6
Lightmeter Models CA811 and CA813
CHAPTER 3
SPECIFICATIONS
Light Conversion Formulas
1 foot-candle (lumens/foot
2
) = 10.764 lux
1 lux (lumens/meter
2
) = 0.0929 foot-candles
The Inverse-square Law
The inverse-square law states that the illuminant E at a point on a surface
varies directly with the intensity I of a point source, and inversely as the square
of the distance d between the source and the point. If the surface at the point is
normal to the direction of the incident light, the law is expressed by E=I/d
2
.
Cosine Law
The cosine law states that the illuminant on any surface varies with the cosine
of the angle of incidence. The angle of incidence q is the angle between the
normal to the surface and the direction of the incident light. The inverse-square
low and the cosine law can be combined as E=(I cos q) /d
2
.
3.1
Environmental Specifications
Range (CA811):
20 lux, 200 lux, 2000 lux, 20k lux
20 fc, 200 fc, 2000 fc, 20k fc
Range (CA813):
20 lux, 200 lux, 2000 lux, 20k lux, 200k lux
20 fc, 200 fc, 2000 fc, 20k fc
Resolution:
0.01 lux, 0.01 fc
Accuracy (Type A 2856K light source):
±5% ± 10cts
Accuracy for Common Light Sources (CA811):
±18% ± 2cts
Accuracy for Common Light Sources (CA813):
±11% ± 2cts
Cosine Angle:
ƒ’
2
<2% cosine corrected (150°)
Spectral Response:
CIE photopic curve
Spectral Accuracy (CA811):
ƒ’
1
<15%
Spectral Accuracy (CA813):
ƒ’
1
<8%
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