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.
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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 %