In photography

Photography

Photography 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...

, light value has been used to refer to a
“light level” for either incident or reflected light, often on a base-2 logarithm

Logarithm

The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...

ic scale.
The term does not derive from a published standard, and has had several
different meanings:

1. An arbitrary value indicated by an exposure meter such as the Weston Master V, discussed in Adams
   Ansel Adams
   Ansel Easton Adams was an American photographer and environmentalist, best known for his black-and-white photographs of the American West, especially in Yosemite National Park....
   
    (1948, 14–18). This may have been the origin of the term. The indicated light value was transferred to the meter's exposure calculator, which then was used to determine camera settings. Ray (2000) uses the term, with the acronym 'LV', in this sense. The Honeywell
   Honeywell
   Honeywell International, Inc. is a major conglomerate company that produces a variety of consumer products, engineering services, and aerospace systems for a wide variety of customers, from private consumers to major corporations and governments....
   
   /Pentax
   Pentax
   Pentax is a brand name used by Hoya Corporation for its medical-related products & services and Pentax Ricoh Imaging Company for cameras, sport optics , etc. Hoya purchased and merged with the Japanese optics company on March 31, 2008. Hoya's Pentax imaging business was sold to Ricoh Company, Ltd...
   
    1°/21° spot meter indicated in “light level” (“LL”), with LL essentially exposure value
   Exposure value
   In photography, exposure value denotes all combinations of a camera's shutter speed and relative aperture that give the same exposure. In an attempt to simplify choosing among combinations of equivalent camera settings, the concept was developed by the German shutter manufacturer in the 1950s...
   
    (EV) for ISO 100 film speed
   Film 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....
   
   . The later Pentax Spotmeter V and Digital Spotmeter indicated directly in EV for ISO 100, but they made no mention of “light level”, “light value”, or LV.
2. A synonym for incident light value, from the Additive system of Photographic EXposure (APEX
   APEX system
   APEX stands for Additive system of Photographic EXposure, whichwas proposed in the 1960 ASA standardfor monochrome film speed, ASA PH2.5-1960,as a means of simplifying exposure computation.-Exposure equation:...
   
   ). Zakia and Stroebel (1993) and Stroebel, Compton, Current, and Zakia (2000) used the term in this sense. Curiously, they used the APEX symbol , normally used for luminance value.
3. An apparent synonym for luminance value, from APEX. Stroebel, Compton, Current, and Zakia (2000) referred to “scene illuminance” in the text of the article, but the example used units of luminance. The defining equation used the symbol . The table at the end of the article used units of illuminance and the symbol , as noted above.
4. A synonym for exposure value (EV) (e.g., Kyoritsu calibrated light sources, for which the luminance ranges are specified in terms of “LV at ISO 100”. Interestingly, Kyoritsu specify the luminance ranges of their multi-function camera testers in EV, presumably at ISO 100).
5. A synonym for “EV at ISO 100 film speed
   Film 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....
   
   ”. This usage appears on many web pages, usually without attribution of an authoritative source. Eads (2000) proposed a revised APEX
   APEX system
   APEX stands for Additive system of Photographic EXposure, whichwas proposed in the 1960 ASA standardfor monochrome film speed, ASA PH2.5-1960,as a means of simplifying exposure computation.-Exposure equation:...
   
    in which luminance value was equal to EV for ISO 100 speed, but he did not use the term light value.
6. Literally speaking, the English term light value could be translated as “Lichtwert” in German language, however, this is not what the term Lichtwert, as it was introduced by the German shutter manufacturer in 1954 and defined as Belichtungswert in DIN 19010, is used for in German-speaking countries. Instead, the established term Lichtwert (abbreviated as “LW”) describes what became known as exposure value
   Exposure value
   In photography, exposure value denotes all combinations of a camera's shutter speed and relative aperture that give the same exposure. In an attempt to simplify choosing among combinations of equivalent camera settings, the concept was developed by the German shutter manufacturer in the 1950s...
   
    (EV) elsewhere.

In APEX

APEX system

APEX stands for Additive system of Photographic EXposure, whichwas proposed in the 1960 ASA standardfor monochrome film speed, ASA PH2.5-1960,as a means of simplifying exposure computation.-Exposure equation:...

, luminance value and incident light value are numerically equal, so that for a given “light value”, either meaning 2 or 3 would result in the same camera settings.