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9 Factors to Consider when Choosing an Infrared Thermal Imaging Lens

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9 Factors to Consider when Choosing an Infrared Thermal Imaging Lens

Issue Time:2021-07-22
 nine factors that need to be considered when choosing an infrared thermal imaging lens

The infrared thermal imaging lens is an indispensable part of the infrared thermal imager. Its function is to converge the infrared radiation of the target on the infrared detector, through photoelectric conversion and image processing, and finally form an image with good contrast. The pros and cons of an infrared thermal imaging lens largely determine the performance of an infrared thermal imaging camera. The following are nine factors that need to be considered when choosing an infrared thermal imaging lens.

Waveband

Infrared thermal imaging cameras generally work in three wavebands: short wave, mediumwave and longwave. Some thermal imaging cameras used on special occasions also need to work in multiple bands. The infrared lens should be specially designed according to its working band to optimize performance. The infrared materials used in the infrared lenses of different wavebands are also different.

Vignetting

In general, the focal plane of the infrared detector is rectangular or square, and the image formed by the infrared lens is a rotationally symmetrical circular area. The infrared thermal imaging lens must create a diagonal line with a diameter equal to or larger than 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 result in a reduction in the energy of the field of view at the edge of the image.

Generally speaking, infrared lenses do not allow vignetting. For the lens used in the infrared cooling detector, if the lens has to vignette, then the 100% cold diaphragm efficiency design principle cannot be met, because stray radiation will affect the performance of the infrared thermal imager.

Focal length and field of view

Infrared lenses are usually identified by their focal length. As the focal length increases, the field of view of the lens narrows. Conversely, as the focal length decreases, the field of view becomes wider.

Infrared lenses can generally be divided into single-field lenses, multi-field lenses, and continuous zoom lenses. Since the infrared continuous zoom lens can realize target search and continuous tracking of targets at different distances, it has been widely used in many fields.

F-number

The F-number of the infrared lens determines how much the target radiant energy enters the infrared thermal imager. The smaller the F-number, the larger the size of the infrared lens under the same focal length. When matched with the corresponding detector, the more infrared radiation can be obtained, and the higher the sensitivity of the infrared thermal imager.

However, on some occasions with strict requirements on weight and volume (such as UAV photoelectric pod), under the premise of meeting system indicators, some large F-number infrared thermal imaging cameras are more and more widely used, and medium wave F5.5 is adopted. The number of devices and small optoelectronic pod systems for lenses is increasing day by day.

For uncooled infrared detectors, there is no cold screen like in the dewar of refrigerated detectors. For the infrared lens of uncooled infrared detectors, the F-number is relatively flexible, but the sensitivity of uncooled detectors is a low, Generally select infrared lens with a small F-number.

Depth of field

The depth of field is the range of the farthest distance and the shortest distance that the lens can see clearly without focusing. The depth of field is not only related to the focal length of the lens, F-number, imaging quality, and its set alignment imaging distance 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 greater the depth of field. For different alignment planes, the depth of field range is also different.

The closest imaging distance of the lens and the depth of field are two concepts. The closest imaging distance is the closest object distance at which the lens can clearly image when the lens is focused.

Image quality

Generally, the optical transfer function (MTF), distortion, and point spread function are used to evaluate the imaging quality of the lens. The imaging quality of the lens should be selected to match the pixel size of the detector as much as possible. If it cannot be matched, it should be judged whether the infrared camera is a system with limited optics or a system with limited detectors to determine the pair of infrared cameras. The ability to detect and recognize targets.

Transmittance

Most infrared materials have a high refractive index, and the lens in the infrared lens needs to be coated with a high-efficiency antireflection coating to increase the transmittance of the infrared lens. As the number of lenses in the lens increases, the transmittance of the lens gradually decreases. Lens absorption and residual reflection are the two major factors that reduce transmittance, and residual reflection will introduce interference (infrared interference is inevitable from the mechanism unless the lens has 100% transmittance), which affects the sensory effect of the infrared thermal imager And performance.

Athermalization

Since the refractive index of the infrared material changes greatly with temperature, when the ambient temperature changes, the infrared lens will produce a corresponding defocus. The infrared lens also adopts active and passive two ways to achieve athermalization to ensure that the focal position of the lens does not move when the temperature changes.

If the lens does not allow user intervention during use (such as installed in an unattended environment), the infrared lens must be athermalized.

Interface

The optical interface of the infrared lens should match the infrared detector used, especially the infrared lens used to cool the infrared detector, which involves the F-number, the distance from the cold screen to the focal plane, and the detailed parameters of the window.

The mechanical interface of the infrared lens is the connection form with the infrared movement, generally in the form of a flange, thread, bayonet, etc. Generally speaking, the flange installation method is reliable and can ensure the consistency of the installation position of the detector.

The infrared thermal imaging lens produced by Qunhom has strong monitoring capabilities in the dark at night, can accurately identify hidden targets, and has a very strong ability to penetrate harsh conditions such as haze, rain, snow, and smoke.

If you want to know more about infrared thermal imaging lenses after reading the above content, you can get professional solutions by contacting us. At the same time, the infrared thermal imaging lenses we produce are of excellent quality and various types, which can meet your diverse needs.

With excellent technology and high-quality products, Quanhom has become one of the leading manufacturers of opto-electromechanical components. We focus on the production of various thermal infrared lenses (including LWIR, MWIR, and SWIR). We have a professional production team and a strict quality inspection system, and we carry out all aspects of quality control from product design to export. And we will also provide thoughtful one-stop service and effective solution technology according to the needs of customers. If you are interested in our infrared thermal imaging lens, please contact us immediately!