Editing Light meter (section)

===Calibration constants===<!-- This section is linked from [[Exposure value]] -->

Determination of calibration constants has been largely subjective; [[#CITEREF ISO2720:1974|ISO 2720:1974]] states that

<blockquote>
The constants <math>K</math> and <math>C</math> 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.
</blockquote>

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.

[[#CITEREF ISO2720:1974|ISO 2720:1974]] recommends a range for <math>K</math> of 10.6 to 13.4 with
luminance in cd/m<sup>2</sup>.  Two values for <math>K</math> are in common use: 12.5 ([[Canon Inc.|Canon]], [[Nikon]], and
[[Sekonic]]<ref>Specifications for Sekonic light meters are available on the [http://www.sekonic.com/ Sekonic] web site under "Products."</ref>) and 14 ([[Minolta]],<ref name="KM_2006" /> [[Kenko (company)|Kenko]],<ref name="KM_2006">[[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.</ref> and [[Pentax]]); the difference between the two values is approximately {{frac|6}} [[Exposure value|EV]].

The earliest calibration standards were developed for use with wide-angle averaging reflected-light meters
([[#CITEREF JonesCondit1941|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 <math>K</math> 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, [[#CITEREF ISO2720:1974|ISO 2720:1974]] recommends a range for <math>C</math> 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 ([[#CITEREF Norwood1938|Norwood 1938]]), the objective was

<blockquote>
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.
</blockquote>

and the meter provided for "measurement of the effective illumination obtaining at the position of the subject."

With a hemispherical receptor, [[#CITEREF ISO2720:1974|ISO 2720:1974]] recommends a range for <math>C</math> 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 <math>C</math> = 330 will indicate an exposure approximately 0.40 step greater than that indicated by a flat receptor with <math>C</math> = 250.  With a slightly revised definition of illuminance, measurements with a hemispherical receptor indicate "effective scene illuminance."