Page 293 - Modern Optical Engineering The Design of Optical Systems

272 Chapter Twelve

2
not 1/2 lumen ster 1 ft ). The brightness of a number of sources is
tabulated in Fig. 12.8 and natural illumination and reflectance levels
are tabulated in Fig. 12.9.
The terminology of photometry has grown through engineering
usage, and is thus far from orderly. Special terms have derived from

Source Brightness, candles cm -2
5
Sun (zenith) through atmosphere 1.6 x 10 cd/cm 2
Sun (zenith) above atmosphere 2.75 x 10 5
Sun (horizon) 6 x 10 2
Blue sky 0.8
Dark cloudy sky 4 x 10 –3
Night sky 5 x 10 –9
Moon 0.25
Exteriors—daylight (typical) 1
Exteriors —night (typical) 10 –6
Interiors —daylight (typical) 10 –2
Mercury arc—laboratory 10
Mercury arc—high pressure 5 x 10 5
4
Xenon arc 1.5 x 10 to 1.5 x 10 5
4
Carbon arc 10 to 10 5
Tungsten—3655 K (melting point) 5.7 x 10 3
3500 K 4.2 x 10 3
3000 K 1.3 x 10 3
Tungsten filament – ordinary lamp 5 x 10 2
– projection lamp 3 x 10 3
Blackbody—2042 K 60.0 (by definition)
—4000 K 2.5 x 10 4
—6500 K 3 x 10 5
Fluorescent lamp 0.6
Sodium lamp 6
Flame—candle, kerosene 1
Least perceptible brightness 5 x 10 –11
–8
Least perceptible point source 2 x 10 cd @ 3 m distance
Star Sirius 1.5 x 10 6
Atom bomb 10 8
Lightning 8 x 10 6
Ruby laser 10 14
Metal halide lamp 4 x 10 4
Figure 12.8 Typical values for the brightness (luminance) of a number of sources.