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A routing signal indicates which of the detectors detected the currently processed photon. The TCSPC module puts the photons from different detectors into different memory segments Detectors Amplifiers Discriminators Encoder PMT 1 A1 D1 PMT .. 2 .. . A2 D2 'Channel' Detector number ... .. 'Disable count' PMT n An Dn to TCSPC module Photon Pulse As Summing Amplifier Fig. 3 Routing module for multidetector TCSPC. For each photon, the routing module delivers the photon pulse and a „channel“ signal that indicates in which detector the photon was detected The routing module consists of a number of amplifiers, A1 through An, connected to discriminators, D1 through Dn, a digital encoder circuit, and a summing amplifier, As.

G. by a photodiode receiving the pulses of the excitation laser. Often a gate pulse generator and a delay generator are used to control the gate pulse duration and delay. However, often the photons need to be counted only in a narrow, fixed time interval within the signal pulse period. In these cases it is sufficient to derive a gate pulse from the laser pulse via a fast photodiode and a discriminator [5, 27]. In practice the gate is often combined with the first counter stage. The principle is shown in Fig.

5 Multiplexed TCSPC operation. Several signals are actively multiplexed into the detector. The destination in the TCSPC memory is controlled by a multiplexing signal at the „channel“ input. For each multiplexing channel a separate photon distribution is built up over the signal time period and the sequencer coordinates Several optical signals are multiplexed on the microsecond or millisecond time scale. Multiplexing of signals can be accomplished by switching several diode lasers, either electronically or by fibre switches, or by rotating elements in an optical system.