Skinny Lens Makes Cheap Surveillance Camera for Home Use - SYS

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Dark alleys might not feel so dangerous someday thanks to a new

ultra-thin type of lens, which could pave the way to making smaller and

cheaper heat-sensing imagers. A team of French researchers has found a

way to make a thermal infrared (IR) camera with a lens made of silicon,

a much less expensive material than is commonly used today.

The new silicon lens is as thick as a fingernail with a diameter less

than that of a No. 2 pencil. Although its resolution is not superb, the

lens is good enough to reveal the presence of a person and some general

features. The researchers describe their new work, which includes the

development of a working infrared camera, in a paper published today in

The Optical Society’s (OSA)

journal Optics

Letters.

Potential applications for the imager include more affordable

surveillance, particularly for home use, the researchers say.

“It could detect people in a room, on a street corner, or in a dark

alleyway,” said first author Tatiana Grulois, a Ph.D. candidate at the

French aerospace lab ONERA, who developed the prototype with researchers

from the French thermal imaging sensor company ULIS as well as the

Institut d’Optique and the French National Center for Scientific

Research (CNRS). Their new IR camera has a wide field of view—130

degrees—and an f-number of 1.5, meaning it performs well in low-light

conditions. It is effective over a wide range of infrared light

wavelengths, from 7 to 14 micrometers, which are highly sensitive to

differences in temperature.

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.

“In recent years, huge efforts have been made to reduce the price of

uncooled long-wavelength IR detectors,” which were typically thousands

of dollars for the higher-resolution detectors, Grulois said. “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.”

Previously, materials such as silicon could not be used as a lens for

thermal infrared light (of 7 to 14 micrometers) because too much of the

light would be absorbed by the material instead of passing through. But

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

“lighthouse” Fresnel 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 “devastating

chromatic problems” when you try to image multiple wavelengths of light

at the same time. Grulois says 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-degree 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 design has been patented, but further work remains to make it

marketable at a truly affordable price, Grulois said. The silicon lens

was made by an expensive process called direct diamond turning; this

process was used to etch the lens’ rings, whose depth in this design is

50 micrometers, or about half the thickness of a human hair. By finding

cheaper ways to manufacture the lenses—for example, using photo printing

or molding of the appropriate materials—Grulois says her team hopes to

reduce the cost of fabricating the optics and drive down the price of

the whole imager.

Extremely inexpensive low-resolution long wavelength IR sensors are

still new to the market, so a direct comparison to similar imagers is

difficult, Grulois said. Nevertheless, new thermal IR imagers such as

this one will “open completely new and exciting fields of application,”

she said.

The research is funded by the French procurement agency.

Paper: “Extra-thin

infrared camera for low cost surveillance applications,” T. Grulois

et al., Optics Letters, vol. 39, issue 11, pp. 3169-3172

(2014).

EDITOR’S NOTE: Images are available to members of the media upon

request. Contact Angela Stark, astark@osa.org.

About Optics Letters

Published by The Optical Society (OSA), Optics Letters offers

rapid dissemination of new results in all areas of optics with short,

original, peer-reviewed communications. Optics Letters covers the

latest research in optical science, including optical measurements,

optical components and devices, atmospheric optics, biomedical optics,

Fourier optics, integrated optics, optical processing, optoelectronics,

lasers, nonlinear optics, optical storage and holography, optical

coherence, polarization, quantum electronics, ultrafast optical

phenomena, photonic crystals, and fiber optics. This journal, edited by

Xi-Cheng Zhang of the University of Rochester and published twice each

month, is where readers look for the latest discoveries in optics. Visit www.OpticsInfoBase.org/OL.

About OSA

Founded in 1916, The Optical Society (OSA) is the leading professional

society for scientists, engineers, students and business leaders who

fuel discoveries, shape real-world applications and accelerate

achievements in the science of light. Through world-renowned

publications, meetings and membership programs, OSA provides quality

research, inspired interactions and dedicated resources for its

extensive global network of professionals in optics and photonics. For

more information, visit www.osa.org.

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