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Researchers use laser light to form a tiny "optical pinhole" inside a nonlinear crystal, which also turns the infrared image into a visible image that a traditional silicon-based camera sensor can detect. With this setup, the researchers captured clear, wide-depth images without using any lenses, even in very low light. Credit: Kun Huang, East China Normal University
Researchers have used the centuries-old idea of pinhole imaging to create a high-performance mid-infrared imaging system without lenses. The new camera can capture extremely clear pictures over a large range of distances and in low light, making it useful for situations that are challenging for traditional cameras.
"Many useful signals are in the mid-infrared, such as heat and molecular fingerprints, but cameras working at these wavelengths are often noisy, expensive or require cooling," said research team leader Heping Zeng from East China Normal University. "Moreover, traditional lens-based setups have a limited depth of field and need careful design to minimize optical distortions. We developed a high-sensitivity, lens-free approach that delivers a much larger depth of field and field of view than other systems."
Writing in Optica, the researchers describe how they use light to form a tiny "optical pinhole" inside a nonlinear crystal, which also turns the infrared image into a visible one. Using this setup, they acquired clear mid-infrared images with a depth of field of over 35 cm and a field of view of more than 6 cm. They were also able to use the system to acquire 3D images.
"This approach can enhance night-time safety, industrial quality control and environmental monitoring," said research team member Kun Huang from East China Normal University. "And because it uses simpler optics and standard silicon sensors, it could eventually make infrared imaging systems more affordable, portable and energy efficient. It can even be applied with other spectral bands such as the far-infrared or terahertz wavelengths, where lenses are hard to make or perform poorly."
The video shows the mid-infrared imaging system capturing clear images of a resolution test target as it is moved 9 cm away, demonstrating the large depth-of-field capability of the lensless configuration. Credit: Kun Huang, East China Normal University
Pinhole imaging reimagined
Pinhole imaging is one of the oldest image-making methods, first described by the Chinese philosopher Mozi in the 4th century BC. A traditional pinhole camera works by letting light pass through a tiny hole in a lightproof box, projecting an inverted image of the outside scene onto the opposite surface inside. Unlike lens-based imaging, pinhole imaging avoids distortion, has an infinite depth of field and works across a wide range of wavelengths.
To bring these advantages to a modern infrared imaging system, the researchers used an intense laser to form an optical hole, or artificial aperture, inside a nonlinear crystal. Because of its special optical properties, the crystal converts the infrared image into visible light, so that a standard silicon camera can record it.
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