When the human eye searches for a target in a certain background or a target image on the display of the imaging system, the continuous response of the eye is divided into four levels: detection (discovery), classification, recognition and discrimination. composed of interrelated steps. Here, detection refers to distinguishing a target from its background or other targets; classification is to roughly classify the detected target, such as a vehicle or an airplane; identification is to subdivide the classified target, such as a tank or a car; The identification is to identify the target identified by oneself, such as the M-60 tank or the T-72 tank, then the specific details of the target can be seen.

In the detection, classification, recognition and identification of objects, people's subjective judgments are experienced, so there is a problem of search probability in object search. The size of the search probability provides an estimate of the performance of the photoelectric imaging system to a certain extent. Of course, the search probability is also related to the state of the observer. The search probability is related to a certain search task, such as detection or recognition.