
88 OPTICS FOR ENGINEERS
TABLE 4.1
f-Numbers
F,Indicatedf -number 1.4 2 2.8 4 5.6 8 11
Actual f -number
√
2
1
√
2
2
√
2
3
√
2
4
√
2
5
√
2
6
√
2
7
NA 0.3536 0.2500 0.1768 0.1250 0.0884 0.0625 0.0442
, sr 0.3927 0.1963 0.0982 0.0491 0.0245 0.0123 0.0061
Note: Typical f-numbers and the associated numerical apertures in image space are shown for
|
m
|
1.
where
η, the quantum efciency, is the probability of a received photon producing an electron in
the detector (for a detailed discussion of detectors, see Chapter 13)
t is the frame time of the camera
For η = 0.4 and t = (1/30) s, we expect about 2.7 million electrons.
We note that we could reduce the number of electrons by using ...