ABSTRACT
A light meter or photometer is a hand-held instrument using a photodetector which is a non-imaging photocell. Determination of film camera exposure is by means of light measurements or photometry related to the subject, using a hand-held or in-camera opto-electronic light metering system, commonly referred to as an exposure meter. The techniques of using a light meter or metering system have been detailed by Dunn and Wakefield. Non- single-lens reflex cameras may have the metering system separately attached or located externally to read directly from the subject. The superiority and convenience of in-camera metering using through the lens measurements of scene luminance are well established. Contemporary cameras may use one or two separate photocell locations to provide different alternative forms of metering. A perforated main mirror may be preferred, since the polarized state of transmitted light is unaffected; otherwise, a circular polarizing filter is necessary over the camera lens.
