Infrared optical lens design

Infrared optical lens design, different lenses have different methods

Infrared imaging is to gather the detected optical signal to the detector through the lens group, and then convert the optical signal to the electrical signal by the detector and its back channel. Depending on the detector and field of application, the lens materials used in the optical system are also different. According to the characteristics of different infrared lens materials, the processing methods and process equipment are also different.

Infrared Optical Lens Design

The design of the infrared lens needs to be designed according to the pixel and size of the detector, the specific application occasion, and the cost performance and other factors. Generally speaking, infrared crystals, especially Ge and ZNS raw materials, are relatively expensive. Therefore, in many designs, designers will use aspheric or diffractive surfaces to reduce the number of lenses while taking into account excellent imaging effects. Since the processing and inspection of aspheric and binary surfaces are also relatively expensive, try not to use multiple aspheric or diffractive surfaces.

SWIR lens

For SWIR lenses, especially extended-band SWIR short-wave infrared lenses, the materials that can be used need to consider factors such as wide-band transmission, machinability, and visible light transmission. Therefore, ZNSE, ZNS, and CaF2 have become common choices. In addition, there are some rare glass.

MWIR lens

MWIR lenses are usually used with medium-wave cooling detectors, and the aperture is placed behind the lens, so the lens is relatively large, and the so-called cold aperture effect (ghosting, reflection, also called cold screen effect) must also be considered. Although the cooling lens and detector are bulky, the detection distance can reach very far. For example, the focal length is 150 mm and 300 mm, and the distance from 10 km to 30 km can be seen.

LWIR lens

The design of LWIR is commercial-oriented, that is, it should be cheap and effective. Therefore, the use of aspheric surfaces is more common. In addition, with the popularization of commercial applications, such as vehicle night vision, gun sights, mobile phones, etc., chalcogenide glass has become the darling of such applications.

Under extremely cold and hot conditions, especially when the temperature difference is large, the curvature of the infrared lens, the thickness of the lens, the lens barrel and the refractive index changes of the lens material will cause the lens to defocus. In order to ensure a clear image, the lens needs to be refocused. Electric or manual focus adjustment is required. In order to eliminate the adverse effects of temperature changes, an athermal design is required. Usually, designers use different optical materials for optical compensation (temperature difference), or use mechanical materials and optical materials. In contrast to the design, optomechanical compensation is performed. There is also an AF automatic focusing design, which is to continuously approach the focal plane through software technology, so as to automatically adjust the focus automatically.

Dual field of view, triple field of view design

Dual field of view and triple field of view designs refer to a set of lenses that can obtain fixed focal lengths at different positions, such as 25 mm, 50 mm, and 75 mm. The optical design of this type of lens only needs to consider the imaging quality at the three focal points, without considering the image quality change during the change process. This is a very practical design. For example, it is easy to find the target at a small focal length of 25 mm, and quickly switch to 50 mm or 75 mm focal length to zoom in and see the target clearly.

Continuous zoom lens

Continuous zoom lens (continuous zoom) can be regarded as the most challenging lens, and the higher the zoom ratio, the more difficult it is. The 20x zoom lens currently on the market comes from a company called Ophir. The lens uses three optical path switching to achieve a continuous zoom effect of 15-300 mm. In addition to the high challenge of optics, the difficulty of mechanical structure is also extremely high.

Reflective design

Reflective designs generally appear on lenses with long focal lengths. This is to reduce the size of the lens (too long), and a reflective lens is added to the optical path for return to shorten the length. Because it is a wide infrared band, the general metal film can achieve the effect of reflection.

Temperature measurement, imaging lens

The design of temperature measurement lens and the design of imaging lens have some different considerations. Due to the need for temperature measurement, the focus of lens design is to reduce reflections or stray light, etc., and the position and size of the aperture will be properly handled. Its relative illuminance uniformity is one of the most important indicators for evaluating the quality of a lens. A good lens design needs to comprehensively consider the effect of optical-mechanical coordination, both of which are indispensable. There are also very good optical-mechanical simulation software on the market, which are designed to improve the effect of optical-mechanical coordination.

It can be said that the mechanical design is very important, and the lightweight design has strict requirements on the vehicle and airborne, not to mention the application in the military. Recently, commercial use of infrared has become more and more widespread. Consumers do not want to carry bulky infrared lenses, and they prefer products that are as light as plastic. Therefore, it is very important to pay attention to the design of infrared optical lenses.