Page 291 - Modern Optical Engineering The Design of Optical Systems

270 Chapter Twelve

Note that not all sources are continuous emitters. Gas discharge
lamps at low pressure emit discrete spectral lines; the plot of spectral
radiant emittance for such a source is a series of sharp spikes,
although there is usually a low-level background continuum. In high-
pressure arcs, the spectral lines broaden and merge into a continuous
background with less pronounced spikes.

Color temperature
Before leaving the subject of blackbody radiation, the concept of color
temperature should be mentioned. The color temperature of a source
of light is a colorimetric concept related to the apparent visual color of
a source, not its temperature. For a blackbody, the color temperature
is equal to the actual temperature in Kelvin. For other sources, the color
temperature is the temperature of the blackbody which has the same
apparent color as the source. Thus, exceedingly bright or dim sources
may have the same color temperature, but radically different radiances
or intensities. Color temperature usually runs about 150 K higher
than filament temperature. Color temperature is extremely important
in colorimetry and in color photography where fidelity of color rendi-
tion is important, but is little used in radiometry.

12.9 Photometry

Photometry deals with luminous radiation, that is, radiation which
the human eye can detect. The basic photometric unit of radiant power is
the lumen, which is defined as a luminous flux emitted into a solid angle
1
of one steradian by a point source the intensity of which is ⁄60 of that of
2
1 cm of a blackbody at the solidification temperature of platinum
(2042 K). From the preceding section, we know that a blackbody radiates
energy throughout the entire electromagnetic spectrum. Chapter 8
indicated that the eye was sensitive to only a small interval of this
spectrum and that its response to different wavelengths within this
interval varied widely. Thus, if a source of radiation has a spectral
1
power function P( ) (W m ), the visual effect of this radiation is
obtained by multiplying it by V( ),* the visual response function which
is tabulated in Fig. 8.9. The effective visual power of a source is, therefore,

*Note that V( ) is customarily the photopic (normal level of illumination and bright-
ness) visual response curve. Under conditions of complete dark adaptation, the visual
response for scotopic vision would be used. The conversion constant in Eq. 12.18
becomes about 1746 instead of 680.