1. Infrared Detector Resolution
That is, the number of pixels of thermal imaging. The higher resolution means more observation and temperature measurement points, thus smaller target at longer distance can be observed and measured. Usually the resolution of infrared thermal imaging ranges from 256x192, 384x288, 640x512, 800x600, 1024x768, 1280x1024, etc. With higher resolution, the cost of detector will be more. Using larger array detectors can:
2. Field of View (FOV)
Field of View (FOV): it refers to the two-dimensional field of view of the object space observed by the optical system of infrared thermal imager. Taking the horizontal FOV as an example, assuming that the detector array size is A×B, the pixel size is d, and the lens focal length is f, then the horizontal FOV angle θ=2×acrtan (A×d/2f).
After the detector array and pixel size are selected, the field of view only changes with the focal length of the optical system: with longer focal length, the field of view will be narrower; with shorter focal length, the field of view will be wider.
3. Spatial Resolution
As an indicator of the clarity of thermal image, it shows the ability to resolve the spatial shape of the target and is usually expressed in mrad (milliradian).
Spatial resolution is a measure of the smallest object that can be resolved by the sensor, or the ground area imaged for the instantaneous field of view (IFOV) of the sensor. The IFOV is affected by the size of the individual detecting element, d and the focal length of the lens, f.
That is iFOV = d / f
For a given distance, the smaller spatial resolution value is, the smaller target and richer details can be seen. For a given target, the smaller spatial resolution value is, the longer range it will be detected.
4. DRI Range
It is a means of measuring the distance at which an infrared detector can produce an image of specific target and can be divided into detection range, recognition range, identification range.
D (Detection): ability to distinguish an object from the background
R (Recognition): ability to classify the object class (animal, human, vehicle, boat …)
I (Identification): ability to describe the object in details (a man with a hat, a deer, a Jeep …)
According to Johnson's criteria, when the probability of target detail visible at the DRI distance is 50%, the minimum number line pairs of the target is 1:3:6 (or 1:4:8), and the corresponding minimum number of pixels is 2:6:12 (or 2:8:16).
Assuming that the target diameter is H, the focal length is f, the pixel size is d, and the number of line pairs is n, then the view distance L=H×f/(2n×d)
Summary: lenses with different focal lengths are selected according to the size, range and details of the observed target, so as to obtain different spatial resolutions, FOV and distance.
For example, a product uses a 640×512/17μm infrared core with 50mm lens. According to the above calculation method, a 1.8-meters-tall person within a field of view of 12.4°×9.9° from 882m away can be identified. (According to Johnson critieria 1:3:6, 3-pair identification calculation.)
NETD is the Noise Equivalent Temperature Difference, which is the temperature difference between the target and the background when the detector output signal-to-noise ratio is equal to 1. It is an indicator to measure the sensitivity of infrared detector.
The smaller NETD an infrared detector has, the higher thermal sensitivity and better image quality will it has.
The smaller temperature difference between target and background is, the higher thermal sensitivity of the detector is required.
6. Frame Rate
Frame rate is the number of frames per second of a complete image produced by the infrared module. If the target has fast moving speed or rapid changing temperature, thermal module with higher frame rate should be selected, otherwise the measurement accuracy and observation effect will be affected.
GST thermal modules support multiple frame rates:
Uncooled thermal module (Standard): 25Hz/30Hz/50Hz/60Hz etc.
Cooled thermal module (Standard): 50Hz/100Hz/200Hz etc.
Various Interfaces: LVDS/DVP/USB2.0/USB3.0/HDMI etc.
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