A light meter (or illuminometer) is a device used to measure the amount of light. In photography, an exposure meter is a light meter coupled to either a digital or analog calculator which displays the correct shutter speed and f-number for optimum exposure, given a certain lighting situation and film speed. Similarly, exposure meters are also used in the fields of cinematography and scenic design, in order to determine the optimum light level for a scene.
Light meters also are used in the general field of architectural lighting design to verify proper installation and performance of a building lighting system, and in assessing the light levels for growing plants.
If a light meter is giving its indications in luxes, it is called a "luxmeter".Merriam-Webster Dictionary - luxmeter
Evolution
Actinometers
The earliest exposure meters were called actinometers (not to be confused with the scientific instrument with the same name), described as early as 1840 but developed in the late 1800s after commercial photographic plates became available with consistent sensitivity. These photographic actinometers used light-sensitive paper; the photographer would measure the time required for the paper to darken to a control value, providing an input to a mechanical calculation of shutter speed and aperture for a given plate number. They were popular between approximately 1890 and 1920.
Extinction types
The next exposure meters, developed at about the same time but not displacing actinometers in popularity until the 1920s and 1930s, are known as extinction meters, evaluating the correct exposure settings by variable attenuation. One type of extinction meter contained a numbered or lettered row of neutral density filters 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. In another example, sold as Heyde's Aktino-Photometer starting from the early 1900s, the photographer views the scene through an eyepiece and turns the meter to vary the effective density until the scene can no longer be seen. The letter or number corresponding to the filter strength causing the "extinction" of the scene was used as an index into a chart of appropriate aperture and shutter speed combinations for a given film speed.
Extinction meters tended to provide inconsistent results because they depended on subjective interpretation and the light sensitivity of the human eye, which can vary from person to person.
Photoelectric types
Starting in 1932, electronic light meters removed the human element and relied on technologies incorporating (in chronological order) selenium, CdS (1960s), and silicon (semiconductor, 1970s) photodetectors. Most modern light meters use silicon sensors. They indicate the exposure either with a needle galvanometer or on an LCD screen.
Selenium 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.
CdS light meters use a photoresistor sensor whose electrical resistance decreases proportionately to the intensity of light exposure. These require a battery to operate, but are significantly more sensitive to low light, able to detect lighting levels approximately of the lower sensitivity limit of selenium cells. However, CdS sensors fell out of favor due to their slower response and extended sensitivity to red and infrared wavelengths.
Semiconductor sensors are also photovoltaic, but the voltage generated is much weaker than selenium cells and semiconductor-based light meters need an amplification circuit and therefore require a power source such as batteries to operate. These are usually named after the materials and filtration used to ensure the spectral response is similar to the human eye or photographic film, such as 'Silicon Blue Cell' (SBC) or ''.
Many modern consumer still and video cameras 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 cinematographers 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.
Reflected and incident measurements Exposure meters generally are sorted into reflected-light or incident-light types, depending on the method used to measure the scene.
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 the 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 (but not in the setting-sun case) is avoided by incident-light meters which measure the amount of light falling on the subject using a diffuser with a flat or (more commonly) hemispherical field of view 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 specialized 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 System.
Many modern cameras include sophisticated multi-segment metering systems that measure the luminance 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 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. Densitometers 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 (f-number) is the exposure time ("shutter speed") in seconds is the average scene luminance is the ISO arithmetic speed 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 is the incident-light meter calibration constant
Calibration constants
Determination of calibration constants has been largely subjective; ISO 2720:1974 states that
The constants and shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a large number of observers, of a number of photographs, for which the exposure was known, obtained under various conditions of subject manner and over a range of luminances.
In practice, the variation of the calibration constants among manufacturers is considerably less than this statement might imply, and values have changed little since the early 1970s.
ISO 2720:1974 recommends a range for of 10.6 to 13.4 with luminance in cd/m2. Two values for are in common use: 12.5 (Canon, Nikon, and SekonicSpecifications for Sekonic light meters are available on the Sekonic web site under "Products.") and 14 (Minolta, Kenko,Konica Minolta Photo Imaging, Inc. left the camera business on March 31, 2006. Rights and tooling for Minolta exposure meters were acquired by Kenko Co, Ltd. in 2007. Specifications for the Kenko meters are essentially the same as for the equivalent Minolta meters. and Pentax); the difference between the two values is approximately EV.
The earliest calibration standards were developed for use with wide-angle averaging reflected-light meters (Jones and Condit 1941). Although wide-angle average metering has largely given way to other metering sensitivity patterns (e.g., spot, center-weighted, and multi-segment), the values for determined for wide-angle averaging meters have remained.
The incident-light calibration constant depends on the type of light receptor. Two receptor types are common: flat (cosine-responding) and hemispherical (cardioid-responding). With a flat receptor, ISO 2720:1974 recommends a range for of 240 to 400 with illuminance in lux; a value of 250 is commonly used. A flat receptor typically is used for measurement of lighting ratios, for measurement of illuminance, and occasionally, for determining exposure for a flat subject.
For determining practical photographic exposure, a hemispherical receptor has proven more effective. Don Norwood, inventor of incident-light exposure meter with a hemispherical receptor, thought that a sphere was a reasonable representation of a photographic subject. According to his patent (Norwood 1938), the objective was
to provide an exposure meter which is substantially uniformly responsive to light incident upon the photographic subject from practically all directions which would result in the reflection of light to the camera or other photographic register.
and the meter provided for "measurement of the effective illumination obtaining at the position of the subject."
With a hemispherical receptor, ISO 2720:1974 recommends a range for of 320 to 540 with illuminance in lux; in practice, values typically are between 320 (Minolta) and 340 (Sekonic). The relative responses of flat and hemispherical receptors depend upon the number and type of light sources; when each receptor is pointed at a small light source, a hemispherical receptor with = 330 will indicate an exposure approximately 0.40 step greater than that indicated by a flat receptor with = 250. With a slightly revised definition of illuminance, measurements with a hemispherical receptor indicate "effective scene illuminance."
Calibrated reflectance