The operating characteristics of digital optical computers utilizing spatial light modulator based shadow casting are reviewed. The geometric and physical limitations of this method are examined. Because of the highly parallel nature of this approach, such systems are capable of truth-table processing. A detailed analysis of the relative advantages of both direct and associative truth table processing is presented. For each case, the use of binary or of multiple-valued logic is considered. The respective merits from the point of view of the number of computations per second and the energy required per computation are outlined. Switching energy considerations based on the use of multiple wavelengths in these systems are also discussed. The characteristic features of this type of optical logic are compared to those of electronic logic.