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Optical Correction of Infrared Lenses

There are two meanings of the term "镜头" in the field of film and television: one refers to the optical component of movie cameras and projectors used to generate images, which is composed of multiple lenses. Different types of lenses have different shapes and characteristics, and their application in cinematography constitutes a means of optical expression. The other meaning refers to a continuous shot taken from start to finish or a segment between two editing points, also called a "镜头" in Chinese. These two meanings are completely different concepts. To distinguish between the two, the optical component is often referred to as a "光学镜头" (optical lens), and the shot is referred to as a "镜头画面" (shot frame). The main function of a lens is to collect the reflected light from the object being photographed and focus it on the CCD. The image projected onto the CCD is inverted, but the camera circuit has the function of reversing it, and its imaging principle is the same as that of the human eye.

Understanding Infrared Lenses

The lenses of ordinary cameras are usually made of optical glass, and their main function is to refract light (in simple terms, to enlarge objects). Optical glass is generally artificially synthesized. Infrared lenses are typically made of germanium glass. This type of glass has a high refractive index. It is only transparent to infrared light and opaque to visible light and ultraviolet light, so it can distinguish between animals and plants in very dark environments. The lenses of ordinary cameras are made of optical glass, and their main function is to refract light, or enlarge objects. Optical glass is generally artificially synthesized.

Compared to the lenses used in ordinary cameras, infrared lenses use rare elements such as germanium that are dispersed in small quantities. Although the natural reserves of germanium are not low, the extraction of high concentrations of germanium is extremely difficult, so the production cost of germanium lenses is higher and their market price is often higher than that of ordinary camera lenses.

Understanding the Optical Correction of Infrared Lenses

In general, a single spherical lens cannot correct chromatic aberration. Positive lenses produce negative chromatic aberration, while negative lenses produce positive chromatic aberration. In order to reduce chromatic aberration, glass combinations with different refractive indices and chromatic aberrations are used to eliminate chromatic aberration. A double-lens group or double-bonded lens group composed of positive and negative lenses with appropriate shapes is a simple structure that can eliminate chromatic aberration. This method of optical correction is commonly used in infrared lenses.

After optical correction, due to the reduction of dispersion, the separation of the refraction of infrared light and visible light is reduced, and the distance between the focusing planes is narrowed (but cannot be eliminated). For lenses in the short focal length range, after optical correction, the deviation of the infrared and visible light focal planes is small, and the impact on image clarity is small. However, as the focal length increases, dispersion will gradually become magnified, and the deviation of the infrared and visible light focal planes will become larger. Optical correction cannot simultaneously achieve clear imaging for both types of light.

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