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Technical Papers
Characterizing the quality parameters of imagers such as the optical distortion is a key point for applications such as defence and security, especially for wide-angle systems. Here, we developed a compact setup using pinhole image projected by a mirror-based off-axis collimator in front of the unit under test (UUT). The former is fixed while the latter is mounted on a 2-axis rotation platform. The collimator projects at infinity a pinhole while the UUT rotates, allowing a perfectly controlled imaging on the desired area. Here, we present this system and the postprocessing method correcting the rotation of the UUT, allowing to reach a top-notch measurement accuracy. Our setup has been successfully deployed and can be used for visible imagers or thermal imagers by easily switching the source (blackbody or integrating sphere) set to the collimator
Low light night vision systems based on I² tubes have been expanding rapidly over the past few years, due to a combination of the growing advancement of this technology and the increased pressure in the current climate. The design of a single optical bench able to fully characterize night vision devices is presented into this paper, focused more specifically on spot defects and goggle axes parallelism tests.
These criteria are indeed very important: misalignment between the two binocular images may be one source of visual fatigue and could degrade task performance of the night vision user, and spot defects can act as visual distractions and may be large enough to mask critical information pilots need to conduct normal night vision operations.
Thanks to HGH’s IRCOL bench, these two tests are integrated on the same support. Spot defect measurement utilizes machine vision algorithms to determine the size and location of the defects, and the parallelism measurement identifies the angular misalignment between the two channels under test. The spot defect test has also been completely automatized compared to the only visible test previously available
All these results will be compiled and directly integrated into a computer-generated report that can be easily used for quality control or for maintenance applications.
Modulating Transfer Function (MTF) has always been very important and useful for objectives quality definition and focal plane settings. This measurand provides the most relevant information on the optimized design and manufacturing of an optical system or the correct focus of a camera. MTF also gives out essential information on which defaults or aberrations appear on an optical objective, and so enables to diagnose potential design or manufacturing issues on a production line or R&D prototype.
Test benches and algorithms have been defined and developed in order to satisfy the growing needs in optical objectives qualification as the latter become more and more critical in their accuracy and quality specification. Many methods are used to evaluate the Modulating Transfer Function. Slit imaging and scanning on a camera, MTF evaluation thanks to wavefront measurement or imaging fixed slanted knife edge on the detector of the camera. All these methods have pros and cons, some lack in resolution, accuracy or don’t enable to compare simulated MTF curves with real measured data. These methods are firstly reminded in this paper.
HGH has recently developed an improved and mixed version of a scanning technique used on a slanted knife edge giving a more accurate, ergonomic, high resolution and precise Line Spread Function (LSF) and one axis MTF measurement of a camera. A selected single pixel corresponding to a precise field point of the camera is scanned with sub pixelic resolution by the tilted knife edge thus enabling an optimized accuracy for LSF and MTF curves. The experimental protocol which requires a high-performance collimator, a scanning wheel device and a camera set up is detailed in this paper. Explained simulations are done to prove the under 1% accuracy of this method with regards to the different characteristics of the camera. All the parameters of this improved measurement technique are described and their effect criticized to give out all the result influence of these variables. These simulations and the algorithms used are then confronted to real measurements on a camera thanks to a mirror-based collimator and a scanning wheel device equipped with a slanted knife edge target.
Blackbodies are the appropriate tools for IR sensors calibration and test. A well-known property of these object is their emissivity factor equals 1 while their transmission and reflection factors equal 0. Though some high emissive coatings with emissivity higher than 0.99 are now available on the market, a residual reflectivity factor always remains. The first part of this paper demonstrates the influence of the reflectivity factor on the radiated energy of a blackbody especially for blackbodies radiating at temperatures close to or below the ambient temperature. It happens that the difference between this radiated temperature, or apparent temperature, and the measured temperature maybe of several tenths of degrees! Such a difference leads to great uncertainty in the calibration procedure of thermal sensors. The case of sensors tested and calibrated into climatic chambers for outdoor applications is particularly critical.
The usual method to compensate this difference is to take emissivity and consequently reflectivity factor into the calculation of the theoretical irradiance received by the sensor. This calculation requires to have a live knowledge of the ambient temperature. While this may not always be the case, calculating the true irradiance i.e. the apparent temperature radiated by the reference source remains a complex calculation for major users of blackbodies. Indeed, they expect their blackbody source to be reliable and an actual reference source whatever the conditions of use. The second part of this paper presents a reminder about this calibration method of the absolute temperature of IR reference source and the correction method when ambient temperature doesn’t change. The third part describes the integrated compensation method of the ambient temperature into the new controller of HGH’s blackbody sources making these sources actual IR reference sources whatever the operating conditions.
Some significant progresses have been made in performance on Infrared Detectors over the past years. Currently existing MCT (mercury cadmium telluride) and InSb Focal Plane Arrays commonly reach NETD (Noise Equivalent Temperature Difference) values lower than 20 mK. Type II Superlattice (T2SL) FPAs, supposed to be the next generation infrared detector technology, are also now able to compete with the above described models. Even the processing of microbolometers is also continuously improving, leading to frequent NETD lower than 30 mK. It can be assumed this performance race will never stop and testing equipment of future detectors must be in accordance with these expected results.
Infrared reference sources, i.e. blackbodies, are key elements of thermal imager test bench.
Measured results are highly dependent on blackbodies characteristics, such as the temperature accuracy, the emissivity, the thermal uniformity and stability. The general rule of metrology considers the contribution of the reference instrument to the uncertainty of the tested device must be at most ¼ of this uncertainty. Most of the time a 1/10 factor is preferred in order to really consider the contribution of the reference instrument as negligible. Considering the case of the IR detectors, the above rule means the temporal stability of the blackbody must be now lower than 2 mK.
This paper first lists an overview of current performances and announced developments in IR detectors and the corresponding expected NETD and stability performances already existing or available in the near future. It demonstrates the existing testing sources available on the market have performances restricting the test of the new detectors. The second part of the paper describes the improvements brought to our next generation of IR reference sources in order to strongly reduce by 4 the temporal instability and shows this next generation of blackbodies is now compatible with the expected NETD values of the next generation of IR detectors.
The accurate knowledge of IR detectors specifi cations becomes of higher importance whatever the application. Among these specifi cations is the relative spectral response. The usual method of relative spectral response measurement uses a source spectrally defi ned by the wavelength selection through a grating-based monochromator. This simple and proven method has a limited spectral resolution since the signal received by the tested detector is proportional to the width of the wavelength selection slit i.e. the spectral resolution.
Another method consists in using a Fourier Transform IR Spectrometer (FTIR) easily allowing a 1 cm-1 spectral resolution even in the Long Wave IR range. However, the implementation of this method requires a meticulous analysis of all the elements of the bench and all the parameters to avoid any misinterpretation of the results. Among the potential traps are the frequency dependence of the signals and the parasitic fringes effect on the curves. Practical methods to correct the frequency dependence of the reference detector and to remove parasitic interference fringes are presented in this paper.
The accurate knowledge of IR detectors specifi cations becomes of higher importance whatever the application. Among these specifi cations is the relative spectral response. The usual method of relative spectral response measurement uses a source spectrally defi ned by the wavelength selection through a grating-based monochromator. This simple and proven method has a limited spectral resolution since the signal received by the tested detector is proportional to the width of the wavelength selection slit i.e. the spectral resolution.
Another method consists in using a Fourier Transform IR Spectrometer (FTIR) easily allowing a 1 cm-1 spectral resolution even in the Long Wave IR range. However, the implementation of this method requires a meticulous analysis of all the elements of the bench and all the parameters to avoid any misinterpretation of the results. Among the potential traps are the frequency dependence of the signals and the parasitic fringes effect on the curves. Practical methods to correct the frequency dependence of the reference detector and to remove parasitic interference fringes are presented in this paper.
Infrared reference sources such as blackbodies are used to calibrate and test IR sensors and cameras. Applications requiring a high thermal uniformity over the emissive surface become more and more frequent compared to the past applications. Among these applications are non uniformity correction of infrared cameras focused at short distance and simultaneous calibration of a set of sensor facing a large area blackbody. Facing these demanding applications requires to accurately measuring the thermal radiation of each point of the emissive surface of the reference source. The use of an infrared camera for this purpose turns out to be absolutely inefficient since the uniformity of response of this camera is usually worse than the uniformity of the source to be measured. Consequently, HGH has developed a testing bench for accurate measurement of uniformity of infrared sources based on a low noise radiometer mounted of translating stages and using an exclusive drift correction method. This bench delivers a reliable thermal map of any kind of infrared reference source.
Vacuum blackbodies have to combine performance of traditional infrared reference sources with specific features in order to operate in vacuum chamber. As their usual applications are calibration and tests of IR sensors to be loaded on satellites, earth or space radiation simulation and test of IR sensors for scientific applications, their usual features are emission over an ultra extended temperature range, knowledge of the radiated temperature with a high accuracy, extremely high uniformity of the emissive surface and extremely high emissivity. HGH developed tools to demonstrate such performances since they surpass the accuracy of usual tools.
Calibrating IR sensors for thermography or imaging applications requires covering a wide temperature range from temperatures below ambient to extremely high temperature.
Such a wide range cannot be covered with a single IR source. It usually requires two sources:
• A low temperature source covering temperatures around ambient temperature. Usual temperature range is from 0°C to 150°C.
• A high temperature cavity source from 100°C to 1200°C
For the first type of sources, the required electric power is much higher compared to the emitted optical power. For the second type of sources, the stabilization time is usually prohibitive at the junction temperature, i.e. below 150°C. To pass through these two inconvenients, HGH has developed two methods to improve the range of temperature of its blackbodies, to speed up the cooling/heating time and to improve the power efficiency of the blackbodies.