A
light meter is a device used to measure the amount of
lightLight or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
. In
photographyPhotography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
, 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. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance over a specified area.In photographic jargon, an exposure...
for a photograph. Typically a light meter will include a computer, either
digitalA digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
or
analogAn analog computer is a form of computer that uses the continuously-changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved...
, 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 camera's shutter is open....
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....
.
A
light meter is a device used to measure the amount of
lightLight or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
. In
photographyPhotography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
, 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. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance over a specified area.In photographic jargon, an exposure...
for a photograph. Typically a light meter will include a computer, either
digitalA digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
or
analogAn analog computer is a form of computer that uses the continuously-changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved...
, 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 camera's shutter is open....
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....
.
Light meters are also used in the fields of
cinematographyCinematography is the making of lighting and camera choices when recording photographic images for 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 lighting, 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 defines light pollution as:...
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 can be a colorless or grey filter. An ideal neutral density filter reduces and/or modifies intensity of all wavelengths or colors of light equally, giving no changes in hue of color rendition.The purpose of standard photographic...
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....
.
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 atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
,
CdSCadmium sulfide is the inorganic compound with the formula CdS. Cadmium sulfide is a yellow solid. It occurs in nature with two different crystal structures as the rare minerals greenockite and hawleyite, but is more prevalent as an impurity substituent in the similarly structured zinc ores...
, and
siliconSilicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
photodetectorPhotosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:*Active pixel sensors are image sensors consisting of an integrated circuit that contains an array of pixel sensors, each pixel containing a both a light sensor and an active amplifier...
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 one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
to operate. CdS light meters use a sensor based on
photoresistanceA photoresistor or light dependent resistor is a resistor whose resistance decreases with increasing incident light intensity. It can also be referred to as a photoconductor or CdS device, from "cadmium sulfide," which is the material from which the device is made and that actually exhibits the...
, 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 in a magnetic field. .Galvanometers were...
or on an LCD screen.
Many modern consumer still and video
cameraA camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
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 distance approaches 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 circular angle of view. 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. Adams described how the Zone System was developed: "I take this opportunity to restate that the Zone System is not an invention of mine; it is a codification...
. 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
orthochromatic- 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. The increased blue sensitivity causes blue objects to appear lighter and red ones darker...
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 apertureIn 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 speedIn photography, shutter speed is a common term used to discuss exposure time, the effective length of time a camera's shutter is open....
”) in seconds
-
is the average scene 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...
-
is the ISO arithmetic 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....
-
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 illuminanceIn 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