Meetings and Conferences

Discussion of Issues in the Sensitometry of Black-and-White Photographic Materials

The Commission on Scientific Photography and Cinematography of the Academy of Sciences of the USSR held its regular plenum in Leningrad on January 26–28, 1953, devoted to discussion of issues in the sensitometry of black-and-white photographic materials.

Photographic sensitometry is a special branch of metrology, namely the study of the measurement of the properties of photographic materials. This field of measuring technique is highly distinctive. Indeed, in the present case we are dealing with the only field of photochemical reactions that has broad technical significance. In no other field of knowledge do we have such a degree of detailed development of methods for the quantitative study of the interaction of light and matter. The concepts and methods of photographic sensitometry can to a considerable extent be extended to fields of technology related to photography and, first of all, to television.

Photographic technique itself (including cinematography and aerial photography) is developing and improving rapidly, and the requirements for the quality of photographic images are continuously increasing. Obtaining high-quality photographic images in mass production, under variable conditions of photography (in field conditions, etc.), requires the organization of thorough control of all stages of the photographic process. This task is also solved by photographic sensitometry.

All of the above makes the further development of sensitometry and, in particular, critical discussion of its principal problems highly timely.

The scope of the present discussion was deliberately limited in two respects. First, consideration was given to the sensitometry only of black-and-white (i.e., the most widespread) photographic materials. The sensitometry of color photographic materials, which is considerably less developed and has many specific features, should be the subject of a special discussion. Second, within black-and-white sensitometry itself, only the following four problems were considered:

1) reproduction of detail by photographic layers,
2) sensitometric control and the problem of the criterion of photosensitivity of photographic layers,
3) densitometry (methods of measuring blackening), and
4) spectral sensitometry.

On the problem of reproduction of detail by photographic layers, the plenum of the commission considered 4 reports.

MEETINGS AND CONFERENCES

A. T. Ashcheulov reported on work carried out by him jointly with T. A. Pavliuchuk and M. D. Khukhrina on the development of projection and interference methods for determining the resolving power of photographic layers. At present, to measure this most important property of light-sensitive materials, the projection method is predominantly used: a reduced image of a test chart, obtained by means of a high-quality objective, is projected onto the layer under test, using for this purpose the well-known resolving-power tester of F. L. Burmistrov. This instrument, which in its time was a significant achievement of domestic measuring technology, at present has ceased to satisfy all requirements, since photographic materials with very high resolving power (250 mm\(^{-1}\) and more) have become widespread. In this case the quality of the optical image provided by the objective of the existing resolving-power tester proves insufficient: groups of chart lines with the highest frequencies are reproduced with considerably lower contrast than the groups with larger line widths. Thus the conditions for testing materials of different resolving power prove photometrically different: high values of resolving power are underestimated.

The authors have made a substantial step forward in the development of the projection method by using an objective practically free of aberrations, with a high aperture (relative aperture 1:1.5). Such an objective gives an optical image of the test chart even at very high frequencies with a contrast almost indistinguishable from the contrast of the chart itself. Having built a special instrument with such an objective (the instrument was demonstrated to the participants in the discussion), the authors were able to obtain, without distortions by a fall in contrast in the optical image, values of resolving power up to 350 mm\(^{-1}\). An essential feature of the new resolving-power tester is the possibility of determining resolving power over a wide range of apertures.

On the other hand, A. T. Ashcheulov carried out experiments on determining the resolving power of layers by the interference method. Using an interferometer in Lloyd’s arrangement, the speaker obtained photographs of interference fringes with frequencies up to 700 mm\(^{-1}\). However, a number of technical difficulties currently make this method less convenient than the projection method.

I. I. Breido made a parallel communication on a third possible method for determining resolving power—the contact method. This method is attractive because of its simplicity: its implementation requires only a small test chart and an elementary optical device for producing an approximately parallel illuminating beam. The values of resolving power obtained by this method are always higher than when the projection method is used, and by a greater amount the higher the resolving power. This is apparently explained by the fact that in this case there is no lowering of the contrast of the optical image of the chart imposed on the layer in comparison with the contrast of the chart itself. However, the contact method is not without significant drawbacks. Thus, at present it is inapplicable for testing layers with resolving power above 250 mm\(^{-1}\), since there are great technical difficulties in making very small charts with high line frequencies and with a sufficiently thin layer.

Yu. N. Gorokhovskii reported on a new instrument for measuring the graininess of plates—a projection granulometer. This instrument, demonstrated at the plenum, serves for the rapid determination of the graininess of developed photographic layers by the photographic projection method developed by the author. A distinctive feature of the method is that the complex phenomenon of graininess is here assessed under conditions close to those in which it is visually perceived.

...to the conditions under which this harmful effect manifests itself in static photography of broad application. The instrument is a specialized photographic enlarger with a high-quality short-focus objective, equipped with mechanisms for automatic focusing over a wide range of enlargement scales and for automatic maintenance of a constant level of illumination in the focal plane. On photographic paper of standardized contrast placed in this plane, a series of images of the blackening under investigation is obtained, at a specified image scale, in which no graininess is yet visible; these are then evaluated.

All three methodological communications were the subject of a lively discussion, from which it followed that improvement in the technique for determining resolving power and graininess is very essential for the creation of high-resolution and fine-grained photographic materials. In particular, the importance of accurately determining the resolving power of multilayer color-photographic materials was emphasized.

The communication by G. A. Istomin was devoted to the question of the relationship between the conditions for reproducing micro- and macro-details by photographic layers. Proceeding from the fact that visual perception of the details of a photographic image under otherwise equal conditions is determined by two factors—namely, the contrast of the detail relative to the background and the sharpness of the boundary between them—the author showed that increasing the degree of development of a layer does not lead to a significant improvement in the perception of details, since the growth of contrast in this case is accompanied by an expansion of the transition zone from detail to background. The best reproduction of details occurs in the middle portion of the characteristic curve of the negative, at a density of about unity; here the action of the two oppositely acting factors is balanced.

Next, the question of the rational expression of photographic latitude was considered. The author regards untenable all methods of evaluation based on a single and identical definite value of the gradient of the characteristic curve in the initial and final portions of the latter; in the final (upper) portion the visually perceived contrast of details is much lower than in the initial portion. Photographic latitude is closely connected with the nature of light scattering in the emulsion layer, i.e., with the resolving power of the latter. Experiments by the author have shown that the reproduction of large and small details obeys the same laws—there is no fundamental distinction between detailmetry and resolvometry. Therefore it may be recommended that photographic latitude be evaluated by the ratio of exposures between which the spatial frequency resolvable by the photographic layer has a prescribed or greater value.

The problem of sensitometric control and the criterion of light sensitivity of photographic layers was central in this discussion and was the subject of especially heated debate. Three communications were made here.

S. S. Gilev spoke about the experience of introducing into industry the original Soviet system of sensitometry (GOST 2817-50) and about the production-control tasks connected with it. It should be pointed out that at present the Soviet photochemical industry has completely switched over to marking photographic materials by light sensitivity in the GOST system, replacing the obsolete Hurter and Driffield system.

G. A. Istomin, in his report on the comparative evaluation of light-sensitivity criteria, criticized the criterion of light sensitivity now accepted in the GOST system (density of blackening 0.2 above fog),

as oriented toward establishing the correct exposure for the darkest details of the photographed object. Meanwhile, in the speaker’s opinion, the criterion of light sensitivity must be linked with the mean brightness of the photographed object, since only under such conditions is it possible properly to use the readings of photoelectric exposure meters, which, as is known, measure the mean brightness of the object. Proceeding from his extensive experimental experience, G. A. Istomin proposed adopting, as the basic criterion of light sensitivity for contrast negative materials, a point lying not in the initial but in the middle part of the characteristic curve, namely a density of blackening of 0.85 above fog. Acceptance of such a proposal places on a rational basis the solution of the exposure-metering problem—the problem of establishing the correct exposure in photographic shooting.

V. I. Sheberstov reported on his experiments in comparing light-sensitivity numbers determined by both of the above-mentioned criteria. Distinct regularities were established in the change of light sensitivity with increasing contrast coefficient during development, and corresponding equations for these cases were also proposed.

The report by V. Ya. Mikhailov dealt with questions of sensitometric control of photographic images under field conditions; for this purpose the author had developed and introduced into the practice of aerial-geodetic enterprises an appropriate methodology.

The communication by V. G. Pel’ was devoted to control of illumination in cinematography and to the sensitometric questions connected with it. Proceeding from the premise that the requirements for the technical quality of a motion-picture image and of a photographic image differ substantially, the speaker formulated the problem of choosing the correct exposure in cinematography as the problem of obtaining on negatives specified densities of blackening ($0.8$–$1.0$) for the details of the motion-picture image most important for the subject—the human face. This requirement is close to the requirement formulated, on the basis of other considerations, by G. A. Istomin.

Next, methods were considered for solving the exposure-metering problem in cinematography in two fundamentally different cases: under forced lighting conditions (location shooting without fill light, a number of types of newsreel shooting indoors) and under controlled lighting conditions (shooting in a film studio, location shooting with fill light, etc.). Data were presented on the total reflection coefficients of a number of reflecting surfaces specific to motion-picture technology, and exposure formulas were proposed for the case of assessing light sensitivity by density 0.2 and by the inertia point. Universal exposure meters for cinematographic purposes, developed in the speaker’s laboratory, were described and demonstrated.

In the discussion that developed around the problem of the light-sensitivity criterion, a number of comments were made both in favor of the criterion established by GOST 2817-50 (density 0.2 above fog) and in favor of the criterion lying in the middle part of the characteristic curve. However, the majority of those who spoke considered the second criterion more consistent with the requirements of practice. At the same time, attention was drawn to the insufficient validity of the proposed concrete value of the second criterion and to the need for careful study of this question.

On the problem of densitometry, one communication was discussed. V. A. Korndorf gave a very detailed report on the measurement of optical densities and scattering of light by photographic blackenings.

As is known, the optical density of blackening is by no means an unambiguous concept, since, owing to the strong scattering of the light passing through the developed layer, this quantity changes greatly when the conditions of illumination of the blackening and the conditions of reception of the transmitted beam by the radiation receiver are changed.

Using very precise means of measurement, the author of the report showed that indicatrices of scattering determined goniophotometrically make it possible to compute with sufficient reliability the magnitude of the optical density for a receiving device (photometer) with any preassigned aperture angle. A fact important for the creation of new models of densitometers was established: when the blackening being measured is illuminated by a directed beam of any aperture and when all the light emerging from the blackening is integrated, values of optical density are obtained that are practically no different from those obtained when the blackening is illuminated by diffuse light and when the receiver perceives only part of the emerging beam (within an arbitrarily limited aperture angle). Thus, the so-called integral and diffuse optical densities of blackenings are practically equal to one another.

During the discussion, a model of a photoelectric wedge densitometer manufactured by industry was demonstrated; it operates on the differential principle and is therefore free from the instability of readings often observed in photoelectric densitometers with direct readout. This instrument is equipped with a gray circular wedge, two photoelements with blocking layers—one of which is practically in tight contact with the negative being measured—and a mirror galvanometer operating as a null instrument. It makes it possible to measure, very quickly and accurately, integral densities which, as already indicated, are equal to diffuse densities.

A report on the problem of spectral sensitometry was given by Yu. N. Gorokhovskii. The report considered methodological and instrumental questions of measuring curves of absolute spectral photosensitivity of black-and-white and color photographic materials. Spectrosensitometers for the visible and infrared regions of the spectrum, on the one hand, and for the ultraviolet region of the spectrum, on the other, were demonstrated in diagrams and as an industrial prototype.

Further, on the basis of an investigation of the spectral distribution of the photosensitivity of a wide assortment of modern photographic materials, the speaker proposed a draft system for classifying photographic materials according to their spectral properties. This draft proposes abandoning the historically established nonrational terminology that has developed for commercial reasons (orthochromatic, panchromatic, etc., layers) and moving to a classification and terminology based on analysis of the physical properties of photographic materials: establishing groups of blue-sensitive, green-sensitive, red-sensitive, equal-sensitive, and infrared-sensitive layers, with subsequent subdivision into subgroups according to the position in the spectrum of the maximum of photosensitivity.

Finally, the report considered questions of how the effect of optical sensitization of photographic emulsions should be evaluated from the data of spectral sensitometry, and of what method should be used to calculate, most accurately, from spectral photosensitivity curves, the sensitivity of a photographic material to light of complex spectral composition.

In the course of discussion of this problem, the proposal for a new classification of photographic layers was supported by many speakers.

In general, the discussion took place at a high scientific level with the participation of a large number of specialists from Leningrad, Moscow, and other cities of the Union, and contributed to the clarification of many important questions of photographic sensitometry.

Yu. N. Gorokhovskii