Infallible light meter Wynne. It is a small box-shaped pocket watch, in which a paper disc is placed sensitive to light. A small part of the paper is exposed in a window; paper darkens with exposure. A small area of a fixed shade (in some models, two different shadows) is provided by the window for comparison. To take a reading, the user first turns the paper disc to expose a fresh area and measures the time it takes to darken the paper to match the fixed hue. This time, along with the speed of the film or plate and opening the desired lens is used to determine the exposure time to the scale of the meter. No timer in the meter; the user uses a separate clock, a pendulum or a simple counting to measure time reading. Because sensitive paper was not perfectly gray meter could be used with a tinted filter, designed to prevent color differences interfere with the comparison tone. The Infalible meters are very similar to those meters Watkins Bee and precede them [1] the like meters Watkins, were available with scales Autochrome and other color plates, and for different scales opening EE .S. which is then used.