The first camera to feature automatic exposure was the selenium light meter-equipped, fully automatic Super Kodak Six-20 of 1938, but its extremely high price (for the time) of $225 (3715 in present terms[11]) kept it from achieving any degree of success. By the 1960s, however, low-cost electronic components were commonplace and cameras equipped with light meters and automatic exposure systems became increasingly widespread. The next technological advance came in 1960, when the German Mec 16 SB subminiature became the first camera to place the light meter behind the lens for more accurate metering. However, through-the-lens metering ultimately became a feature more commonly found on SLRs than other types of camera; the first SLR equipped with a TTL system was the Topcon RE Super of 1962. 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. Typically a light meter will include a computer, either digital or analog, 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 used in the general field of lighting, where they can help to reduce the amount of waste light used in the home, light pollution outdoors, and plant growing to ensure proper light levels. In photography, exposure is the amount of light allowed to fall on each area unit of a photographic medium (photographic film or image sensor) during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance in a specified region. In photographic jargon, an exposure generally refers to a single shutter cycle. For example: a long exposure refers to a single, prot acted shutter cycle to capture enough low-intensity light, whereas a multiple exposure involves a series of relatively brief shutter cycles; effectively layering a series of photographs in one image. For the same film speed, the accumulated photometric exposure (Hv) should be similar in both cases. Through-the-lens (TTL) metering is a photographic term describing a feature of cameras capable of measuring light levels in a scene through their taking lenses, as opposed to a separate metering window. This information can then be used to select a proper exposure (average luminance), and control the amount of light emitted by a flash connected to the camera.Through-the-lens metering is most often associated with single-lens reflex (SLR) cameras. In most film and digital SLRs, the light sensor(s) for exposure metering are incorporated into the pentaprism or pentamirror, the mechanism by which a SLR allows the viewfinder to see directly through the lens. As the mirror is flipped up, no light can reach there during exposure, therefore, these light sensors can be used for ambient light TTL metering only. In newer SLRs as well as in almost all DSLRs, they can also be utilized for preflash TTL metering, where the metering is carried out before the mirror flips up using a preflash and the necessary amount of flash light is precalculated and then applied during the exposure without any real-time feedback. There were a few particularly sophisticated film SLRs including the Olympus OM-2, the Pentax LX, the Nikon F3, and the Minolta 9000, where metering cells located at the bottom of the mirror box were used for ambient light metering, depending on model either instead or in addition to metering cells in the roof of the camera. Depending on model, the light was reflected down there either by a secondary mirror behind the half-transparent main mirror, a special reflective coating of the first shutter curtain, the surface of the film itself, or combinations thereof.