Denis Dufour

Resolution capability comparison of infrared and terahertz imagers

Infrared and terahertz are two imaging technologies that differ fundamentally in numerous aspects. Infrared imaging is an efficient passive technology whereas terahertz technology is an active technology requiring some kind of illumination to be efficient. Whats more, the detectors are also different and yield differences in the fundamental physics when integrated in a complete system. One of these differences lies in the size of the detectors. Infrared detectors are typically larger than the infrared wavelengths whereas terahertz detectors are typically smaller than the wavelength of illumination. This results in different constraints when designing these systems, constraints that are imposed by the resolution capabilities of the system. In the past INO has developed an infrared imaging camera core of 1024×768 pixels and tested some microscanning devices to improve its sampling frequency and ultimately its resolution. INO has also engineered detectors and camera cores specifically designed for active terahertz imaging with smaller dimensions (160×120 pixels). In this paper the evaluation of the resolution capabilities of a terahertz imager at the pixel level is performed. The resolution capabilities for the THz are evaluated in the sub-wavelength range, which is not actually possible in the infrared wavebands. Based on this evaluation, the comparison between the resolution limits of infrared detectors and the terahertz detectors at the pixel level is performed highlighting the differences between the wavebands and their impact on system design.

Introducing sub-wavelength pixel THz camera for the understanding of close pixel-to-wavelength imaging challenges

Conventional guidelines and approximations useful in macro-scale system design can become invalidated when applied to the smaller scales. An illustration of this is when camera pixel size becomes smaller than the diffraction-limited resolution of the incident light. It is sometimes believed that there is no benefit in having a pixel width smaller than the resolving limit defined by the Raleigh criterion, 1.22 λ F/#. Though this rarely occurs in todays imaging technology, terahertz (THz) imaging is one emerging area where the pixel dimensions can be made smaller than the imaging wavelength. With terahertz camera technology, we are able to achieve sub-wavelength pixel sampling pitch, and therefore capable of directly measuring if there are image quality benefits to be derived from sub-wavelength sampling. Interest in terahertz imaging is high due to potential uses in security applications because of the greater penetration depth of terahertz radiation compared to the infrared and the visible. This paper discusses the modification by INO of its infrared MEMS microbolometer detector technology toward a THz imaging platform yielding a sub-wavelength pixel THz camera. Images obtained with this camera are reviewed in this paper. Measurements were also obtained using microscanning to increase sampling resolution. Parameters such as imaging resolution and sampling are addressed. A comparison is also made with results obtained with an 8-12 μm band camera having a pixel pitch close to the diffractionlimit.

Case study of concealed weapons detection at stand-off distances using a compact, large field-of-view THz camera

The detection of concealed weapons in crowd situations is a critical need and solutions are being sought after by security agencies at the federal, state and municipal levels. Millimeter waves have been evaluated for these kinds of applications, but the currently available technologies are typically too large and bulky to allow for widespread deployment. Alternatively soft X-rays have been considered but safety issues hinder their acceptance. Terahertz technology is ideally suited for such an application as it has the ability to see through clothing, and offers higher resolution than in the millimeter band, also being more compact. THz photons have lower energy than infrared and do not show the ionizing properties of X-ray radiation. The longer Terahertz waves penetrate deeper into various materials then their visible and infrared counterparts. Though the wavelength is longer it has been shown that high resolution in a small form factor can be obtained in the THz wavebands thanks to the use of small pixel pitch detectors. In this paper, a case study for the use of a compact THz camera for active see-through imaging at stand-off distances is presented. More specifically, the cases of seeing through packages and clothing are analyzed in the perspective of concealed weapons detection. The paper starts with a review of the characteristics of a high resolution THz camera exhibiting small pixel size and large field-of-view. Some laboratory results of concealed object imaging along with details of a concept for live surveillance using a compact see-through imaging system are reviewed.

Towards very high-resolution infrared camera core

In various military, space and civilian infrared applications, there is an important need for fast prototyping. For example, detectors with small pitch compared to the diffraction limited spot radius are now available and their specificities must be studied to optimize the design of the next imaging systems. At the very heart stands a requirement for flexible camera modules that provide a multitude of output formats as well as fast adaptability. Based on this concept, INO has developed an advanced compact camera module IRXCAM that can provide both raw data as well as fully processed images under a variety of outputs: NTSC, DVI, VGA, GigE and Camera Link. This tool can be used to perform a rapid demonstration of concept for a specific application. IRXCAM now supports the bolometric detectors INO IRM160A (160 x 120 52 μm pitch pixels, LWIR and THz), Ulis 04 17 1 (640 x 480 25 μpitch pixels, LWIR) and Ulis 05 25 1 (1024 x 768 17 μm pitch pixels). Reduction of the pixel pitch is a way to improve the compromise between the spatial resolution and the dimensions of an imaging system, mainly by reducing the required optical focal length with constant numerical aperture. Microscanning is another way that provides excellent results in terms of spatial resolution for pixel pitches as small as 25 μm in the LWIR range for F/1 optics. Microscanning also preserves the field of view without increasing the number of pixels of the detector. Finally, microscanning is an efficient way to reduce the aliasing effect of a non unity filling factor, a parameter that becomes increasingly important for small pixels. This paper presents the IRXCAM-1024 camera module, its performances, and its use for microscanning with 17 μm pitch pixels and commercial F/1 and F/0.86 refractive optical lenses.

Reflection imaging in the millimeter-wave range using a video-rate terahertz camera

The ability of millimeter waves (1-10 mm, or 30-300 GHz) to penetrate through dense materials, such as leather, wool, wood and gyprock, and to also transmit over long distances due to low atmospheric absorption, makes them ideal for numerous applications, such as body scanning, building inspection and seeing in degraded visual environments. Current drawbacks of millimeter wave imaging systems are they use single detector or linear arrays that require scanning or the two dimensional arrays are bulky, often consisting of rather large antenna-couple focal plane arrays (FPAs). Previous work from INO has demonstrated the capability of its compact lightweight camera, based on a 384 x 288 microbolometer pixel FPA with custom optics for active video-rate imaging at wavelengths of 118 μm (2.54 THz), 432 μm (0.69 THz), 663 μm (0.45 THz), and 750 μm (0.4 THz). Most of the work focused on transmission imaging, as a first step, but some preliminary demonstrations of reflection imaging at these were also reported. In addition, previous work also showed that the broadband FPA remains sensitive to wavelengths at least up to 3.2 mm (94 GHz). The work presented here demonstrates the ability of the INO terahertz camera for reflection imaging at millimeter wavelengths. Snapshots taken at video rates of objects show the excellent quality of the images. In addition, a description of the imaging system that includes the terahertz camera and different millimeter sources is provided.

Subwavelength imaging challenges in the infrared and THz wavebands

Subwavelength imaging has recently seen increased interest in multiple fields. There are various applications and distinct contexts for performing subwavelength imaging. The technological ways to proceed as well as the benefits obtained are as various as the applications foreseen. To benefit from subwavelength imaging a way around standard imaging procedure is often required. INO is also involved in this activity mainly for the infrared and the THz wavebands. In the infrared band a detector with 17 um pixel pitch, larger than the pixel, was used in conjunction with a microscanning device to oversample the image at a pitch much smaller than the wavelength. In this case the pixel size is in the order of the wavelength but the sampling is at subwavelength level. In the THz band a 35 um pixel pitch is used at wavelength ranging from 70 um to 1,063 mm to perform imaging through various objects. In this case, the pixel itself is smaller than the wavelength. Subwavelength imaging is not without its challenges, though. For instance, while the use of ultra-fast optics provides better definition, their design becomes more challenging as the models used are at their very limits. Questions about information content of images can be raised as well. New research avenues are being investigated to help address the challenges of subwavelength imaging with the goal of achieving higher imaging system performance. This paper discusses aspects to be considered, review some results obtained and identify some of the key issues to be further addressed.

Portable LWIR hyperspectral imager based on MEMS Fabry-Perot interferometer and broadband microbolometric detector array

The fast growing consumer electronics market of connected and wearable devices is driving a wealth of new applications. Personal capability for detecting and monitoring substances part of our everyday life (food, cosmetics, drugs, etc.) by spectroscopic means will soon become a reality as a number of new miniature spectrometers are being reported. These devices mostly operate in the visible and near infrared spectral region due to the readily available lowcost detectors in these spectral regions. However, enhanced selectivity is achievable in the molecular fingerprint spectral region (7-20 μm), allowing for applications that would be difficult or impossible at lower spectral wavelengths. To this end, a compact, portable, Long-Wave Infrared (LWIR) hyperspectral imager was developed. It is based on INO’s MICROXCAM-384 camera featuring a 384 x 288 pixel, 35 μm pitch uncooled bolometric broadband Focal Plane Array (FPA) and Fraunhofer ENAS’ 2 mm x 2 mm aperture MEMS tunable Fabry-Pérot Interferometer (FPI). The INO’s broadband FPA exhibits a Noise Equivalent Temperature Difference (NETD) lower than 25 mK (for the 8-12 μm range at 300 K, 50 fps and f/1) and a flat spectral response from 3 to 14 μm. The footprint of the hyperspectral imager is 7 cm x 8 cm x 10 cm excluding the source. The spectral resolution varies from 55 to 220 nm depending on the type of FPI used. The Noise Equivalent Spectral Radiance (NESR) is 430 mW/(m2 .sr.μm) at 9 μm. Using this hyperspectral imager, spectra of various substances including polymers were recorded in the transmission, reflection and transflectance configurations. A good agreement was found with spectra obtained by applying the FPI transfer function to spectra recorded with a commercial FTIR spectrometer. The LWIR configuration of the imaging spectrometer will be described and test results presented.

A low resource imaging radiometer for nanosatellite based fire diagnosis

Details of a multispectral imaging radiometer specially designed to retrieve fire characteristics from a nanosatellite platform are presented. The instrument consists of an assembly of three cameras providing co-registered midwave infrared, longwave infrared, and visible image data. Preliminary evaluation of the instrument budgets showed approximately a mass of 12 kg, an envelope of 220×240×200 mm3, and an average power consumption of 13 W. A method was devised to stagger two linear arrays of 512×3 VOx microbolometers in each infrared detector assembly. Investigation of the first completed detector assemblies showed an alignment accuracy better than 10% of pixel pitch and a response uniformity achieved across 92% of the pixels. Effects of the thermal environment seen by the pixels were evaluated to optimize the radiometric packaging design. It was found that the resulting thermal stability of the arrays, combined with the available electronic dynamic range, allows acquisition of targets with temperatures as high as 750 K with the desired accuracy and without saturation. The detector assemblies were able to withstand extreme environments with vibration up to 14 grms and temperatures from 218 to 333 K. Exposing the assembly’s window and bandpass filter to proton and Co-60 gamma radiation with successive dose of 10 krad and 100 Gy resulted in no adverse effect on their transmittance characteristics. Performance characteristics of the assembled midwave and longwave infrared telescopes were consistent with modeling predictions. Results of the point spread function measurement supported the conclusion that the lenses alignment had been achieved within mechanical tolerances for both telescopes.

Customized packaged bolometers in niche applications at INO

This paper reviews recent developments in customized packaged bolometers at INO with an emphasis on their applications. The evolution of INOs bolometer packages is also presented. Fully packaged focal plane arrays of broadband microbolometers with expanded absorbing range are shown, for applications in spectroscopic and THz imaging. This paper also reports on the development of customized packaged bolometer focal plane arrays (FPAs) for space applications such as a multispectral imaging radiometer for fire diagnosis, a far infrared radiometer for in-situ measurements of ice clouds and a net flux radiometer for Mars exploration.

Novel vacuum packaged 384×288 broadband bolometer FPA with enhanced absorption in 3-14μm wavelength

This paper reports on the development of a fully packaged focal plane array of broadband microbolometers. The detector makes use of a gold black thin film to expand its absorption range from 3 to 14 μm. A low temperature packaging process was developed to minimize sintering of the gold black absorber during vacuum sealing of the bolometer array package. The gold black absorber was also laser trimmed to prevent lateral diffusion of heat and promote a better MTF. The resulting FPAs show a NETD lower than 25 mK at a frame rate of 50 Hz