Tektronix LumaColor II J18, Instruction Manual

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J18 Photometer Instruction Manual A–1
Appendix A: Photometry and Radiometry
Reference
This appendix summarizes convenient relationships and formulas
used in photometry and radiometry.
Photometric Relationships
In photometry, the spectral sensitivity of the sensor is matched to the
average human eye, with a peak at 555 nm.
H
An isotropic light source (a source that emits light uniformly in
all directions) of 12.6 lumens is 1 candela (1 candle power).
H
A 1 candela source at a distance of 1 meter provides an
illuminance of 1 lux on a surface, regardless of the reflectance of
the surface.
H
A 1 candela source at a distance of 1 foot provides an illumi-
nance of 1 footcandle on a surface, regardless of the reflectance
of the surface
H
Moving a light source further from a surface reduces the
illuminance proportionally with the square of the distance
(inverse square law). For example, moving a 1 candela source
from 1 foot to 2 feet will reduce the illuminance to
0.25 footcandle.
H
A perfectly white, diffuse surface illuminated by 1 footcandle has
a surface luminance of 1 footlambert.
H
A diffuse surface that has a reflectance of less than 100% will
have a surface luminance, in footlamberts, equal to the
illuminance in footcandles multiplied by the reflectance factor.
H
Measurement of the luminance of a large uniformly illuminated
surface is essentially independent of distance, since the area
viewed by the sensor increases with the square of the distance,
exactly compensating for light falloff due to the inverse square
law.
J18 Photometer Instruction Manual A–1
Appendix A: Photometry and Radiometry
Reference
This appendix summarizes convenient relationships and formulas
used in photometry and radiometry.
Photometric Relationships
In photometry, the spectral sensitivity of the sensor is matched to the
average human eye, with a peak at 555 nm.
H
An isotropic light source (a source that emits light uniformly in
all directions) of 12.6 lumens is 1 candela (1 candle power).
H
A 1 candela source at a distance of 1 meter provides an
illuminance of 1 lux on a surface, regardless of the reflectance of
the surface.
H
A 1 candela source at a distance of 1 foot provides an illumi-
nance of 1 footcandle on a surface, regardless of the reflectance
of the surface
H
Moving a light source further from a surface reduces the
illuminance proportionally with the square of the distance
(inverse square law). For example, moving a 1 candela source
from 1 foot to 2 feet will reduce the illuminance to
0.25 footcandle.
H
A perfectly white, diffuse surface illuminated by 1 footcandle has
a surface luminance of 1 footlambert.
H
A diffuse surface that has a reflectance of less than 100% will
have a surface luminance, in footlamberts, equal to the
illuminance in footcandles multiplied by the reflectance factor.
H
Measurement of the luminance of a large uniformly illuminated
surface is essentially independent of distance, since the area
viewed by the sensor increases with the square of the distance,
exactly compensating for light falloff due to the inverse square
law.