This paper discusses some aspects related to the capability and limitations of the human observer as an integral part of a photo-optical system. The subject matter is more or less divided into two parts covering respectively the visual mechanism of the human eye, its functional behavior, its resolution limits and visual perception and the system engineering aspects as related to display systems and sensors. Most of the material covered in the first part is available in current publications or textbooks. The main emphasis here has been on correlating and organizing existing material in a comprehensive manner to expose the scientist-engineer to this perhaps somewhat unfamiliar discipline. Very often the human observer or human operator is considered as the magical "black box" somewhere in the loop of the overall system, be it airborne or earthbound. Such a black box approach usually considers only the input signals or stimuli and the corresponding output signals or responses. Nevertheless it seems appropriate, at least once, to dig into this black box, i.e., the human operator, and have a closer look at its (or his) internal mechanism. Perhaps it may help us in our evaluation of the human performance as a vital link in the overall system. The latter part of the discussion is more related to the human ability to detect and recognize targets in real-time exploitation of airborne sensor data. This will be treated in terms of graphical presentations of critical target dimension versus observation time. It is proposed that such presentations can be employed for effective utilization of various airborne sensors. A simple analysis of display system parameters relevant to dynamic imagery presentation and interpretation will also be discussed.