Page 294 - Modern Optical Engineering The Design of Optical Systems

Principles of Radiometry and Photometry 273

Source Illumination, footcandles

Direct sunlight 10,000
Open shade 1,000
Overcast/dark day 10–100
Twilight 0.1–1.0
Full moon 0.01
Starlight 0.0001
Dark night 0.00001
(a)

Material Reflectance

Asphalt 0.05
Trees, grass 0.20
Red brick 0.35
Concrete 0.40
Snow 0.85
Aluminum building 0.65
Glass window wall 0.70
Parking lot with cars 0.40

(b)
Figure 12.9 (a) Illumination levels produced by sources in
nature. (b) Reflectance of a number of exteriors.

special usages, and many such terms have survived. A tabulation of
photometric units is given in Fig. 12.10.
Photometric calculations may be carried out exactly as are radio-
metric calculations, using the relationships presented in Secs. 12.2
through 12.6. If lumens are substituted for watts in all the expres-
sions, the computations are straightforward. When the starting and
final data must be expressed in the special terminology of photometry
(as opposed to what one might term the rational units of lumens,
steradians, and square centimeters), then conversion factors may be
necessary for each relationship. A very simple way of avoiding this dif-
ficulty is to convert the starting data to lumens, steradians, and square
centimeters, complete the calculation, and then convert the results into
the desired units.
For convenience, the basic relationships are repeated here in both
radiometric (left column) and photometric (right column) form: