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Application Design of Infrared Optical Lenses

Infrared optical devices, commonly known as IR optical devices, are used to collect, focus, or collimate spectra in near-infrared (NIR), short-wave infrared (SWIR), medium-wave infrared (MWIR), or long-wave infrared (LWIR). The wavelength range of infrared optics is between 700 and 16000nm. wavelength. Wavelength Optoelectronics provides a variety of high-performance infrared optical devices for life sciences, security, machine vision, thermal imaging, and industrial applications.

SWIR lens

Compared with visible light and other thermal radiation bands, the SWIR lens band has common imaging advantages. It is mainly used in electronic board inspection, material/food sorting, solar cell inspection, quality inspection, military applications, and industrial machine vision categories. SWIR lenses are also used in situations where other detectors or cameras are not flexible enough to stop limited detail recognition.

SWIR lenses, especially extended-band SWIR lenses, can use materials that need to consider wide-band passability, processability, and transparency to visible light, so ZnSe, ZnS, CaF2, etc. become common choices.

MWIR lens

Mid-wave infrared achromatic lenses are available for designers and researchers working in the 3μm to 5μm spectral range. These lenses have near-diffraction-limit performance and are suitable for FTIR (Fourier Transform Infrared) spectroscopy and mid-wave infrared thermal imaging, as well as tunable quantum cascade lasers.

Mid-wave infrared lens is usually used in conjunction with the mid-wave refrigerant detector, and the aperture is placed in front of the lens, so the lens is relatively large, and the so-called cold light effect (ghosting, reflection) should also be considered. Although the volume of refrigerated lenses and detectors is large, the detection distance can reach far, such as a focal length of 150 mm and 300 mm, which can see distances of 10 km to 30 km.

LWIR lens

LWIR lenses are usually non-cooled, and therefore less flexible. They allow users to see through dust or smoke, making them particularly valuable in some environments and applications. The field of view of the lens mainly depends on the focal length and detector size.

The design of LWIR lens is commercially dominant, that is, they must be cheap and effective. Therefore, aspherical use is quite common. In addition, with the improvement of commercial applications, such as vehicle night vision, gun sighting, mobile phones, etc., sulfide glass has become the favorite of such applications. Due to its low-temperature molding, if the number of sulfide glass lenses is large, the price can be very low.

In extremely cold and hot conditions, especially when the temperature difference is large, changes in the curvature, thickness, refractive index of the lens material, lens barrel, and lens material of the infrared lens cause defocusing of the lens. To ensure clear imaging, electric or manual focus adjustment is required. To eliminate the adverse effects of temperature changes, non-thermal designs are required. Usually, designers will use different optical materials to perform optical compensation (temperature difference), or use a material and optical material with opposite changes in design, to perform optomechanical compensation.

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