In recent years the European Space Agency (ESA) has been pursuing studies dedicated to Earth imaging from space in the Long Wave Infrared region for applications ranging from monitoring of evapotranspiration, and water resources management to the development of urban heat island and monitoring of high temperature events. Among the various solutions being studied is also that of a low cost instrument with moderate needs in terms of resources. . One potential enabler for such type of mission could be the technology of microbolometer detectors. The latest generation of microbolometer arrays now available offer large formats (XGA) and small pixel sizes which are favourable for keeping the instrument size within reasonable limit while addressing larger swath compared to VGA format. A major concern however, in using commercial microbolometers in space is their ability to sustain the radiation environment of space but also the harsh mechanical environments. COTS microbolometers are potentially susceptible to SEE (single even effects) because of the use of commercial CMOS technology/libraries and no implementation of specific design rules (i.e. space tailored rad hardened). In the past, and in the context of their national program, CNES has performed a space evaluation of COTS microbolometer arrays of 640x480 with 25 μm pitch[3]. Despite successful gamma irradiations and vibration tests; degradation of the ROIC has been evidenced during the heavy ions tests, which makes the full qualification of COTS microbolometers for future space programmes mandatory. Similar tests have been performed on an even earlier device (384x288 with a pitch of 35 μm) under the ESA EarthCARE programme[2]. ESA and Thales Alenia Space have recently run an activity with the objective to validate a third-generation COTS microbolometer offered by ULIS (France) against the relevant environment for a candidate Thermal InfraRed (TIR) space mission. The micro-bolometer selected is the PICO 1024E[1], which offers 1024x768 pixels of size 17 μm square. The validation sequence included the main types of irradiation tests required by a space application as well as vibration and shock tests. Ageing tests are included and synergetic effects are also investigated. The detector performances were tested before, after and during any test sequence. In this paper, the results of this activity achieved in the beginning of 2017 are reported.
Silicon-based vacuum packaging is a key enabling technology for achieving affordable uncooled Infrared Focal Plane Arrays (IRFPA) required by a promising mass market that shows momentum for some extensive consumer applications, such as automotive driving assistance, smart presence localization and building management. Among the various approaches studied worldwide, CEA, LETI in partnership with ULIS is committed to the development of a unique technology referred to as PLP (Pixel Level Packaging). In this PLP technology, each bolometer pixel is sealed under vacuum using a transparent thin film deposition on wafer. PLP operates as an array of hermetic micro caps above the focal plane, each enclosing a single microbolometer. In continuation of our on-going studies on PLP for regular QVGA IRFPAs, this paper emphasizes on the innate scalability of the technology which was successfully demonstrated through the development of an 80 × 80 pixel IRFPA. The relevance of the technology with regard to the two formats is discussed, considering both performance and cost issues. We show that the suboptimal fill factor inherent to the PLP arrangement is not so critical when considering smaller arrays preferably fitted for consumer applications. The discussion is supported with the electro-optical performance measurements of the PLP-based 80×80 demonstrator.
In order to evaluate the impact of technological evolutions on the spectral responsivity of microbolometer FPAs (Focal Plane Arrays) as well as to find out a way to estimate the mechanical stability of microbolometric pixel membranes, ULIS is proposing a new method to measuring the spectral response of the detector array over a large region (area of pixels) simultaneously. This is done by tweaking the standard protocol of a commercial FTIR (Fourier Transform InfraRed) spectrometer where the IR detector is replaced by the array to be measured. All the calculations (i.e. interferogram processing) are taken care of externally. We use this new set up to measure the angular spectral response of the detector array and to analyse the relationship between spectral response and mechanical behaviour of the pixel. Firstly the setup of this measurement is presented and some preliminary technical issues are outlined. Then we focus on the results obtained from the measurements on 17μm pitch pixels over a wide range of angles of incidence (from normal to 45° incidence). Finally, we share some theoretical insights on both those results and the inherent limitations of this protocol using a simple optical cavity model.
In uncooled LWIR microbolometer imaging systems, temperature fluctuations of FPA (Focal Plane Array) as well as lens and mechanical components placed along the optical path result in thermal drift and spatial non-uniformity. These non-idealities generate undesirable FPN (Fixed-Pattern-Noise) that is difficult to remove using traditional, individual shutterless and TEC-less (Thermo-Electric Cooling) techniques. In this paper we introduce a novel single-image based processing approach that marries the benefits of both statistical scene-based and calibration-based NUC algorithms, without relying neither on extra temperature reference nor accurate motion estimation, to compensate the resulting temperature-dependent non-uniformities. Our method includes two subsequent image processing steps. Firstly, an empirical behavioral model is derived by calibrations to characterize the spatio-temporal response of the microbolometric FPA to environmental and scene temperature fluctuations. Secondly, we experimentally establish that the FPN component caused by the optics creates a spatio-temporally continuous, low frequency, low-magnitude variation of the image intensity. We propose to make use of this property and learn a prior on the spatial distribution of natural image gradients to infer the correction function for the entire image. The performance and robustness of the proposed temperature-adaptive NUC method are demonstrated by showing results obtained from a 640×512 pixels uncooled LWIR microbolometer imaging system operating over a broad range of temperature and with rapid environmental temperature changes (i.e. from –5°C to 65°C within 10 minutes).
a-Si (amorphous Silicon) microbolometer FPAs (Focal Place Arrays) with TEC-less (without Thermo-Electric Cooler)
and shutterless capabilities have become the technology of choice for low cost, high resolution and low SWaP (Size,
Weight and Power) uncooled LWIR (Long Wave Infrared) cameras used in mobile applications. Over the past 10 years,
a-Si microbolometric FPAs have seen a steady reduction in pixel pitch from 45μm to 17μm as well as an increase in
pixel count from 160x120 to 1024x768. Next-generation arrays are projected to feature 12μm pixel pitch and resolution
up to 1440x1080. However, microbolometer technology scaling has detrimental effects on pixel performance and the
imaging system's optical complexity, which does not always yield a better infrared image quality. In this paper, we
describe, from an information-theoretic perspective, the benefits of using computational imaging technologies and more
specifically pupil function engineering to compensate for the optical resolution and noise sensitivity problems caused by
shrinking pixel geometry in microbolometer FPAs. Computational imaging is a developing field in which the image
acquisition process is shared between the optics and post-capture digital processing (cf. encoding-decoding scheme).
In this paper we present an approach to obtain an extended Depth-of-Field (DoF) for cell phone miniature camera by
jointly optimizing optical system and post-capture digital processing techniques. Using a computational imaging
approach, we demonstrate how to increase, to a useful operating range, the effective DoF of a specifically designed fixed
focus lens operating e.g. at f/2.8. This is achieved with a lens design where the longitudinal chromatic aberration has
been increased. This increase is controlled so as to have, for any distance within the extended DoF, at least one colour
channel of a RGB image which contains the in-focus scene information (e.g. high frequencies). By determining the
sharpest colour (for each region in the digital image) and reflecting its sharpness on the others, we show that it is possible
to get a sharp image for all colours through the merged DoF of the three of them. We compare our technique with other approaches that also aimed to increase the DoF such as Wavefront coding.
A general trend in the CMOS image sensor market is for increasing resolution (by having a larger number of pixels)
while keeping a small form factor by shrinking photosite size. This article discusses the impact of this trend on some of
the main attributes of image quality. The first example is image sharpness. A smaller pitch theoretically allows a larger
limiting resolution which is derived from the Modulation Transfer Function (MTF). But recent sensor technologies
(1.75μm, and soon 1.45μm) with typical aperture f/2.8 are clearly reaching the size of the diffraction blur spot. A second
example is the impact on pixel light sensitivity and image sensor noise. For photonic noise, the Signal-to-Noise-Ratio
(SNR) is typically a decreasing function of the resolution. To evaluate whether shrinking pixel size could be beneficial to
the image quality, the tradeoff between spatial resolution and light sensitivity is examined by comparing the image information capacity of sensors with varying pixel size. A theoretical analysis that takes into consideration measured and predictive models of pixel performance degradation and improvement associated with CMOS imager technology scaling, is presented. This analysis is completed by a benchmarking of recent commercial sensors with different pixel
technologies.
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