The efficiency of the S-20 photocathode is shown
in Figure 
. The probability that an electron emitted by the
photocathode is counted (the counting efficiency) is of
order 60 per cent. The overall efficiency of the IPCS is the product
of these two numbers.
It can be seen that the peak efficiency
of the IPCS is 11 % at a wavelength of about 4000 Å,
falling to 8 % at 5000 Å, and 
2.5 % at 6500 Å.
[ TIFF ]
Figure: Responsive Quantum Efficiency as a function of wavelength for the
S-20 photocathode used in the IPCS. The overall efficiency of the IPCS is equal to the efficiency
of the photocathode multiplied by the counting efficiency, which
is probably about 60 %