Concepts, Terminology, and Notation for Optical Modulation

In the early 1960s, the National Bureau of Standards aided the photographic and printing industries by conducting research on precise measurement of optical transmission and reflection density, providing physical standards to calibrate instruments, and writing national and international documentary standards on optical density measurement. Optical density is a logarithmic measure of the darkness of a photograph or printed image. Although the science was hardly new, it suffered from loose concepts and imprecise terminology. The same problems were encountered in photometry, radiometry, colorimetry, and spectrometry. This publication [1] represented a major step forward in removing the confusion and promoting the use of precise concepts in optical measurements. Physics textbooks defined “reflectance” as the ratio of the amount of light reflected from a surface to the amount of incident light. Workers who said they were measuring reflectance actually measured the ratio of the amount of reflected light to the amount of light reflected from a standard diffuse white surface. They generally regarded the measurement of the light reflected from the white surface as merely a convenient way of measuring the incident light. There was no generally accepted terminology to distinguish these two kinds of measurements. The concepts thus were blurred. The degree of confusion may be illustrated by an example. Light incident on white paper is diffusely reflected in all directions, only a minute fraction being reflected to the pupil of the eye. The ratio of the amount reflected in that direction to the amount incident is a very small number, perhaps under 0.001. On the other hand, the ratio of the amount reflected in that direction to the amount reflected in the same direction from a standard diffuse white surface may be as much as 0.9. These are two different concepts—two different physical quantities, with vastly different numerical values. Such considerations led to a thorough analysis of the basic concepts and terminology in this field. The term “reflectance” was retained for the concept defined in textbooks. The measurement relative to a white standard was called “reflectance factor.” That term is now used internationally in photography, printing, and color science. Considerations of transmission measurements had even more important consequences. The textbook definition of “transmittance” is the ratio of the amount of transmitted light to the amount of incident light. The most important applications of transmission are motionpicture projection, slide projection, projection printing (enlarging), and the viewing of x rays and other transparencies on viewing boxes. A projector forms an image on a screen because the film absorbs or scatters some light, so the illumination at each point is some fraction of what it would be without the film. It might appear that one would need no more than the concept “transmittance” to quantify this process. However, some projectors were made to view microfilm images that scattered light. The light source was moved to one side, so the light beam passed through the film gate but was not directed toward the projection lens. With no film in the gate, the screen was dark. When film was inserted, some light was scattered in the direction of the projection lens and the screen became brighter. In Concepts, Terminology, and Notation McCamy defined “transmittance factor” as the ratio of the screen luminance with the film in place to the screen luminance without film. The concept called “transmittance factor” had not been previously differentiated from “transmittance.” In the projector just described, the transmittance factor would be much greater than one. By definition, transmittance cannot be greater than one. “Transmittance” and “transmittance factor” are different concepts—two different physical quantities, with different numerical values. There was no generally accepted collective term for ratios such as those describing reflection, transmission, or some combination of them, so the general term “modulation” was introduced, based on the idea that objects modulate the flow of light. The combination of light source and optics directing light to a specimen was called an “illuminator” or “irradiator” and the optical system collecting and evaluating the light reflected or transmitted in a specified direction was called a “receiver.” Light flowing from an illuminator to a specimen was called “influx” and that evaluated by the receiver was called “efflux.” A guiding principle was that the physical quantity measured was a function of the