SummaryLight Source Considerations to Secure Single PhotoelectronsOur measurements of the amplitude distribution of photomultiplier anode pulses due to the emission of single-electrons from the cathode consistently show a peak. It is significant that the peak position agrees with that of a calculated distribution based on a Poisson distribution of secondary electrons at each dynode. The integral distribution, obtained by counting single-electron pulses, tends to show a plateau. In low-light-level counting applications, one can set the discriminator so that a majority of photomultiplier single-electron pulses will be counted. Further increase in the sensitivity will eventually increase the noise rate faster than the counting efficiency.The techniques for measuring photomultiplier single-electron statistics are useful for obtaining comparative collection efficiencies • By singleelectron measurements, one can adjust focusingelectrode potentials to maximize overall collection efficiency.It is believed that there is some correlation between the amplitude and time distributions. Further work is necessary to show the extent of the correlation.

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Low -power LASERs have been adapted for underwater use in order to solve a variety of measurement problems.Several systems, using HeNe plasma tubes, have been designed to determine range, measure size, and provide visual orientation information for photographic recording systems.Semi -conductor LASERs have been used for precise measurements of optical transmissivity and scatterance of seawater, in -situ. A number of applications will be described and future developments of these techniques will be discussed.