Sunlight and Skylight as Determinants of Photographic Exposure II Scene Structure, Directional Index, Photographic Efficiency of Daylight, Safety Factors, and Evaluation of Camera Exposure (1949)

Following a brief review of Part I of this communication, additional factors involved in the computation of correct camera exposure are discussed. The first of these is the structure of the scene. Two aspects of structure, namely, spatial structure and local illumination conditions, are recognized. Four categories of spatial structure, definable, at least in a qualitative manner, in terms of distance between scene elements and camera, are described and illustrated pictorially. Local illumination conditions are defined in terms of the extent of shading and four definitely recognizable aspects are described.The minimum scene luminance, Bo min., is determined to a considerable extent by aspects of scene structure, and quantitative values corresponding to combinations of structural elements are given.When an appreciable part of the light entering the scene space comes from a source which subtends a small solid angle, the minimum scene luminance is dependent to some extent upon the direction, relative to the optical axis of the camera system, from which this unidirectional illumination enters the scene space. The angular relationships between this component of the incident light and the optical axis of the camera which results in conditions defined as front lighting, side and top lighting, and back lighting are defined and relative values of Bo min. are given corresponding to these three directional lighting conditions.The speed of photographic negative materials is defined in terms of radiant energy having a specified spectral composition which has been standardized by international agreement. The spectral composition of the radiant energy which, on the average, is reflected from the scene elements of minimum luminance is not identical to that used in the measurement of speed. It is therefore necessary to include a term in the formula for the computation of correct camera exposure, the magnitude of which depends upon the difference in the spectral qualities of the two radiant energies. This factor is expressed in terms of the photographic efficiency of the radiant energy reflected on the average from the scene elements of minimum luminance. This efficiency can be computed, if certain functional relations are known, or it may be measured by methods of photographic photometry. Evidence shows that the values obtained by the two methods are in fairly good agreement. However, it seems advisable to use a value derived by the direct methods of photographic photometry. In this connection the spectral composition of radiant energy in sunlight and skylight is considered in detail. The use of the term color temperature as a significant specification for the spectral composition of radiant energy emitted by natural and artificial sources is examined critically and the conclusion is reached that in the majority of cases a color temperature specification is not adequate for defining the quality, i.e., spectral composition, from the standpoint of photographic efficiency.As a final step in the derivation of the formula for correct camera exposure, certain safety factors are considered and recommended. Up to this point the treatment has been analytical, involving the consideration of many factors. To make the information of practical use, the treatment must be integrated and simplified in such a way that values of correct camera exposure can be determined simply and expeditiously. Three possible methods are discussed for this purpose: a purely arithmetic, a purely logarithmic, and a combination of arithmetic, logarithmic, and simple slide rule operation. The last of these possible techniques turns out to be the simplest and probably the most easily applied in practical work.