1 Optical system entrance pupil aperture D0 and focal length f'

The D0 and f' of the optical system of the thermal imager are important factors that determine its performance, volume and weight.

2. Instantaneous field of view

When the optical axis does not move, the spatial range that the system can observe is the instantaneous field of view. It depends on the size of the unit detector and the focal length of the infrared objective, which determines the highest spatial resolution of the system.

If the detector is rectangular and the size is axb, then

α=a/f’

β=b/f’

It is the plane angle of the instantaneous field of view (often expressed in rad or mrad).

3. Total Field of View

The total field of view refers to the maximum viewing area of the thermal imager. Usually described by two plane angles in the horizontal and vertical directions

4. Frame period Tf, and frame frequency fp

The time Tf that the system takes to form a complete picture is called cycle or hour (measured in seconds); and the number of frames formed in one second is called frame rate or frame rate fP (measured in frequency). Therefore:

fp=1/Tf

 

5 Scanning efficiency n

When the thermal imager images the scene, since it takes time for synchronous scanning, retrace, and DC recovery, and no video signal is generated during these periods, it is summed up as the no-load time T'f. Therefore, the difference (T-T'f) is the effective scanning time, and the ratio of its frame period is the scanning efficiency, that is,

n= Tf-T' f/Tf

 

6. Dwell time

The time that the optical axis of the system takes to sweep a detector is called the dwell time, denoted as Td, which is a key parameter of the optical-mechanical scanning thermal imager.

If the frame period is Tf, the scanning efficiency is n, and the thermal imager uses a unit detector, the detector dwell time Td1 is

Td1= nTf α β/AB

In the formula, A and B are the field angles of the thermal imager in the horizontal and vertical directions respectively; α and β are the instantaneous field angles.

 

When the detector is a line column composed of n unit detectors orthogonal to the row scanning direction, the dwell time is

Td=NTd1= nŋTfaß /AB

It can be seen that under the condition of the same frame period and scanning efficiency, if n identical unit detectors are arranged in a line along the direction orthogonal to the row scanning, the dwell time on a single detector will be extended to n times. , which is beneficial to improve the signal-to-noise ratio of the thermal imager.

It must be noted that the dwell time of the detector should be greater than its time constant.