A light meter, also commonly referred to as an exposure meter, is a device containing a light sensitive sensor that measures the intensity of light in a scene and uses that measurement to calculate and indicate the correct combination of aperture, shutter speed, and ISO settings required to produce a correctly exposed photographic image. It is one of the most fundamental and important tools in photography, providing the objective measurement of light levels that allows the photographer to set their camera with confidence rather than relying on estimation or trial and error.
Light meters operate on two principal measuring methods, each suited to different photographic situations and approaches. Reflected light metering measures the light reflected from the subject back towards the camera and meter, and is the method used by the through the lens metering systems built into virtually all modern cameras. The meter reads the brightness of the scene as seen from the camera position and calculates an exposure based on the assumption that the average tonal value of the scene corresponds to a mid-grey tone of approximately eighteen percent reflectance - a standard known as the eighteen percent grey card standard. While this assumption produces accurate results for scenes of average tonal distribution, it can lead to exposure errors with subjects that are predominantly very light or very dark, as the meter attempts to render them as mid-grey rather than as the light or dark tones they actually are.
Incident light metering measures the light falling on the subject rather than reflected from it, using a translucent white dome or flat diffusing disc placed over the meter's sensor to integrate light arriving from a wide range of angles. Because incident metering measures the illumination at the subject position independently of the subject's own tonal values and reflectance characteristics, it is inherently unaffected by the tonal distribution of the scene and produces consistent, accurate exposure readings regardless of whether the subject is light, dark, or of average tone. Incident metering is the preferred method of many professional photographers, particularly in studio and controlled lighting situations where accurate and consistent exposure across multiple setups is essential.
Light meters are available in several physical forms to suit different working methods and applications. Built in through the lens meters are incorporated into virtually all modern film and digital cameras, providing instant, automatic, or semi-automatic exposure calculation without the need for a separate handheld device. Handheld exposure meters offer greater versatility and measurement sophistication than built in camera meters, typically supporting both reflected and incident measurement modes, spot metering capabilities for measuring small areas of a scene with great precision, and flash metering functions for measuring the output of studio electronic flash equipment - a measurement that built in camera meters cannot perform as the flash duration is too brief for the camera's metering sensor to respond to before the shutter closes.
Spot meters are a specialised category of reflected light meter that measure the light reflected from an extremely small and precisely defined area of the scene - typically a one or two degree angle of view - allowing the photographer to measure and compare the brightness of specific highlight and shadow areas individually and calculate an exposure that places them correctly within the tonal range of the film or sensor. Spot metering is an essential tool for practitioners of the Zone System and other systematic exposure approaches that require precise knowledge of the luminance values of specific areas within a scene.