The thermal performance parameter of the thermal imaging system is a laboratory parameter. When the system is used for the detection of actual targets, the target characteristics and environmental conditions do not meet the laboratory standard conditions, and various observation levels and probabilities will also affect the observation effect. The and some other parameters must be corrected.
(1) Atmospheric transmission attenuation
For the detection of the actual target, the infrared image information of the target is always attenuated by the atmospheric transmission, and its influence cannot be ignored, and the actual atmospheric attenuation is one of the most important influence items. For the detection of a small temperature difference target image, the thermal imaging system receives a signal generated by the target and background radiation power difference in proportion to the temperature difference between them. Let the apparent temperature difference of zero lines of sight between the black object and the background be. When the atmospheric transmission of the distance reaches the thermal imaging system, the equivalent temperature difference between the target and the background can be approximated. Expressed as
Where and are the average extinction coefficient and the average atmospheric transmittance of the atmospheric transmission in the working band of the thermal imaging system, respectively, along with the target direction distance. Atmospheric transmission attenuation affects the line of sight of thermal imaging systems, and the attenuation produced by different atmospheric conditions is quite different. Therefore, in the technical and tactical indicators of the thermal imaging system, there should be clear atmospheric conditions (such as atmospheric pressure, temperature, relative humidity, visibility distance, transmission path, etc.).
(2) Determination of observation level
The observation level is a visual division method that combines system performance with human vision and needs to be completed by visual psychological test. The currently accepted division method is the Johnson criterion, which relates the observation problem of the target to the observation problem of the equivalent strip pattern according to the experiment, and divides the visual observation level into discovery, recognition, and recognition, and the definition is as shown in the table.
|Johnson criterion for visual observation level|
|Observation level||definition||Number of strips required|
|Find||Find a target in the field of view|
|Identification||Can classify targets|
|Recognize||Differentiate the target model and other characteristics|