Study of asymmetric or decentered multi-view designs for uncooled infrared imaging applications.

Multi-view architectures using lens arrays can bring interesting features like 3D or multispectral imagery over single aperture cameras. Combined with super-resolution algorithms, multi-view designs are a way to miniaturize cameras while maintaining their resolution. These optical designs can be adapted for thermal infrared imagery and can thus answer the size, weight and power (SWAP) challenge with advanced imagery functions. However, in this spectral range, the choice of an uncooled microbolometer detector imposes a high numerical aperture for the system which increases the size of the optics and makes difficult a multi-channel arrangement combined with a single focal plane array (FPA). In this paper, we theoretically investigate several asymmetric or decentered multi-view designs that allow both a high aperture for the optical channels and the use of a single FPA for the sub-images. Ray-traced designs will illustrate this study and their image quality will be checked with modulation transfer functions (MTF) for different field points.

Compact multichannel infrared camera integrated in an operational detector dewar cooler assembly.

We present an ultracompact infrared cryogenic camera integrated inside a standard Sofradir's detector dewar cooler assembly (DDCA) whose field of view is equal to 120°. The multichannel optical architecture produces four nonredundant images on a single SCORPIO detector with a pixel pitch of 15 µm. This ultraminiaturized optical system brings a very low additional optical and mechanical mass to be cooled in the DDCA: the cool-down time is comparable to an equivalent DDCA without an imagery function. Limiting the number of channels is necessary to keep the highest number of resolved points in the final image. However, optical tolerances lead to irregular shifts between the channels. This paper discusses the limits of multichannel architectures. With an image-processing algorithm, the four images produced by the camera are combined to process a single full-resolution image with an equivalent sampling pitch equal to 7.5 µm. Experimental measurements on the modulation transfer function and noise equivalent temperature difference show that this camera achieves good optical performance.

Pixel-sized infrared filters for a multispectral focal plane array.

We present a theoretical study of guided-mode-resonance filters made of two sub-wavelength metallic gratings and a dielectric waveguide, with lateral geometries compatible with the size of infrared focal plane array pixels. Contrary to most of the studies described in the literature, we consider here a focused beam on finite-sized filters, and we investigate the optical properties of a mosaic of 30 µm-long filters. We demonstrate the spectral filtering ability and low crosstalk of such components. We also study the impact of an oblique beam onto a filtering mosaic. We finally discuss the opportunities offered by these results for a new generation of multispectral infrared cameras, and we give an example of a simple architecture.

Fabrication of concave and convex potassium bromide lens arrays by compression molding.

A new simple and cost-effective method has been developed for the fabrication of both plano-convex and plano-concave lens arrays with potentially important sag heights. The process is based on the use of potassium bromide (KBr) powder. At ambient temperature and under pressure, KBr powder is compressed on a molding die with the desired shape to form a solid lens array. The quality of the lens arrays has been assessed, and we present the first image produced by a converging KBr lens array.

Compact infrared cryogenic wafer-level camera: design and experimental validation.

We present a compact infrared cryogenic multichannel camera with a wide field of view equal to 120°. By merging the optics with the detector, the concept is compatible with both cryogenic constraints and wafer-level fabrication. The design strategy of such a camera is described, as well as its fabrication and integration process. Its characterization has been carried out in terms of the modulation transfer function and the noise equivalent temperature difference (NETD). The optical system is limited by the diffraction. By cooling the optics, we achieve a very low NETD equal to 15 mK compared with traditional infrared cameras. A postprocessing algorithm that aims at reconstructing a well-sampled image from the set of undersampled raw subimages produced by the camera is proposed and validated on experimental images.

Random phase mask in a filamentation regime: application to the localization of point sources.

We present a optical system with an extended point-spread function (PSF) for the localization of point sources in the visible and IR spectral ranges with a subpixel precision. This compact system involves a random phase mask (RPM) as its unique component. It exhibits original properties, because this RPM is used in a particular regime, called the "filamentation regime," before the speckle region. The localization is performed by calculating the phase correlation between the PSF and the image obtained under off-axis illumination. Numerical simulations are presented to assess the basic optical properties of this RPM in the filamentation regime.

Modulation transfer function measurement of a multichannel optical system.

We present a new method to measure the modulation transfer function (MTF) beyond the Nyquist frequency of a multichannel imaging system for which all the channels have parallel optical axes. Such a multichannel optical system produces a set of undersampled subimages. If the subimages contain nonredundant information, high spatial frequencies are folded between low spatial frequencies, leading to the possible extraction of frequencies higher than the Nyquist frequency. The measurement of the MTF of the multichannel system leads to the estimation of the resolution enhancement of the final image that can be obtained by applying a postprocessing algorithm to the collection of undersampled subimages. Experimental images are presented to validate this method.