In
photometry Photometry can refer to:
* Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision
* Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electro ...
, illuminance is the total
luminous flux incident on a surface, per unit
area
Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape or planar lamina, while '' surface area'' refers to the area of an ope ...
. It is a measure of how much the incident
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
illuminates the surface, wavelength-weighted by the
luminosity function
A luminous efficiency function or luminosity function represents the average spectral sensitivity of human visual perception of light. It is based on subjective judgements of which of a pair of different-colored lights is brighter, to descri ...
to correlate with human brightness perception.
[International Electrotechnical Commission (IEC): ''International Electrotechnical Vocabulary.']
ref. 845-21-060, illuminance
/ref> Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.
In SI units illuminance is measured in lux (lx), or equivalently in lumens per square metre ( lm· m−2). Luminous exitance is measured in lm·m−2 only, not lux.[
International Electrotechnical Commission (IEC): ''International Electrotechnical Vocabulary.']
ref. 845-21-081, luminous exitance
/ref> In the CGS system, the unit of illuminance is the phot
A phot (ph) is a photometric unit of illuminance, or luminous flux through an area. It is not an SI unit but rather is associated with the older centimetre–gram–second system of units. The name was coined by André Blondel in 1921.Parry Moo ...
, which is equal to . The foot-candle
A foot-candle (sometimes foot candle; abbreviated fc, lm/ft2, or sometimes ft-c) is a non- SI unit of illuminance or light intensity. The foot-candle is defined as one lumen per square foot. This unit is commonly used in lighting layouts in pa ...
is a non-metric unit of illuminance that is used in photography
Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employe ...
.
Illuminance was formerly often called brightness
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. The perception is not linear to luminan ...
, 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 , while at the bright end, it is possible to read large text at 108 lux, or about 1000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimage
AfterImage is a Filipino rock band formed in 1986, best known for their songs "Habang May Buhay", "Next in Line", and "Mangarap Ka". They disbanded in 1997 and became active again in 2008 after they reunited and released their fourth studio alb ...
s.
Common illuminance levels
Astronomy
In astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
, the illuminance stars cast on the Earth's atmosphere is used as a measure of their brightness. The usual units are apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...
s in the visible band. V-magnitudes can be converted to lux using the formula
:,
where ''E''v is the illuminance in lux, and ''m''v is the apparent magnitude. The reverse conversion is
:.
Relation to luminance
The luminance of a reflecting surface is related to the illuminance it receives:
where the integral covers all the directions of emission , and
* v is the surface's luminous exitance
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 ...
* v is the received illuminance, and
* is the reflectance
The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...
.
In the case of a perfectly diffuse reflector (also called a Lambertian reflector), the luminance is isotropic, per Lambert's cosine law
In optics, Lambert's cosine law says that the radiant intensity or luminous intensity observed from an ideal diffusely reflecting surface or ideal diffuse radiator is directly proportional to the cosine of the angle ''θ'' between the directi ...
. Then the relationship is simply
See also
* Irradiance
*Exposure value
In photography, exposure value (EV) is a number that represents a combination of a camera's shutter speed and f-number, such that all combinations that yield the same exposure have the same EV (for any fixed scene luminance). Exposure value is ...
* Luminance
References
External links
Illuminance Converter
* Knowledgedoor, LLC (2005
* Kodak's guide t
Estimating Luminance and Illuminance
using a camera's exposure meter. Also available i
PDF form
{{Authority control
Physical quantities
Photometry