Focal Plane Array Camera Research Articles

Water-film thickness imaging based on time-multiplexed near-infrared absorption with up to 500 Hz repetition rate.

We demonstrate high-repetition-rate imaging of the liquid-film thickness in the 50-1000µm range resulting from impinging water droplets on a glass surface. The pixel-by-pixel ratio of line-of-sight absorption at two time-multiplexed near-infrared wavelengths at 1440 and 1353nm was detected with a high-frame-rate InGaAs focal-plane array camera. Frame rates of 1kHz and thus measurement rates of 500Hz could be achieved, well suited to capture the fast dynamics of droplet impingement and film formation. The droplets were sprayed onto the glass surface using an atomizer. Suitable absorption wavelength bands for water droplet/film imaging were determined from Fourier-transform infrared (FTIR) spectra of pure water between 298 and 338K. At 1440nm, the water absorption is nearly temperature-independent, making the measurements robust against temperature fluctuations. Time-resolved imaging measurements capturing the dynamics of the water droplet impingement and evolution were successfully demonstrated.

An experimental study on terahertz light field data acquisition and digital refocusing

In recent years, terahertz imaging has attracted great attention due to its advantages including penetrability and nondestructive property. The field imaging technology within the terahertz range is expected to enhance the terahertz image quality and improve its application effect. In this paper, an experiment on the data acquisition and digital refocusing of the terahertz light field was conducted. Firstly, the basic principle, system structure, and the method of reconstructing light field imaging were analyzed. Secondly, the terahertz focal plane array camera was used to collect the data about light field and digital refocusing was used to get the computed imaging. Finally, the reconstructed image was enhanced to obtain higher depth resolution, angle resolution, and object contour resolution. Experimental results showed the feasibility and ability of terahertz light field imaging to improve image quality and enrich retrieval effects.

Imaging high-speed moving targets with a single-pixel detector.

Single-pixel imaging (SPI) has recently been intensively studied as an alternative to the traditional focal plane array (FPA) technology. However, limited by the refresh rate of spatial light modulators (SLM) and inherent reconstruction mechanism, SPI is inappropriate for high-speed moving targets. To break through this limitation, we propose a novel SPI scheme for high-speed moving targets. In our scenario, the spatial encoding for the target is done by the movement of the target relative to a static pseudo-random illumination pattern. In this process, a series of single-pixel signals are generated that corresponds to the overlap between the target and certain parts of the illumination structure. This correspondence can be utilized for image reconstruction in the same way as normal SPI. In addition, compressive sensing and deep learning algorithms are used for reconstruction, respectively. Reasonable reconstructions can be obtained with a sampling ratio of only 6%. Experimental verification together with theoretical analysis has shown that our scheme is able to image high-speed moving targets that could be alternatively achieved by a fast FPA camera. Our scheme keeps the inherent advantages of SPI and meanwhile extend its application to moving targets. It is believed that this technology will have wide application in many situations.

Laser Characteristics for VCSELs for 77 K and 4 K Optical Data Applications

Small-sized vertical-cavity surface-emitting laser (VCSEL) offer the possibility of very low power consumption along with high reliability for cryogenic data transfer. Cryogenic data transfer has applications in focal plane array cameras operating at 77 K, and at the lower temperature of 4 K for data extraction from superconducting circuits. A theoretical analysis is presented for 77 K and 4 K operation based on small cavity, oxide-free VCSEL sizes of 2 to $6~\mu \text{m}$ , that have been shown to operate efficiently at room temperature. Temperature dependent operation for optimally-designed VCSELs are studied by calculating the response of the laser at 77 K and 4 K to estimate their bias conditions needed to reach modulation speed for cryogenic optical links. The temperature influence is to decrease threshold for reducing temperature, and to increase differential gain for reducing temperature. The two effects predict very low bias currents for small cavity VCSELs to reach needed data speed for cryogenic optical data links. Changing the number of top-mirror pairs has also been studied to determine how cavity design impacts speed and bit energy. Our design and performance predictions paves the way for realizing highly efficient, ultra-small VCSEL arrays with applications in optical interconnects.

Radiometric Calibration of Digital Counts of Infrared Thermal Cameras

Infrared thermal cameras have applications in a range of fields including security, agriculture, inspection, and health. In this paper, we are interested in their sensing properties, where we model and estimate the camera parameters required to transform digital counts to temperature values. Specifically, we propose a method to characterize the spatial deviation for the irradiance observed at the image plane for an uncooled focal-plane array camera deprived of an internal measurement of temperature. In our approach, we establish the relationship between the radiance emitted by a temperature varying blackbody chamber and the uncooled camera digital output. The model approximates this relationship as a quadratic polynomial whose variations are themselves approximated with another polynomial expression. Our results suggest a significant improvement over the commonly used Plancklike expression.

Water film thickness imaging based on time-multiplexed near-infrared absorption.

We demonstrate the imaging of the thickness of liquid water thin films in the 100-1500 µm range at a constant temperature by monitoring the pixel-by-pixel ratio of absorbance at two near-infrared (NIR) wavelengths near 1400 nm detected with a fast framing InGaAs focal-plane array camera. Experiments were performed in reflection mode with films of pure water and water/ethanol mixtures supported on opaque surfaces using two illumination-detection configurations. One scheme uses specular reflection of incident and reflected linearly polarized diode-laser light at Brewster's angle, which enables detection of signal light that has twice traversed the liquid film with negligible interference from unwanted partial reflections of the incoming beams at the front surface interfaces (air/window and window/water for films constrained by a cover plate or air/water for free-standing films). The second configuration located the detection camera perpendicular above the surface where the detected light was transmitted through the sample and diffusely scattered from the support surface. Imaging measurements of film thickness using both configurations were successfully demonstrated. Time-resolved measurements capture the dynamics of flowing water films or waves generated by droplet impingement.

Quantitative thermal imaging using single-pixel Si APD and MEMS mirror.

Accurate quantitative temperature measurements are difficult to achieve using focal-plane array sensors. This is due to reflections inside the instrument and the difficulty of calibrating a matrix of pixels as identical radiation thermometers. Size-of-source effect (SSE), which is the dependence of an infrared temperature measurement on the area surrounding the target area, is a major contributor to this problem and cannot be reduced using glare stops. Measurements are affected by power received from outside the field-of-view (FOV), leading to increased measurement uncertainty. In this work, we present a micromechanical systems (MEMS) mirror based scanning thermal imaging camera with reduced measurement uncertainty compared to focal-plane array based systems. We demonstrate our flexible imaging approach using a Si avalanche photodiode (APD), which utilises high internal gain to enable the measurement of lower target temperatures with an effective wavelength of 1 µm and compare results with a Si photodiode. We compare measurements from our APD thermal imaging instrument against a commercial bolometer based focal-plane array camera. Our scanning approach results in a reduction in SSE related temperature error by 66 °C for the measurement of a spatially uniform 800 °C target when the target aperture diameter is increased from 10 to 20 mm. We also find that our APD instrument is capable of measuring target temperatures below 700 °C, over these near infrared wavelengths, with D* related measurement uncertainty of ± 0.5 °C.

Proper orthogonal decomposition preprocessing of infrared images to rapidly assess stress-induced heat source fields

An image processing technique using the proper orthogonal decomposition (POD) of infrared thermal data was developed to improve the speed of assessment of 2D heat source fields accompanying mechanical transformation. This method involved the generation of a reduced orthonormal basis to approximate thermal fields prior to heat source estimation. The robustness of the method was first assessed using a penalising benchmark test. This test involved artificially setting several tricky situations that arise in practice (high diffusivity, low signal-to-noise ratio, complex heat source distribution, etc.). Application of the method to several experimental temperature fields obtained by an infrared focal plane array camera is then presented. The error between the POD approximated solution in terms of heat sources and a reference solution, computed via a local least squares fitting method, was found to be negligible, thus confirming the efficiency and advantages of the POD preprocessing technique – the method enabled us to obtain a reliable estimate of heat sources while drastically reducing the computation cost in terms of CPU time.

Alternative thermal calibrations of focal plane array infrared cameras

In the past decades, the FPA technology has completely and definitively supplanted the scanning infrared cameras; however, if these devices allow better spatial resolution, they induce several calibration problems due to the non-uniformity of the detectors array. For several applications considering the temperature field and not only a mean value (e.g. to realise maps of dissipative sources in thermomechanical engineering, to determine local thermophysical properties…), one can not only consider the global calibration proposed by the manufacturers, and some home-made specific procedure must be developed. Moreover, these built-in calibration are often associated to Non-uniformity correction (NUC) and Bad pixel replacement procedures, that induce a local correlation between pixels, that is incompatible with any data treatment including spatial derivatives (gradient, Laplacian); then, pixel to pixel calibration is the only solution for these kind of applications. In order to help the users needing such a measurement quality to choose how to calibrate their Focal Plane Array camera, different methods for calibration are proposed and tested in this paper. A comparison between these different calibrations and the standard built-in calibration including a non-uniformity correction (NUC) has been realized, and it has been shown that, in each configuration, a pixel to pixel calibration gives better results that global one.

Challenges in using an analog uncooled microbolometer thermal camera to measure crop temperature

Abstract: It has been long known that thermal imaging may be used to detect stress (e.g. water and nutrient deficiency) in growing crops. Developments in microbolometer thermal cameras, such as the introduction of imaging arrays that may operate without costly active temperature stabilization, have vitalized the interest in thermal imaging for crop measurements. In this study, we have focused on the challenges occurring when temperature stabilization is omitted, including the effects of focal-plane-array (FPA) temperature, camera settings and the environment in which the measurements are performed. Further, we have designed and tested models for providing thermal response from an analog LWIR video signal (typical output from low-cost microbolometer thermal cameras). Finally, we have illustrated and discussed challenges which typically occur under practical use of thermal imaging of crops, by means of three cereal showcases, including proximal and remotely based (UAV) data acquisition. The results showed that changing FPA temperature greatly affected the measurements, and that wind and irradiance also appeared to affect the temperature dynamics considerably. Further, we found that adequate settings of camera gain and offset were crucial for obtaining a reliable result. The model which was considered best in terms of transforming video signals into thermal response data included information on camera FPA temperature, and was based on a priori calibrations using a black-body radiation source under controlled conditions. Very good calibration (r2>0.99, RMSE=0.32°C, n=96) was obtained for a target temperature range of 15-35°C, covering typical daytime crop temperatures in the growing season. However, the three showcases illustrated, that under practical conditions, more factors than FPA temperature may need to be corrected for. In conclusion, this study shows that thermal data acquisition by means of an analog, uncooled thermal camera may represent a possible, cost-efficient method for the detection of crop stress, but appropriate corrections of disturbing factors are required in order to obtain sufficient accuracy. Keywords: analog thermal camera, uncooled microbolometer, canopy temperature, cereals, UAV DOI: 10.3965/j.ijabe.20140704.007 Citation: Kusnierek K, Korsaeth A. Challenges in using an analog uncooled microbolometer thermal camera to measure crop temperature. Int J Agric & Biol Eng, 2014; 7(4): 60－74.

Dissipation Assessments During Dynamic Very High Cycle Fatigue Tests

This paper presents an experimental device developed to detect and estimate dissipated energy during very high cycle fatigue tests (VHCF) at high loading frequency (20 kHz) and low stress (i.e. far below the yield stress). Intrinsic dissipation is computed using local expressions of the heat diffusion equation and thermal data fields provided by an infrared focal plane array camera. The results obtained from tests performed on pure copper specimens show that dissipated energy exists whatever the attainable stress range and show that the dissipated energy rate is not constant throughout the test. Both findings are respectively incompatible with the concepts of fatigue limit based on elastic shakedown or on stabilized cyclic state associated with the mechanical hysteresis loop (viscoplastic shakedown).

Errata: Correcting for focal-plane-array temperature dependence in microbolometer infrared cameras lacking thermal stabilization

This article [Opt. Eng.. 52, (6 ), 061304 (2013)] was originally published online on 7 January 2013 with an error in the numerator of Eq. (13) that propagated into Eqs. (14), (15), and (17). The corrected Eq. (13) and subsequent equations are given here:

Dual-band ultraviolet-short-wavelength infrared imaging via luminescent downshifting with colloidal quantum dots

The performance of short-wavelength infrared (SWIR) cameras in the visible and ultraviolet (UV) regions is limited by the absorption of high-energy photons in inactive regions of the imaging array. Dual-band UV-SWIR imaging can be achieved by using PbS colloidal quantum dots (CQD) to downshift incident UV light to the SWIR band. The CQD downshifting layer has minimal impact on the SWIR imaging performance and greatly increases the UV sensitivity of an InGaAs camera. A dual-lens design in which the QDs are incorporated on a removable substrate is demonstrated, which provides UV sensitivity without modification of the InGaAs camera focal plane array. A single-lens design in which the QDs are deposited directly on the focal plane array is demonstrated using both a standard InGaAs focal plane and a substrate-thinned focal plane. Higher UV resolution for the substrate-thinned focal plane is observed.

Dissipative and microstructural effects associated with fatigue crack initiation on an Armco iron

In this study, the first signs of fatigue crack initiation are studied in the HCF domain on a body centered cubic Armco iron (with 80ppm of carbon content). Some tests are performed on plate specimens through a piezoelectric fatigue machine. During fatigue tests, the deformation evolution is observed by optical microscope, and the temperature recording on the specimen surface is achieved by an infrared focal plane array camera. From the temperature recording, the intrinsic dissipation field is calculated using a local expression of the heat diffusion equation. The results show that above a given stress level, Slips Marks can be clearly observed on the surface specimen, and related to the intrinsic dissipation distribution. Observations through a Scanning Electron Microscope on the specimen surface and the fracture surface are related to stage I and stage II of fatigue damage.

Design and Improvement of Host and Backup Selection

In order to resist the Single Event Upset（SEU）in outer space so that the camera focal plane array system could work in a long time, a design and improvement method for the selection of host and backup based on FPGA was introduced. VHDL was used to do the design and simulation, FPGA developed by Altera company was chosen as the hardware design platform. The results of experiment showed that the improved module could reliably get a group of effective data from the two in all cases and the camera focal plane array system could produce reliable driving timing for the CCD camera.

Extensometric and Thermo-Elasticimetric Measurements to Characterize the Damage Evolution in Adhesively Bonded Joints: Application for the Adhesively Bonded Double Scarf Joint

In this article, two nondestructive experimental methods are compared in order to evaluate the influence of a singularity, created by geometry, on the damage evolution in the adhesively bonded joints. The first experimental method uses the “back face technique” to measure the disturbances of the strain fields induced by the cracks. The second experimental method uses an infrared focal plane array camera to measure the thermo-elastic field in the geometrical singularity. First of all, both methods are carried out independently: the analysis of the stress/strain and the thermo-graphic analysis. Then, these two methods enabled us to generate a database, the use of which improves the diagnosis of the damage state of the adhesively bonded joints.

Scene-based nonuniformity correction method using multiscale constant statistics

In scene-based nonuniformity correction (NUC) methods for infrared focal plane array cameras, the statistical approaches have been well studied because of their lower computational complexity. However, when the assumptions imposed by statistical algorithms are violated, their performance is poor. Moreover, many of these techniques, like the global constant statistics method, usually need tens of thousands of image frames to obtain a good NUC result. In this paper, we introduce a new statistical NUC method called the multiscale constant statistics (MSCS). The MSCS statically considers that the spatial scale of the temporal constant distribution expands over time. Under the assumption that the nonuniformity is distributed in a higher spatial frequency domain, the spatial range for gain and offset estimates gradually expands to guarantee fast compensation for nonuniformity. Furthermore, an exponential window and a tolerance interval for the acquired data are introduced to capture the drift in nonuniformity and eliminate the ghosting artifacts. The strength of the proposed method lies in its simplicity, low computational complexity, and its good trade-off between convergence rate and correction precision. The NUC ability of the proposed method is demonstrated by using infrared video sequences with both synthetic and real nonuniformity.

A study of IRFPA camera measurement errors: radiometric artefacts

Infrared Focal Plane Array cameras have several aberrations and measurement artefacts with different material origins: lenses, sensors, Read-Out-Circuits of the sensors. The authors' goal is to characterize each artefact separately to obtain more versatile correction procedures and improve the correction accuracy. This paper focuses on some radiometric artefacts associated with variations of measured illuminance. It presents a new correction algorithm dedicated to Narcissus and vignetting effects, which is identified and validated for two different lenses. It also proposes an innovative experimental protocol to detect low-level contrast-induced artefacts. Several R&D cameras and lenses are tested and some present such artefacts, which can not be corrected by traditional pixel-to-pixel methods. Their origins are shown to derive either from the optical system or the Read-Out-Circuit.

Bilateral filter-based adaptive nonuniformity correction for infrared focal-plane array systems

In scene-based nonuniformity correction (NUC) methods for infrared focal-plane array cameras, the problem of ghosting artifacts widely affects the sensitivity of the imaging system and visibly decreases the image quality. Ghosting artifacts can also degrade the performance of several applications, such as target detection and tracking. We carried out a detailed analysis of the problem using a well-established NUC technique: the least mean square Scribner's algorithm. In order to solve some drawbacks of the original Scribner's algorithm, we introduced in the scheme a new technique that mitigates ghosting. Such technique relies on the employment of an edge-preserving spatial filter for the purpose of computing reliable spatial estimates. We tested the effectiveness of the new technique applying the improved NUC method to an experimental IR sequence of frames acquired in the laboratory. Finally, the performance of the proposed method was discussed and compared to that yielded by a well-established deghosting technique.

Thermal imaging of smart power DMOS transistors in the thermally unstable regime using a compact transient interferometric mapping system

Smart power DMOSes are analyzed under thermally unstable conditions up to the destruction level using a new compact version of the transient interferometric mapping (TIM) method. High accuracy phase measurements are achieved employing superluminescent diodes and focal plane array cameras. Two-dimensional thermal mapping at two time instants during a single stress pulse is performed in the range of 100 μs to few milliseconds. The size of the region where the parasitic bipolar transistor becomes thermally activated at the onset of thermal runaway is determined. The results are correlated to conventional failure analysis.