
RADIOMETRY AND PHOTOMETRY 391
both the ¯y spectrum and the spectral response of the detector, so that the voltage reading is
directly proportional to luminous ux.
12.4.2 Power Measurement: The Integrating Sphere
In Section 12.1, we discussed integrating the radiance over solid angle and area to obtain
radiant ux (e.g., using Equations 12.7 and 12.9 in succession). In some cases, physical imple-
mentation of the integration is easy. For example, given a Gaussian laser beam with a diameter,
d
0
, the beam divergence is (4/π)(λ/d
0
), and the product of A is of the order of λ
2
.Ifthe
beam is small enough, all of the light can easily be captured by a power ...