In analyzing radiation from a communication link, single-photon counting can
be
used to advantage especially at low levels of radiation energy, e.g. in the detection
of optical radiation. Preferred detection techniques include methods in which (i)
received optical radiation is intensity-modulated in accordance with a preselected
code, (ii) wherein it is the optical radiation which is intensity-modulated with
the preselected code, and (iii) wherein the radiation modulated with a preselected
code is received. For registration of the signals received by a sensing element
of a single-photon detector, time of arrival is recorded, optionally in conjunction
with registration of time intervals. Advantageously, in the interest of minimizing
the number of pulses missed due to close temporal spacing of pulses, D-triggers
can be included in counting circuitry.