «dispersion» set earlier are averaged. All roentgenogram Densitometrirutsya by method scannings (see). On a signal of the COMPUTER the recorder applies the average size of optical density of the roentgenogram on paper, as if throwing out half tone. It allows to receive more contrast image of the studied body or fabric. On the roentgenogram processed thus are visible to heterogeneity in the studied structure more accurately.
Bibliography: Gorokhovsky Yu. N. and Levenberg T. M. General sensitometry, M., 1963, bibliogr.; The guide to cytology, under the editorship of A.S. Troshin, t. 1, page 85, M. — L., 1965.
V. I. Belkevich.