A
light meter is a device used to measure the amount of
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
. In
photographyPhotography is the process, activity and art of creating still or moving pictures by recording radiation on a sensitive medium, such as a photographic film, or an electronic sensor...
, a light meter is often used to determine the proper
exposureIn photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph...
for a photograph. Typically a light meter will include a computer, either digital or analogue, which allows the photographer to determine which
shutter speedIn photography, shutter speed is a common term used to discuss exposure time, the effective length of time a shutter is open; the total exposure is proportional to this exposure time, or duration of light reaching the film or image sensor....
and
f-numberIn optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
should be selected for an optimum exposure, given a certain lighting situation and
film speedFilm speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. Relatively insensitive film, with a correspondingly lower speed index requires more exposure to light to produce the...
.
Light meters are also used in the fields of
cinematographyCinematography , is the making of lighting and camera choices when recording photographic images for the cinema. It is closely related to the art of still photography...
and
scenic designScenic design is the creation of theatrical, as well as film or television scenery. Scenic designers have traditionally come from a variety of artistic backgrounds, but nowadays, generally speaking, they are trained professionals, often with M.F.A...
, in order to determine the optimum light level for a scene.
A
light meter is a device used to measure the amount of
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
. In
photographyPhotography is the process, activity and art of creating still or moving pictures by recording radiation on a sensitive medium, such as a photographic film, or an electronic sensor...
, a light meter is often used to determine the proper
exposureIn photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph...
for a photograph. Typically a light meter will include a computer, either digital or analogue, which allows the photographer to determine which
shutter speedIn photography, shutter speed is a common term used to discuss exposure time, the effective length of time a shutter is open; the total exposure is proportional to this exposure time, or duration of light reaching the film or image sensor....
and
f-numberIn optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
should be selected for an optimum exposure, given a certain lighting situation and
film speedFilm speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. Relatively insensitive film, with a correspondingly lower speed index requires more exposure to light to produce the...
.
Light meters are also used in the fields of
cinematographyCinematography , is the making of lighting and camera choices when recording photographic images for the cinema. It is closely related to the art of still photography...
and
scenic designScenic design is the creation of theatrical, as well as film or television scenery. Scenic designers have traditionally come from a variety of artistic backgrounds, but nowadays, generally speaking, they are trained professionals, often with M.F.A...
, in order to determine the optimum light level for a scene. They are used in the general field of
lightingIllumination, an observable property and effect of light, may also refer to:*Illumination , the use of light sources*Illumination , the use of light and shadow in art*Illumination , the artistic decoration of hand-written texts...
, where they can help to reduce the amount of waste light used in the home,
light pollutionLight pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light. The International Dark-Sky Association , "The Light Pollution Authority," defines light pollution as: It obscures the stars in the night sky for city dwellers, interferes with...
outdoors, and plant growing to ensure proper light levels.
Use in photography
The earliest type of light meters were called
extinction meters and contained a numbered or lettered row of
neutral density filterIn photography and optics, a neutral density filter or ND filter is a "grey" filter. An ideal neutral density filter reduces light of all wavelengths or colors equally...
s of increasing density. The photographer would position the meter in front of their subject and note the filter with the greatest density that still allowed incident light to pass through. The letter or number corresponding to the filter was used as an index into a chart of appropriate aperture and shutter speed combinations for a given
film speedFilm speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. Relatively insensitive film, with a correspondingly lower speed index requires more exposure to light to produce the...
.
Extinction meters suffered from the problem that they depended on the
light sensitivity of the human eyeThe luminosity function or luminous efficiency function describes the average visual sensitivity of the human eye to light of different wavelengths. It should not be considered perfectly accurate in every case, but it is a very good representation of visual sensitivity of the human eye and it is...
(which can vary from person to person) and subjective interpretation.
Later meters removed the human element and relied on technologies incorporating
seleniumSelenium is a chemical element with the atomic number 34, represented by the chemical symbol Se, an atomic mass of 78.96. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature....
,
CdSCadmium sulfide is a chemical compound with the formula CdS. Cadmium sulfide is yellow in colour and is a semiconductor. It exists in nature as two different minerals, hexagonal greenockite and cubic hawleyite. Cadmium sulfide is a direct band gap semiconductor and has many applications for...
, and
siliconSilicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon...
photodetectorPhotosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties: *Optical detectors, which are mostly quantum devices in which an individual photon produces a discrete effect....
s.
Selenium and silicon light meters use sensors that are photovoltaic: they
generate a voltage proportional to light exposure. Selenium sensors generate enough voltage for direct connection to a meter; they need no battery to operate and this made them very convenient in completely mechanical cameras. Selenium sensors however cannot measure low light accurately (ordinary lightbulbs can take them close to their limits) and are altogether unable to measure very low light, such as candlelight, moonlight, starlight etc. Silicon sensors need an amplification circuit and require a power source such as
batteriesAn electrical battery is a combination of one or more electrochemical cells, used to convert stored chemical energy into electrical energy. Since the invention of the first Voltaic pile in 1800 by Alessandro Volta, the battery has become a common power source for many household and industrial...
to operate. CdS light meters use a sensor based on
photoresistanceA photoresistor or light dependent resistor or cadmium sulfide cell is a resistor whose resistance decreases with increasing incident light intensity. It can also be referenced as a photoconductor....
, i.e. their electrical resistance changes proportionately to light exposure. These also require a battery to operate. Most modern light meters use silicon or CdS sensors. They indicate the exposure either with a needle
galvanometerA galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil...
or on an LCD screen.
Many modern consumer still and video
camerathumb |right|Cameras from Large to Small, Film to Digital A camera is a device that records images, either as a still photograph or as moving images known as videos or movies...
s 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. Photographers working with controlled lighting and
cinematographerA cinematographer is one photographing with a motion picture camera . The title is generally equivalent to director of photography , used to designate a chief over the camera and lighting crews working on a film, responsible for achieving artistic and technical decisions related to the image...
s use handheld 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 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. Badly underexposed sunset photos are common exactly because of this effect: the brightness of setting sun fools the camera's light meter and, unless the in-camera logic or the photographer take care to compensate, the picture will be grossly underexposed and dull.
This pitfall is avoided by
incident-light meters which measure the amount of light
falling on the subject using an
integrating sphereAn Integrating sphere is an optical component consisting of a hollow cavity with its interior coated for high diffuse reflectivity , having relatively small holes as needed for entrance and exit ports....
(usually, a translucent hemispherical plastic dome is used to approximate this) placed on top of the light sensor. Because the incident-light reading is independent of the subject's reflectance, it is less likely to lead to incorrect exposures for subjects with unusual average reflectance. Taking an incident-light reading requires placing the meter at the subject's position and pointing it in the general direction of the camera, something not always achievable in practice, e.g., in landscape photography where the subject is at infinity.
Another way to avoid under- or over-exposure for subjects with unusual reflectance is to use a
spot meter: a reflected-light meter that measures light in a very tight cone, typically with a one degree angle. An experienced photographer can take multiple readings over the shadows, midrange and highlights of the scene to determine optimal exposure, using systems like the
Zone SystemThe Zone System is a photographic technique for determining optimal film exposure and development, formulated by Ansel Adams and Fred Archer in 1941. The Zone System provides photographers with a systematic method of precisely defining the relationship between the way they visualize the...
. Many modern cameras include sophisticated multi-segment metering systems that measure the
luminanceLuminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
of different parts of the scene to determine the optimal exposure. When using a film whose spectral sensitivity is not a good match to that of the light meter, for example
orthochromaticOrthochromatic refers to any spectrum of light that is devoid of red light.- Orthochromatic photography :Orthochromatic photography refers to a photographic emulsion that is sensitive to only blue and green light, and thus can be processed with a red safelight. Using it, blue objects appear lighter...
black-and-white or infrared film, the meter may require special filters and re-calibration to match the sensitivity of the film.
There are other types of specialized photographic light meters. Flash meters are used in flash photography to verify correct exposure. Color meters are used where high fidelity in color reproduction is required.
DensitometerA densitometer is a device that measures the degree of darkness of a photographic or semitransparent material or of a reflecting surface. The densitometer is basically a light source aimed at a photoelectric cell. It determines the density of a sample placed between the light source and the...
s are used in photographic reproduction.
Exposure meter calibration
In most cases, an incident-light meter will cause a medium tone to be
recorded as a medium tone, and a reflected-light meter will cause
whatever is metered to be recorded as a medium tone. What
constitutes a “medium tone” depends on meter calibration and
several other factors, including film processing or digital image conversion.
Meter calibration establishes the relationship between subject lighting and
recommended camera settings. The calibration of photographic light meters is
covered by ISO 2720:1974.
Exposure equations
For reflected-light meters, camera settings are related to ISO speed and
subject luminance by the reflected-light exposure equation:
where
- is the relative aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
(f-numberIn optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
)
- is the exposure time (“shutter speed
In photography, shutter speed is a common term used to discuss exposure time, the effective length of time a shutter is open; the total exposure is proportional to this exposure time, or duration of light reaching the film or image sensor....
”) in seconds
- is the average scene luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
- is the ISO arithmetic speed
Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. Relatively insensitive film, with a correspondingly lower speed index requires more exposure to light to produce the...
- is the reflected-light meter calibration constant
For incident-light meters, camera settings are related to ISO speed and
subject illuminance by the incident-light exposure equation:
where
- is the illuminance
In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per...
- is the incident-light meter calibration constant