Light meter
light meter used in photography
A light meter (or illuminometer) is a device used to measure the amount of light. In photography, an exposure meter is a light meter coupled to either a digital or analog calculator which displays the correct shutter speed and f-number for optimum exposure, given a certain lighting situation and film speed. Similarly, exposure meters are also used in the fields of cinematography and scenic design, in order to determine the optimum light level for a scene.
Light meters also are used in the general field of architectural lighting design to verify proper installation and performance of a building lighting system, and in assessing the light levels for growing plants.
If a light meter is giving its indications in luxes, it is called a "luxmeter".
Evolution
Actinometers
The earliest exposure meters were called actinometers (not to be confused with the scientific instrument with the same name), described as early as 1840 but developed in the late 1800s after commercial photographic plates became available with consistent sensitivity. These photographic actinometers used light-sensitive paper; the photographer would measure the time required for the paper to darken to a control value, providing an input to a mechanical calculation of shutter speed and aperture for a given plate number. They were popular between approximately 1890 and 1920.
Extinction types
The next exposure meters, developed at about the same time but not displacing actinometers in popularity until the 1920s and 1930s, are known as extinction meters, evaluating the correct exposure settings by variable attenuation. One type of extinction meter contained a numbered or lettered row of neutral density filters of increasing density.
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