image intensifier 170 of a ccd camera 180 the filter pack 160 contains a combination of appropriate notch dichroic and glass filters to give an acceptable signal to background noise ratio the image intensifier 170 is gated synchronously with the laser beam 60 pulses such that it is switched off during the laser pulse and is switched on in the period between pulses the gating of the intensifier is controlled by the pulse generator the timing of the gating with respect to the laser pulse is indicated schematically in fig4 the pulse generator triggers the laser pulse which has a duration peak half width of t 1 the laser pulse occurs during the time interval t 2 during which time the intensifier is switched off after a suitable delay the intensifier gate is switched on and is kept on for a time t 3 during which time a digital signal which is spatially representative of the luminescence image is captured after time t 3 it is switched off again for a further period t 2 during which time the next laser pulse appears the pulsing is repeated until the desired number of digital signals have been collected with signals being preferably continuously summed to provide the final image signal the summing operation to produce the final image is preferably performed as follows during the periods between the pulses i e during periods defined by t 3 each pixel on the photodetector ccd.