When developing a new type of infrared thermal imaging instrument, engineers and their managers must consider factors such as application conditions, operating bands, minimum resolution, pixel size, environmental adaptability, batch production capacity, etc. in the process of forming its solution. However, the most important factor that affects these factors is the ir lens.
The ir lens is an indispensable component in the infrared thermal imager. Its function is to converge the target's infrared radiation onto the infrared detector. After photoelectric conversion and image processing, a high-contrast image is finally formed. The performance of the infrared thermal imager is largely determined by the quality of the ir lens.
Infrared thermal imagers generally operate in three bands: short-wave, medium-wave, and long-wave. Some infrared thermal imagers used for special occasions also need to work in multiple bands. The ir lens should be specially designed according to the operating band to optimize performance. The infrared materials used for ir lenses for different bands are also different.
The lens forms an image to fill the infrared detector. In general, the focal plane of the infrared detector is rectangular or square, while the image formed by the ir lens is a rotationally symmetrical circular area.
The lens must create a diameter equal to or greater than the diagonal of the focal plane array at the focal plane of the detector. If the image cannot completely fill the detector area, the resulting effect is called vignetting, which will reduce the energy of the edge field of the image.
Generally speaking, ir lenses cannot be vignetted. For lenses used for infrared cryogenic detectors, if there is vignetting, then they cannot meet the design principle of 100% cold aperture efficiency, and stray radiation will affect the performance of the infrared thermal imager.
ir lenses are usually identified by their focal length. When the focal length increases, the field of view of the lens becomes narrower, and vice versa, as the focal length decreases, the field of view becomes wider.
ir lenses can generally be divided into single-field-of-view lenses, multi-field-of-view lenses, and continuous zoom lenses. Since infrared continuous zoom lenses can achieve continuous tracking of targets at different distances, they are widely used in many fields.
The F number of the ir lens determines how much radiation energy of the target enters the infrared thermal imager. The smaller the F number, the larger the size of the ir lens under the same focal length. When used in conjunction with the corresponding detector, the more infrared radiation is obtained, and the higher the sensitivity of the infrared thermal imager.
However, in some cases where weight and volume are strictly required (such as UAV optoelectronic pods), the application of some large F-number infrared thermal imagers is becoming more and more common under the premise of meeting system indicators. The small-size optoelectronic pod systems using MWIR F5.5 devices and lenses are increasing.
There is no cold screen in the ir lens for non-cooled infrared detectors. For the ir lens of non-cooled infrared detectors, the choice of F number is relatively flexible, but due to the low sensitivity of non-cooled detectors, ir lenses with small F numbers are generally used.
The depth of field is the range of the farthest and closest distances that the ir lens can see clearly without adjusting the focus. The depth of field is not only related to the focal length, F number, imaging quality, and the alignment imaging distance set by the lens but also related to the pixel size of the detector.
Generally speaking, the larger the F number, the shorter the focal length, and the larger the pixel size of the detector, the larger the depth of field. The range of the depth of field is also different for different alignment planes.
The closest imaging distance of the lens and the depth of field are two concepts. The closest imaging distance of the lens is the closest object distance that the lens can clearly image when focusing.
The imaging quality of the lens is generally evaluated by the optical transfer function (MTF), distortion, and point spread function. The choice of lens imaging quality should match the pixel size of the detector as much as possible. If it cannot match, it should be judged whether the infrared thermal imager is an optically limited system or a detector-limited system to determine the detection and recognition capabilities of the infrared thermal imager for the target.
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