The detective_quantum_efficiency (DQE)is defined as the
square of the signal-to-noise ratio in the recording device
divided by the square of the signal-to-ratio in the electron beam:
(SIGNAL/NOISE)2 recording device
DQE = -------------------------------
(SIGNAL/NOISE)2 electron beam
A DQE value of 1 indicates a perfect recorder. "DQE = 0.25" menas
that the signal-to-noise ratio is reduced by half in the
recording step.
(0.5)**2
DQE = --------- = 0.25.
(1.0)**2