Image‐intensified photodiode array as a fluorescence detector in cw‐laser experiments

Abstract

Imaging systems based on image‐intensified photodiode‐array cameras are excellent detectors for laser‐induced fluorescence experiments in fluid mechanics and combustion science. The principles of operation of such a system are described. Special attention is given to the use of an image intensifier in conjunction with cw‐laser experiments. In that mode, ghost images caused by the finite phosphor decay time can contribute major systematic errors. Measurements of the phosphor decay times for exposure times between 0.1 and 100 ms (a typical range for cw‐laser experiments) were conducted and show that the decay time increases with exposure time. Methods for circumventing the ghosting problem are suggested. The signal and noise analysis points to analog‐to‐digital converter noise (ADC) or quantization error of the camera and to photon shot noise as the dominating noise sources. The image intensifier improves time resolution and signal‐to‐noise ratio (SNR) by moving the system from the camera noise limit to the shot‐noise limit. Once the shot‐noise limit is reached, the SNR can only be improved by increasing the quantum efficiency of the intensifier, not by increasing the intensifier gain. The spatial resolution of such a system is generally limited by the photodiode array, but can be dominated by focusing errors, if lenses with low f numbers are used. Within a certain range of imaging magnifications, the use of a fiber‐optic minifier between intensifier and array in lieu of a 1:1 fiber bundle can improve the collection efficiency, and thereby both signal and camera‐limited SNR. Use of a minifier will always improve the shot‐noise‐limited SNR.