Editing Light meter (section)

===Calibrated reflectance===

It is commonly stated that reflected-light meters are calibrated to an 18% reflectance,<ref> Some authors ([[#CITEREF Ctein1997|Ctein 1997]], 29) have argued that the calibrated reflectance is closer to 12% than to 18%. </ref> but the calibration has nothing to do with reflectance, as should be evident from the exposure formulas.  However, some notion of reflectance is implied by a comparison of incident- and reflected-light meter calibration.

Combining the reflected-light and incident-light exposure equations and rearranging gives

:<math>\frac {L} {E} = \frac {K} {C}</math>

[[Reflectance]] <math>R</math> is defined as

:<math>R = \frac {\mbox {flux emitted from surface}} {\mbox {flux incident upon surface}}</math>

A uniform perfect diffuser (one following [[Lambert's cosine law]]) of luminance <math>L</math> emits a flux density of <math>\pi</math><math>L</math>; reflectance then is

:<math>R = \frac {\pi L} {E} = \frac {\pi K} {C}</math>

Illuminance is measured with a flat receptor.  It is straightforward to compare an incident-light measurement using a flat receptor with a reflected-light measurement of a uniformly illuminated flat surface of constant reflectance.  Using values of 12.5 for <math>K</math> and 250 for <math>C</math> gives

:<math>R = \frac {\pi \times 12.5} {250} \approx 15.7\%</math>

With a <math>K</math> of 14, the reflectance would be 17.6%, close to that of a standard 18% neutral test card.  In theory, an incident-light measurement should agree with a reflected-light measurement of a test card of suitable reflectance that is perpendicular to the direction to the meter.  However, a test card seldom is a uniform diffuser, so incident- and reflected-light measurements might differ slightly.

In a typical scene, many elements are not flat and are at various orientations to the camera, so that for practical photography, a hemispherical receptor usually has proven more effective for determining exposure.  Using values of 12.5 for <math>K</math> and 330 for <math>C</math> gives

:<math>R = \frac {\pi \times 12.5} {330} \approx 11.9\%</math>

With a slightly revised definition of reflectance, this result can be taken as indicating that the average scene reflectance is approximately 12%.  A typical scene includes shaded areas as well as areas that receive direct illumination, and a wide-angle averaging reflected-light meter responds to these differences in illumination as well as differing reflectances of various scene elements. Average scene reflectance then would be

:<math>\mbox{average scene reflectance} =
\frac {\mbox {average scene luminance} } {\mbox {effective scene illuminance }}
</math>

where "effective scene illuminance" is that measured by a meter with a hemispherical receptor.

[[#CITEREF ISO2720:1974|ISO 2720:1974]] calls for reflected-light calibration to be measured by aiming the receptor at a transilluminated diffuse surface, and for incident-light calibration to be measured by aiming the receptor at a point source in a darkened room.  For a perfectly diffusing test card and perfectly diffusing flat receptor, the comparison between a reflected-light measurement and an incident-light measurement is valid for any position of the light source.  However, the response of a hemispherical receptor to an off-axis light source is approximately that of a [[cardioid]] rather than a [[cosine]], so the 12% "reflectance" determined for an incident-light meter with a hemispherical receptor is valid only when the light source is on the receptor axis.