Editing Light meter (section)

== Evolution ==
<gallery mode="packed" heights="120px" widths="160px" caption="Photographic exposure meters">
File:Watkins Bee Meter.jpg|Watkins Bee Meter - a type of actinometer
File:Leudi extinction meter.jpg|Leudi extinction meter<ref>{{cite web |url=http://www.earlyphotography.co.uk/site/entry_E20.html |title=Leudi, 1934 |website=Early Photography |access-date=8 September 2023}}</ref>
File:METROPHOT.jpg|Metrophot
Norwood Director IMG20240628134530.jpg|Norwood Director <br />Model No. 56646
Norwood Director Pat56646 IMG20240628134530.jpg|Norwood Director<br /> Pat. 2,214,238
File:SEKONIC TWINMATE L208.jpg|Sekonic "Twinmate" L208
File:KODALUX.jpg|Kodalux
File:TESSINA METER.jpg|Tessina Meter
File:Voigtlander VC METER.jpg|Voigtlander VC Meter
File:LEICA IIIC WITH VOIGTLANDER VC METER II.jpg|Leica IIIC with Voigtlander VC Meter II
File:Sekonic L-358 Flash Master.jpg|alt=A handheld digital light meter showing an exposure of {{frac|200}}th at an aperture of {{f/|11}}, at ISO 100. The light sensor is on top, under the white diffusing hemisphere|Sekonic L-358 "Flash Master"
</gallery>

===Actinometers===
[[File:Watkins Standard Exposure Meter.jpg|thumb|Watkins Standard Exposure Meter - a type of actinometer]]
The earliest exposure meters were called ''actinometers'' (not to be confused with the [[Actinometer|scientific instrument with the same name]]), described as early as 1840<ref name=Clerc70>{{cite book |url=https://archive.org/details/photographytheor0000cler |title=Photography: Theory and Practice |last1=Clerc |first1=Louis-Philippe |last2=Mannheim |first2=L.A. |editor1-last=Spencer |editor1-first=D.A. |volume=3: Films {{!}} Subject and Exposure |date=1970 |publisher=Focal Press |edition=Completely revised |isbn=0-240-50706-1 |url-access=registration}}</ref>{{rp|415}} but developed in the late 1800s after commercial [[photographic plate]]s 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.<ref name=Secret-Exposure/>{{rp|69}} They were popular between approximately 1890 and 1920.<ref name=EP-meters/>

===Extinction types===
[[File:Dremo Light Meter (1931) (15494003563).jpg|thumb|right|upright|Dremo extinction meter<ref>{{cite web |url=http://www.earlyphotography.co.uk/site/entry_E277.html |title=Dremo, 1931 |website=Early Photography |access-date=8 September 2023}}</ref>]]
The next exposure meters, developed at about the same time<ref name=Clerc70/>{{rp|415}} but not displacing actinometers in popularity until the 1920s and 1930s, are known as ''[[Complex index of refraction|extinction]] meters'', evaluating the correct exposure settings by variable attenuation.<ref name=EP-meters>{{cite web |url=http://www.earlyphotography.co.uk/site/meters.html |title=Exposure Meters |website=Early Photography |access-date=8 September 2023}}</ref> One type of extinction meter contained a numbered or lettered row of [[neutral density filter]]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. 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.<ref>{{cite web |url=http://www.earlyphotography.co.uk/site/entry_E3.html |title=Heyde Aktino-Photometer, 1904 |website=Early Photography |access-date=8 September 2023}}</ref> 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]].<ref name=Secret-Exposure>{{cite book |url=https://archive.org/details/secretofexposure00fraprich/page/72/mode/2up |title=The Secret of Exposure |series=Practical Photography |volume=1 |editor=Fraprie, Frank R. |date=1915 |publisher=American Photographic Publishing Company |location=Boston, Massachusetts |quote=In [the Heyde] instrument, blue-glass prisms are used to cut out the light reflected from the object. One looks through the eyepiece and turns the thicker portion of the prisms (one or both, according to the luminosity of the object) into position until the shadow details are suppressed. By reference to Tables, the necessary exposure may readily be found. |access-date=8 September 2023}}</ref>{{rp|72}}

Extinction meters tended to provide inconsistent results because they depended on subjective interpretation and the [[Luminosity function|light sensitivity of the human eye]], which can vary from person to person.<ref>{{cite book |url=https://archive.org/details/exposuremanual0000dunn/ |title=Exposure Manual |chapter-url=https://archive.org/details/exposuremanual0000dunn/page/82/mode/2up |chapter=3: Extinction Meters |author1=Dunn, Jack F. |author2=Wakefield, George L. |edition=Third |date=1974 |publisher=Fountain Press |location=Hertfordshire, England |isbn=0-85242-361-6 |pages=82–86 |url-access=registration |access-date=8 September 2023}}</ref>

===Photoelectric types===
[[File:Gossen Lunasix 3 front.jpg|thumb|upright|Gossen Lunasix 3 (in US: Luna Pro S) using {{chem|Cd|S|link=Cadmium sulfide}} photoresistor with analog readout; available from 1961 to 1977]]
Starting in 1932,<ref>{{cite book |url=https://archive.org/details/guideperfectexpo0000unse/page/20/mode/2up |title=Guide to Perfect Exposure |first1=George B. |last1=Wright |first2=Cora |last2=Wright |edition=Fourth |publisher=Chilton Book Company |location=Philadelphia |date=1967 |url-access=registration |lccn=67-17795}}</ref>{{rp|20}} electronic light meters removed the human element and relied on technologies incorporating (in chronological order) [[selenium]], [[Cadmium sulfide|CdS]] (1960s), and silicon ([[semiconductor]], 1970s) [[photodetector]]s.<ref name=Dunn-Wakefield-ch4>{{cite book |url=https://archive.org/details/exposuremanual0000dunn/ |title=Exposure Manual |chapter-url=https://archive.org/details/exposuremanual0000dunn/page/86/mode/2up |chapter=4: Photoelectric Reflected-light Meters |author1=Dunn, Jack F. |author2=Wakefield, George L. |edition=Third |date=1974 |publisher=Fountain Press |location=Hertfordshire, England |isbn=0-85242-361-6 |pages=87–121 |url-access=registration |access-date=8 September 2023}}</ref> Most modern light meters use silicon sensors.<ref name=Hicks99>{{cite book |url=https://archive.org/details/perfectexposuref0000hick/ |chapter-url=https://archive.org/details/perfectexposuref0000hick/page/88/mode/2up |title=Perfect exposure: from theory to practice |chapter=7: Exposure Meters |last1=Hicks |first1=Roger |last2=Schultz |first2=Frances |date=1999 |pages=88–103 |publisher=David & Charles |isbn=0-7153-0814-9 |url-access=registration}}</ref>{{rp|91}} 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.<ref name=Clerc70/>{{rp|416}}<ref name=Dunn-Wakefield-ch4/>{{rp|87–88}} 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.<ref name=Focal78/>{{rp|56}}

CdS light meters use a [[photoresistor]] sensor whose electrical resistance decreases proportionately to the intensity of light exposure. These require a battery to operate,<ref name=Dunn-Wakefield-ch4/>{{rp|89–90}} but are significantly more sensitive to low light, able to detect lighting levels approximately {{frac|100}} of the lower sensitivity limit of selenium cells.<ref name=Clerc70/>{{rp|417}} However, CdS sensors fell out of favor due to their slower response and extended sensitivity to red and infrared wavelengths.<ref name=Clerc70/>{{rp|417}}<ref name=Focal78/>{{rp|58}}

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 [[battery (electricity)|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 '{{chem|Ga|As|link=Gallium arsenide}}'.<ref name=Focal78>{{cite book |url=https://archive.org/details/focalguidetoexpo0000lync/ |title=The Focalguide to exposure |first=David |last=Lynch |date=1978 |publisher=Focal Press |isbn=0-8038-2364-9 |url-access=registration}}</ref>{{rp|59}}

[[File:Autoexpmeter.JPG|thumb|left|An automatic light meter/exposure unit from an [[8mm film|8 mm]] [[movie camera]], based on a galvanometer mechanism (center) and a [[Cadmium sulfide|CdS]] [[photoresistor]], in opening at left.]]
Many modern consumer still and video [[camera]]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 [[cinematographer]]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.

===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.<ref name=Dunn-Wakefield-ch4/> All in-camera meters are reflected-light meters. Reflected-light meters are calibrated to show the appropriate exposure for "average" scenes.<ref name=Focal78/>{{rp|74}} 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.

[[File:Sekonic L 398.jpg|thumb|left|upright|Sekonic L-398, with opalescent incident light-integrating dome]]
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.<ref name=Dunn-Wakefield-ch5>{{cite book |url=https://archive.org/details/exposuremanual0000dunn/ |title=Exposure Manual |chapter-url=https://archive.org/details/exposuremanual0000dunn/page/122/mode/2up |chapter=5: Photoelectric Incident-light Meters and Attachments |author1=Dunn, Jack F. |author2=Wakefield, George L. |edition=Third |date=1974 |publisher=Fountain Press |location=Hertfordshire, England |isbn=0-85242-361-6 |pages=122–140 |url-access=registration |access-date=8 September 2023}}</ref> 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.

[[File:Asahi Pentax Digital Spotmeter, against color chart (Pcs34560).jpg|thumb|right|upright|Pentax Digital Spotmeter]]
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 [[Elliptical cone|cone]], typically with a one degree circular [[Angle of view#Calculating a camera's angle of view|angle of view]].<ref name=Dunn-Wakefield-ch6>{{cite book |url=https://archive.org/details/exposuremanual0000dunn/ |title=Exposure Manual |chapter-url=https://archive.org/details/exposuremanual0000dunn/page/140/mode/2up |chapter=6: Exposure Photometers |author1=Dunn, Jack F. |author2=Wakefield, George L. |edition=Third |date=1974 |publisher=Fountain Press |location=Hertfordshire, England |isbn=0-85242-361-6 |pages=141–177 |url-access=registration |access-date=8 September 2023}}</ref> 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]].<ref name=Hicks99/>{{rp|102;126}}

Many modern cameras include sophisticated [[Metering mode#Multi-zone metering|multi-segment metering]] systems that measure the [[luminance]] of different parts of the scene to determine the optimal exposure.<ref name=Hicks99/>{{rp|102}} When using a film whose spectral sensitivity is not a good match to that of the light meter, for example [[orthochromatic]] [[Black and White Photography|black-and-white]] or infrared film, the meter may require special filters and re-calibration to match the sensitivity of the film.<ref name=Hicks99/>{{rp|89–92}}

[[File:Gossen-variosix hg.jpg|thumb|left|upright|Gossen Variosix (Luna-Star) F, capable of reflected, incident, and flash measurements]]
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. [[Densitometer]]s are used in photographic reproduction.