
OPTICAL DETECTION 421
which is considerably higher than the photon noise for the visible detector in Table 13.1; this
detector will not be quantum-limited.
13.3.2 Noise-Equivalent Power
It is often useful to ask how much power would have been required at the input to the detector,
to produce a signal equivalent to this noise. This power is called the noise-equivalent power,
or NEP. In general,
NEP =
i
n
ρ
i
, (13.27)
and specically for Johnson noise,
NEP =
4k
B
TB
R
hν
ηe
(Johnson noise). (13.28)
Several other sources of noise may also arise in detection. A typical detector specication
will include the NEP, or more often the NEP per square root of bandwidth,