Silicon infrared lens to lower cost of thermal imaging

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A team of French researchers has found a way to make a thermal infrared camera with a lens made of silicon, a much less expensive material than is commonly used today.

Although its resolution is not superb, the lens is good enough to reveal the presence of a person and some general features. Potential applications for the imager include more affordable surveillance, particularly for home use, the researchers say.

‘In recent years, huge efforts have been made to reduce the price of uncooled long-wavelength IR detectors,’ said first author Tatiana Grulois, a PhD candidate at the French aerospace lab Onera, who developed the prototype with researchers from the French thermal imaging sensor company Ulis, the Institut d’Optique, and the French National Center for Scientific Research (CNRS).

Grulois continued: ‘Thanks to the low thickness of the lens, this work paves the way to inexpensive unconventional materials that are not traditionally used because of their absorption, but which could potentially be massively replicated at low cost.’

Their new IR camera has a 130° field of view and an f-number of 1.5, and is effective over a wide range of infrared light wavelengths, from 7 to 14µm.

The authors say their thin lens design could be a breakthrough in lowering the cost of thermal infrared camera lenses by using materials that are cheaper than traditional ones such as germanium and chalcogenide.

Previously, materials such as silicon could not be used as a lens for thermal infrared light of the wavelengths Grulois is interested in because too much of the light would be absorbed by the material instead of passing through. Researchers reasoned that if they could make the silicon lens thin enough, it would no longer be opaque to thermal IR light. The French team decided to test whether changing the style of optics would allow them to make a thinner lens using silicon.

To make their lens thinner, the researchers turned to a type of lens called a Fresnel lens. One style of Fresnel lens is familiar to many for its frequent use in lighthouses and theatrical lighting. These lenses consist of a material arranged in concentric rings, similar in shape to an array of ring-shaped prisms. By bending the light using the rings of material, the light can be focused using a much thinner lens than a conventional lens would need to be.

Another style, called a diffractive Fresnel lens, is praised for its high-quality imaging applications, although it suffers from chromatic problems when imaging multiple wavelengths of light at the same time.

Grulois stated that her team’s new lens is an intermediate between the lighthouse and diffractive Fresnel lens styles.

The researchers found that by increasing the depth of the lens’ concentric rings and decreasing their number, they could reduce the diffractive lens’ chromatic effects to a tolerable level. In tests of the image quality of their prototype, the team found that the image brightness and sharpness uniformity was good for a 130° ultra-wide field of view imager. They also demonstrated that their camera can provide usable images of people inside an office, for example, after commonly used digital post-processing corrections for non-uniformity and distortion.

The research was unveiled in a paper published in The Optical Society’s (OSA) journal Optics Letters.

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