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Defense & Security

Lensless infrared microcameras

Configuring an IR focal-plane array in a 1cc cold chamber pierced by a small pinhole enables a tiny thermal imager without optics.
27 May 2010, SPIE Newsroom. DOI: 10.1117/2.1201005.002878

There is an emerging demand for compact IR instruments, imagers and/or spectrometers, integrated on ground or air vehicles for security and surveillance applications. However, designing compact or ‘micro’ IR systems is challenging. Certain technological barriers have already been overcome, leading to the development of high-performance IR focal-plane arrays (IRFPAs) with large (megapixel) formats. These are packaged in handheld configurations—called IR-detector dewar-cooler assemblies (IDDCAs)—that contain the cold detector chip and its immediate neighborhood, a dewar envelope, and a cooler. Ultimately, it would be ideal if the entire IR system could be reduced to the size of IDDCAs (approximately 10 × 10 × 10cm3).

For this, fresh ideas are needed, however. For several years, the immediate neighborhood of IRFPAs was inaccessible for the optical engineer, because adding optical elements in the dewar could have disturbed the carefully developed IDDCA architecture. This risk can be overcome if we integrate simple optical designs that are highly compatible with existing IDDCAs. New techniques for characterizing IRFPA response have inspired novel IRFPA-based microsystem architecture. By understanding nonuniform and spatial responses of pixels, we have developed lensless imaging techniques and a camera without optics.

First, we have developed a way to monitor the spectral responses of all pixels simultaneously. When an IRFPA exhibits nonuniform responses, our hyperspectral study can reveal the origins of these nonuniformities.1,2 In particular, we have shown that an IRFPA with a residual prismatic plate in its structure can produce interference fringes that encode the spectral content of the illuminating source, leading to a built-in Fourier-transform spectrometry function (FTIR-FPA).3 Second, we have invented new techniques for measuring the spatial responses of the IRFPA's pixels: the principle is to project periodic arrays of subpixel patterns (lines or spots) using the self-imaging properties of specific diffraction gratings positioned at several millimeters from the IRFPA.4

We have also used these lensless imaging techniques to define new architectures of microcameras. Our novel FTIR-FPA structure is entirely compatible with existing mercury cadmium telluride (HgCdTe) IRFPA technology. At the end of the manufacturing process of a classic HgCdTe IRFPA, the cadmium zinc telluride substrate is usually thinned. Instead of removing the substrate almost completely, we leave a prismatic plate with a small wedge angle. Reflections occur because of abrupt changes in the refractive index at the interfaces, such that an interference pattern builds up inside the wedge. We have realized several components departing from a bulk (unthinned) FPA of TV/4 format (320 × 256 pixels of pitch 30μm). Figure 1(a) shows an example component and Figure 1(b) displays an interferogram produced by the structure.

Figure 1. (a) Fourier-transform IR focal-plane array (IRFPA) component realized by the Electronics and Information Technology Laboratory of the French Atomic Energy and Alternative Energies Commission (CEA LETI) and (b) typical image produced by this IRFPA. The image is an interferogram that encodes the spectral content of the illuminating source. (Credit: French Atomic Energy and Alternative Energies Commission, CEA.)

Designing a camera without optics is the dream of an optical engineer adept at minimalist design. In fact, the mother of the modern camera is a ‘camera obscura,’ a dark chamber pierced by a small pinhole. In an IDDCA, the IRFPA is placed inside a cold shield to minimize background emission. We made a small aperture in the cold shield to get an IR camera obscura5 that can be used for applications requiring a large field of view (up to 180°: see Figure 2). Of course, this simple setup has poor radiometric performance, but it is the father of more optimized configurations: we can replace the pinhole by a circular diffraction grating or add a small lens in the chamber to imitate the structure of the human eye.

Figure 2. (a) Design of an IR pinhole camera compatible with the classic packaging of a cooled IRFPA and (b) a sample image produced by this camera. (Credit: French Aerospace Lab.)

Dewar-level integration of optics is a promising way to develop compact IR cameras from IRFPAs. In collaboration with colleagues at the French Atomic Energy and Alternative Energies Commission (CEA) and the Institute of Optics, we have already demonstrated proof-of-concept microspectrometers and microcameras at the laboratory level. The next step is to develop IR detector optics in a dewar-cooler assembly at an industrial level to demonstrate a real gain in compactness and reliability and open the way for other security applications for cooled IR detectors.

Nicolas Guérineau
French Aerospace Lab (ONERA)
Palaiseau, France

Nicolas Guérineau received an engineering degree from the Optics Institute (Orsay, France) in 1995 and a PhD from Paris XI University (France) in 1999. He has been in charge of ONERA research projects on optics integrated with IRFPAs since 2003.

Guillaume Druart, Sylvain Rommeluère, Jérôme Primot, Joel Deschamps
Theoretical and Applied Optics Department
Palaiseau, France

Guillaume Druart received his PhD in 2009 and is a research scientist in optical design. He is working on new designs for microcameras in the IR spectral range and is also interested in nonconventional optical designs, diffractive optics, multichannel designs, co-design with image processing, and spectro imagery.

Sylvain Rommeluère received his PhD in 2007 and researches optical design. He is working on the characterization of detectors, and on new spectrometer and microspectrometer designs in the IR range.

Jérôme Primot received an engineering degree from the Optics Institute Graduate School in 1985 and a doctoral degree from Paris XI University in 1989, focusing on deconvolution of wavefront sensing in high-angular-resolution astronomical imagery. He holds several patents in the field of optical testing.

Joël Deschamps received his PhD in physics from Franche-Comté University (Besançon, France) in 1982. Since 1990, he has been an expert for the French Armament Delegation on the optronic part of various missiles and airborne programs.

Manuel Fendler
Department of Optronics
Information Technology Laboratory
French Atomic Energy and Alternative Energies Commission
Grenoble, France

Manuel Fendler received his PhD degree from Lille University (France) in microelectronics for his work on optoelectronic components for radio-over-fiber telecommunications systems. He has more than 10 years of experience in research and development, and transfer to manufacturing of opto-electronic products, with a main focus on packaging technology.