Infrared-compatible lenses

Imaging quality for different wavelength ranges 

The modulation transfer functions reflect the imaging quality over a certain wavelength range of light, whereby the proportions of individual wavelengths are included with different weightings. The smaller the wavelength range under consideration, the better the modulation transfer function usually is, as all chromatic aberrations (changes in image position and image size depending on the wavelength) have less influence. A lens that is to deliver good image quality over a very wide wavelength range must therefore have a very complex design and include special glasses for color correction. 

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This ensures that chromatic errors are kept to a minimum and that the image quality hardly changes noticeably over a relatively wide focusing range (for one object distance), which in turn means that there is practically no need to refocus when moving from one wavelength range to another in order to find the best focusing plane. In practice, this means, for example, that an image does not have to be refocused each time when moving from daylight illumination to artificial light illumination to infrared illumination (when focusing in the infrared). 
 

Utilization of the full sensor sensitivity to increase the image brightness 

The sensitivity of a camera-lens combination is also determined by the irradiance that the lens generates on the image sensor. This depends on the transmission of the lens and its relative aperture, which is indicated by the f-number k. Large relative apertures (small f-numbers) allow use down to low object radiance, i.e. down to low light conditions such as at dusk. Good image quality at very small f-numbers can only be achieved with a great deal of design effort. 

The imaging performance of a lens generally decreases with the f-number. Furthermore, the focus difference increases at different wavelength ranges. With extreme relative apertures (very small f-numbers), this can lead to the wavelength range having to be restricted. In daylight, the infrared component would have to be blocked by an IR cut filter. On the other hand, refocusing would be necessary at the transition to the infrared range (without a filter). 

A lens with high imaging performance over a wide wavelength range can also be used at the maximum relative aperture without an IR cut filter. It provides a greater irradiance and therefore a greater detector signal than a lens with an IR cut filter at the same f-number. The additional gain in irradiance from the IR range can be up to 1.5 or more f-stops. 

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This means, for example, that a lens with a minimum f-number of 1.4, which can be used without an IR cut filter due to its high imaging performance in daylight or artificial light, delivers a greater luminous flux than a lens with an aperture one f-stop larger (k=1.0), which must be used with a filter and which also has a lower overall quality for a comparable design effort.
 

Infrared Lens is a rising technology with a limited range of applications. However, with the trend in security, Infrared Lens will rapidly expand.

Infrared Lens technology

Infrared lens uses special optical glass materials and modern optical design methods to eliminate the focus shift between visible light and infrared light, allowing both types of light to be imaged at the same focus point for a clear image. Currently, Infrared cameras on the market mainly use an infrared filter to switch between day and night modes. During the day, the filter blocks infrared light from reaching the CCD, enabling it to detect only visible light. At night or under poor lighting conditions, the filter stops working, allowing infrared light, reflected from objects, to enter the lens and be imaged by the CCD.

Advantages of Infrared Lens technology

However, commonly a clear daytime image becomes blurry under Infrared conditions. This is because the wavelengths of visible and infrared light differ, causing the focus point to be different which results in a blurry image. The latest optical design methods, special optical glass materials, coatings, and advanced materials eliminate the focus shift between visible and infrared light. The special optical glass material addresses the clarity issue of infrared imaging, allowing light to be focused at the same point for both visible and infrared light. Special coatings ensure that infrared light penetrates the lens while suppressing ghosting and flares in backlit conditions, providing high-quality images with high contrast, even under unfavorable backlight conditions.

Applications of Infrared Lens

Infrared Lens has a wide range of applications. It can be used as a specialized lens for Infrared purposes or as a regular lens. Infrared lenses can be easily synchronized with conventional color cameras, white cameras, and day-to-night switchable cameras, depending on the application. White cameras do not have an infrared stop filter, and the sunlight spectrum contains infrared light, so even under daylight conditions, white cameras can be affected by infrared light. Therefore, using an Infrared Lens can significantly improve image quality.

For more information, please visit Infrared Lens for aerospace.