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The performance of a maximum likelihood receiver whose input is the photoelectric current created by a modulated light beam is studied for a number of discrete modulation schemes. The performance is compared with that of a classical microwave receiver, i.e., one designed to optimize detection in a Gaussian channel. Such a receiver has the advantages of availability, simplicity, ease of implementation, and greater tolerance to slow photodetector response and is an obvious candidate for suboptimum detection. Procedures for calculating receiver performance in the Poisson regime by the tilted probability density technique are described and applied specifically to modulation schemes employing one or more subcarriers that are biphase or quadriphase modulated. Results indicate that the classical receiver falls noticeably short of the optimum only at or near 100 percent modulation, where it may require at worst up to 30 percent more light power in order to equal the maximum likelihood receiver. When modulation is incomplete, or when background radiation is significant, the difference in performance is generally much smaller.