Light Meters

A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph. more...

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Typically a light meter will include a computer, either digital or analogue, which allows the photographer to determine which shutter speed and f-number should be selected for an optimum exposure, given a certain lighting situation and film speed.

Light meters are also used in the fields of cinematography and scenic design, in order to determine the optimum light level for a scene. They are also used in the general field of lighting, where they can help to reduce the amount of waste light used in the home, and also light pollution outdoors.

Use in Photography

The earliest type of light meters were called extinction meters. They contained a numbered or lettered row of neutral density filters of increasing density. The photographer would position the meter in front of their subject and note which filter still allowed incident light to pass through. The letter or number corresponding to the filter was consulted on a chart which yielded a list of appropriate aperture and shutter speed combinations.

Extinction meters suffered from the problem that they depended on the light sensitivity of the human eye which varies from person to person.

Later meters removed the human element and relied on technologies incorporating selenium, CdS, and silicon photosensors.

Selenium and silicon light meters use sensors that are photovoltaic. These sensors generate a voltage proportional to light exposure. Selenium sensors generate enough voltage for direct connection to a meter. Silicon sensors need an amplification circuit and require a power source such as batteries to operate. CdS light meters use a sensor based on photoresistance. These also require a battery to operate. Most modern light meters use silicon or CdS sensors. They indicate the exposure either with a needle galvanometer, or on a LCD screen.

Many modern consumer still and video cameras include a built-in meter that measures a scene-wide light level and are able to make an approximate measure of appropriate exposure based on that. Professional photographers and motion picture cinematographers generally use handheld ambient-light meters to precisely measure the light falling on various parts of their subjects, and use suitable lighting to produce the desired exposure levels.

There are two general types of ambient-light meters: reflected-light and incident-light. Reflected-light meters measure the light reflected by the scene to be photographed. All in-camera meters are reflected-light meters. Reflected-light meters are calibrated to show the appropriate exposure for “average” scenes. An unusual scene with a preponderance of light colors or specular highlights would have a higher reflectance; a reflected-light meter taking a reading would incorrectly compensate for the difference in reflectance and lead to underexposure.

Read more at Wikipedia.org