alt=Illuminance diagram with units and terminology.|thumb|372x372px|Illuminance diagram with units and terminology

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance. In the CGS system, the unit of illuminance is the phot, which is equal to . The foot-candle is a non-metric unit of illuminance that is used in photography.

Illuminance was formerly often called brightness, but this leads to confusion with other uses of the word, such as to mean luminance. "Brightness" should never be used for quantitative description, but only for nonquantitative references to physiological sensations and perceptions of light.

The human eye is capable of seeing somewhat more than a 2 trillion-fold range. The presence of white objects is somewhat discernible under starlight, at (50&nbsp;μlx), while at the bright end, it is possible to read large text at 10<sup>8</sup> lux (100&nbsp;Mlx), or about 1000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages.

Common illuminance levels

thumb|right|A [[lux meter for measuring illuminances in work environments]]

{| class="wikitable sortable"

|-

! Lighting condition !! Foot-candles !! Lux

|-

| Sunlight || 10,000 || 100,000

|-

| Shade on a sunny day || 1,000 || 10,000

|-

| Overcast day || 100 || 1,000

|-

| Very dark day || 10 || 100

|-

| Twilight || 1 || 10

|-

| Deep twilight || 0.1 || 1

|-

| Full moon || 0.01 || 0.1

|-

| Quarter moon || 0.001 || 0.01

|-

| Starlight || 0.0001 || 0.001

|-

| Overcast night|| 0.00001 || 0.0001

|}

Astronomy

In astronomy, the illuminance stars cast on the Earth's atmosphere is used as a measure of their brightness. The usual units are apparent magnitudes in the visible band. V-magnitudes can be converted to lux using the formula

<math display="block">E_\mathrm{v} = 10^{(-14.18-m_\mathrm{v})/2.5},</math>

where E<sub>v</sub> is the illuminance in lux, and m<sub>v</sub> is the apparent magnitude. The reverse conversion is

<math display="block">m_\mathrm{v} = -14.18 - 2.5 \log(E_\mathrm{v}).</math>

Relation to luminous intensity

When the light source is sufficiently far away to be treated as a point source, the illuminance on a surface is related to the luminous intensity of light it receives by combining the cosine law with the inverse-square law:

<math display="block">E_\mathrm{v} = \frac{I_\mathrm{v} \cos(\theta)}{D^2}</math>

where

  • <sub>v</sub> is the luminous intensity of the source
  • is the angle of incidence, and
  • is the distance between the source and the surface.

Relation to luminance

thumb|upright=1.5|Comparison of photometric and radiometric quantities

The luminance of a reflecting surface is related to the illuminance it receives:

<math display="block">\int_{\Omega_\Sigma} L_\mathrm{v} \mathrm{d}\Omega_\Sigma \cos \theta_\Sigma = M_\mathrm{v} = E_\mathrm{v} R</math>

where the integral covers all the directions of emission , and

  • <sub>v</sub> is the surface's luminous exitance
  • <sub>v</sub> is the received illuminance, and
  • is the reflectance.

In the case of a perfectly diffuse reflector (also called a Lambertian reflector), the luminance is isotropic, per Lambert's cosine law. Then the relationship is simply

<math display="block">L_\mathrm{v} = \frac{E_\mathrm{v} R}{\pi}</math>

See also

  • Irradiance
  • Exposure value
  • Luminance

References

  • Illuminance Converter
  • Knowledgedoor, LLC (2005) Library of Units and Constants: Illuminance Quantity
  • Kodak's guide to Estimating Luminance and Illuminance using a camera's exposure meter. Also available in PDF form.

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