A light meter is in photography or film photography a photometer, which is either a stand-alone device ( light meter ) or part of a camera ( built-in or internal exposure meter ). With the exposure meter the brightness of the subject is measured and the data for the exposure as the aperture and exposure time is calculated or determined. There is the measure of the incident light flux Φ per unit area in unit Lux (formerly in phot ) on.
Getting methods, the correct exposure of the photographic material by measuring determine originated from the 19th century and used photochemical processes in which the darkness of a treated with a photosensitive substance material with the eye observed and compared for instance with a comparison scale.
Early photographers are supposed to have used the pupil of a cat for exposure measurement.
As optical light meter first stages photometer as the " Lucimeter " also came in the 19th century on the market. For these, by means of stepped superposed papers or glass plates according to the number derselbigen sought where no light penetrated the equipment; from the number could be on the amount of incident light and thus the necessary exposure time conclusions are drawn.
Modern light meter used for measuring light a light sensor. It can be used photoresistors and photodiodes. The light sensor must have a spectral sensitivity that needs to be adapted to the luminosity curve of the eye or of the film.
Older light meter worked with a selenium cell, a photocell or a photodiode based on polycrystalline selenium. The selenium cell fed directly a moving coil instrument; therefore such exposure meter no battery required. Since the photocurrent of the selenium cell as well as other photodiode is linear with the illumination, this light meter often had a range switching ( electrical, filters or pinhole ) to cover all brightnesses, or an adjustment wheel to set off the film speed, shutter speed and aperture. The first stocked with a selenium cell electric meter was brought from the Weston- company in the USA in 1932 on the market; a first German unit also arose in 1932 a collaboration of Dr. Bruno Lange with the company P. Gossen in Erlangen, this was available in spring 1933.
Newer appliances and light meters in cameras often work with a cadmium sulfide ( CdS) photoresistor. The resistance profile of photoresistors is highly non- linear, so that often no range switching is necessary.
Both selenium cells and CdS photo resistors have approximately a spectral sensitivity as the eye and therefore do not require a color filter. One silicon photo diode, however, must be set before a daylight filter, in order to adapt to the spectral sensitivity or to compensate for their higher sensitivity in the red and infrared region.
Lightning exposure meters do not measure the illuminance, but the amount of light that hits them, forming the integral of the light intensity during the time of the flash. Therefore, you need to synchronize the flash: either sync them yourself at the beginning of the flash, or they have a sync terminal to trigger the flash through a cable.
SLR cameras with TTL metering, have a photo receiver in the beam path behind the bezel. You can measure the illumination therefore objectively independent. They have a corrected with the exposure time and the film sensitivity center indicator with LEDs or a superimposed in the viewfinder image pointer.
Automatic cameras can control the flash power during recording based on the amount of light reflected from the object, moreover, often. You finish the flash when enough light has been thrown back.
CCD and CMOS cameras do not require a separate light meter, as they can determine the illuminance based on their image sensors.
Integrated or Internal exposure meter
A light meter is now part of almost all cameras. He is primarily responsible for the correct exposure of a picture, because it calculates the right balance of shutter speed and aperture ( depending on the sensitivity of the film ). Calculated exposure time and / or aperture are then adjusted manually, either from the camera by itself ( automatic) or by photographer (by means of Metered or deducting EV / EV / aperture / time values).
The internal exposure a ( SLR ) camera through the lens is referred to according to the English term "Through The Lens" as TTL metering.
Viewfinder cameras often have a metering, which is built next to the lens or the camera body. For close-ups, this arrangement can lead you through the parallax to incorrect measurements. Single camera designs also make use of the light reflected from the film during the exposure light for exposure control.
As a separate accessory hand exposure meter, especially in photo studios or professional work is used. Here, the integrated light meter is often overwhelmed, mostly because to use multiple sources of artificial light.
When using daughter flash units, the integrated exposure meter of a camera with built in flash can not control the exact amount of light, as the light of the other flash sources only be added when the flash fires, resulting in overexposure. In these situations can measure the exact amount of light only, a separate light meter. The data derived from them may then be set on the camera so that a correct exposure will be obtained.
The measurement takes place either from the camera position or place directly on the object or a gray card. Special spot light meter used to measure individual image sections.
With the aid of a special filter ( cap ), instead of the reflected light at many separate exposure diameters and the light to be measured which is supplied by the light source. The filter is pushed or folded either in the sensor.