IR Lenses Need to Know

When developing a new thermal imager, engineers and their managers must consider factors such as application conditions, operating band, minimum resolution, pixel size, environmental compliance, mass production capabilities, etc. , but affecting these elements is the infrared lens. The infrared lens is an indispensable component in the infrared thermal imager. Its function is to concentrate the target infrared radiation on the infrared detector. After photoelectric conversion and image processing, an image with good contrast is finally formed. The quality of the infrared lens determines the performance of the infrared thermal imager to a large extent. Bands Thermal imaging cameras generally operate in three bands: short-wave, medium-wave and long-wave. Some infrared thermal imagers for special places also need to work in multiple bands. Infrared lenses should be specially designed to optimize performance according to the operating band in which they are located. The infrared data selected by infrared lenses for different wavelengths are also different. Vignetting The lens forms an image to fill the IR detector. The focal plane of the IR detector is normally rectangular or square, while the IR lens forms the image as a rotationally symmetric circular area. The lens must create a diagonal at the detector focal plane with a diameter equal to or greater than the focal plane array. Assuming that the image does not fill the detector area perfectly, the resulting effect is called vignetting, which will incur a reduction in energy in the field of view at the edges of the image. Generally speaking, infrared lenses do not allow vignetting. Regarding the lens used for infrared cooling detectors, assuming that the lens has vignetting, it cannot meet the 100% cold diaphragm efficiency design criterion, and 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 lens's field of view narrows (Narrow). Conversely, as the focal length decreases, the field of view becomes wider (Wide). Infrared lenses can generally be divided into single-field lenses, multi-field lenses, and continuous zoom lenses. Because the infrared continuous zoom lens can complete the search for the target and the continuous tracking of the target at different intervals, it is 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 used with the corresponding detector, the more infrared radiation is obtained, and the higher the flexibility of the infrared thermal imager. Depth of field Depth of field is the range of the farthest and closest distances that the lens can see without focusing. The depth of field is not only related to the focal length of the lens, the F-number, the imaging quality and its set alignment imaging interval, but also to the pixel size of the detector. Generally speaking, the larger the F-number, the shorter the focal length, and the larger the detector pixel size, the larger the depth of field. With respect to different alignment planes, their depth of field ranges are also different. Image quality Generally, the optical transfer function (MTF), distortion and point spread function are used to evaluate the image quality of the lens. The selection of the imaging quality of the lens should be used to match the pixel size of the detector as much as possible. If it cannot be matched, it should be judged whether the infrared thermal imager is an optically limited system or a detector-limited system to ensure that the infrared thermal imager is suitable for Target detection and identification capabilities. Athermalization Since the refractive index of infrared materials varies greatly with temperature, when the ambient temperature changes, the infrared lens will produce a corresponding defocus amount. The infrared lens also uses active and passive methods to achieve athermalization to ensure that the focal position of the lens does not move when the temperature changes. Interface The optical interface of the infrared lens should match the infrared detector used, especially the infrared lens used for cooling the infrared detector, which involves the detailed parameters of the F number, the distance from the cold screen to the focal plane, and the window. The mechanical interface of the infrared lens is the connection method with the infrared movement, generally using flanges, threads, bayonet and other methods. Generally speaking, the flange installation method is reliable and can ensure the uniformity of the installation orientation of the detector.