Page 298 - Modern Optical Engineering The Design of Optical Systems

Principles of Radiometry and Photometry 277

The illumination at E is determined from Eq. 12.11 as before

2
H TN sin
TN
E TB
0.8 0.44 10 4

0.35 10 4 lumen cm 2
929 0.35 10 4 0.032 footcandles

12.10 Illumination Devices
Searchlight
A searchlight is one of the simpler, and at the same time one of the
least understood, illuminating devices. It consists of a source of light
(usually small) placed at the focal point of a lens or reflector. The
image of the source is thus located at infinity. A common misconception
is that the beam of light produced is a “collimated parallel bundle”
which extends out to infinity with a constant diameter and a con-
stant power density. A little consideration of the matter will reveal
the fallacy: the rays from any point on the source do indeed form a
collimated parallel bundle, etc. However, a geometrical point on any
source of finite brightness must emit zero energy, since a point has
zero area, and therefore the “collimated bundle” of rays has zero
energy.
With reference to Fig. 12.11, which shows a source S at the focal
point of lens L, the image (S′) will be located at infinity. Since source
S subtends an angle from lens L, the image S′ will also subtend .
Now the illumination at a point on the axis will be determined by the

L
S
TO S' AT INFINITY
d
ƒ
D
Figure 12.11 The optics of a searchlight.