Tektronix LumaColor II J18, Instruction Manual

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Appendix A: Photometry and Radiometry Reference
A–2 J18 Photometer Instruction Manual
Photometric Formulas
10.764
@
footcandles
+
lux (lumens
ń
meter
2
)
3.426
@
footlamberts
+
nits (candela
ń
meter
2
)
footcandles
@
distance
2
(in feet)
+
candelas
lux
@
distance
2
(in meters)
+
candelas
footcandles
@
surface reflectance factor
+
footlamberts
Radiometric Relationships
In radiometry, an ideal sensor has equal sensitivity to all wavelengths
of light being measured.
H
An isotropic light source (a source that emits light uniformly in
all directions) of 12.6 watts produces a radiant flux of 1 watt/ster-
adian.
H
1 watt/steradian at a distance of 1 meter produces an irradiance of
1 watt/meter
2
.
H
Moving a light source further from a surface reduces the
irradiance proportionally with the square of the distance (inverse
square law). For example, moving a 1 watt/steradian source from
1 meter to 2 meters will reduce the irradiance to
0.25 watt/meter
2
.
H
A perfectly white, diffuse surface illuminated by an irradiance of
1 watt/meter
2
has a surface radiance of .318 watt/meter
2
/stera-
dian (irradiance divided by
p
).
H
A diffuse surface that has a reflectance of less than 100% will
have a surface radiance, in watts/meter
2
/steradian, equal to the
irradiance multiplied by the reflectance factor and divided by
p
.
H
Measurement of the radiance 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.
Appendix A: Photometry and Radiometry Reference
A–2 J18 Photometer Instruction Manual
Photometric Formulas
10.764
@
footcandles
+
lux (lumens
ń
meter
2
)
3.426
@
footlamberts
+
nits (candela
ń
meter
2
)
footcandles
@
distance
2
(in feet)
+
candelas
lux
@
distance
2
(in meters)
+
candelas
footcandles
@
surface reflectance factor
+
footlamberts
Radiometric Relationships
In radiometry, an ideal sensor has equal sensitivity to all wavelengths
of light being measured.
H
An isotropic light source (a source that emits light uniformly in
all directions) of 12.6 watts produces a radiant flux of 1 watt/ster-
adian.
H
1 watt/steradian at a distance of 1 meter produces an irradiance of
1 watt/meter
2
.
H
Moving a light source further from a surface reduces the
irradiance proportionally with the square of the distance (inverse
square law). For example, moving a 1 watt/steradian source from
1 meter to 2 meters will reduce the irradiance to
0.25 watt/meter
2
.
H
A perfectly white, diffuse surface illuminated by an irradiance of
1 watt/meter
2
has a surface radiance of .318 watt/meter
2
/stera-
dian (irradiance divided by
p
).
H
A diffuse surface that has a reflectance of less than 100% will
have a surface radiance, in watts/meter
2
/steradian, equal to the
irradiance multiplied by the reflectance factor and divided by
p
.
H
Measurement of the radiance 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.