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Photometry Tutorial
The practice of Photometry involves the measurement of electromagnetic radiation in terms of its luminous power – that is,
its capacity to stimulate the human visual system, and to be perceived as light. The unit of measurement for such power,
or luminous flux, is the lumen (lm).
As with Radiometry, five fundamental types of photometric measurement may be enumerated, corresponding to five different types
of measurement geometry:
| MEASURAND |
SI UNITS |
NON-SI |
DESCRIPTION |
| Luminous Flux |
lumen
(lm) |
- |
Luminous power produced by a source |
| Illuminance |
lux
[ lm/m2 ] |
footcandle (fc)
[ lm/ft2 ] |
Luminous Flux incident upon a surface, per unit area |
| Luminous Exitance |
lm/m2 |
lm/ft2 |
Luminous Flux leaving a surface, per unit area |
| Luminous Intensity |
candela
(cd)
[ lm/sr ] |
- |
Luminous Flux leaving a point source, per unit solid angle - measured in steradians (sr) |
| Luminance |
nit
[ cd/(m2) ] |
footlambert (fl)
[ cd/(πft2) ] |
Luminous Flux per unit solid angle leaving an extended source in a given direction, per unit projected area |
Photometry is related to Radiometry, and the lumen to the watt, by a simple mathematical model of the spectral sensitivity of the
human visual system. This model is represented by the spectral luminous efficiency function,
V(λ).
By definition, the luminous flux of a source, Φ L,
is related to its spectral radiant flux,
Φ(λ),
by the application of a weighted integral, as indicated in the equation below. Note that - for historical reasons - the unitless
V(λ)
function is normalized at a peak value of 683 (lm/W):
When spectroradiometric data is available, photometric data can be derived by weighted integration, as indicated above. An alternative approach
involves the construction of a sensor with a relative spectral response which closely approximates the
V(λ)
function. Such a sensor is known as a photometric sensor.
UDTi offers a range of equipment for reliable photometric measurement, including:
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