Products > Thermal Imaging
AGEMA Thermovision THV470 camera complete teardown by Fraser
Fraser:
Some time ago I carried out a complete Teardown of an AGEMA Thermovision THV470 in order to better understand its design. The camera was a spares donor unit due to corroded mirrors so the teardown was not wrecking a working camera ! This spares donor has already provided essential parts to repair a THV450 :)
Before the pictures, some background to the THV470 model is in order to better understand the design.....
AGEMA were a major player in the World if thermal imaging and produced some excellent thermal cameras. AGEMA used to be AGA, another well known name in thermal imaging equipment. Early thermal Imaging cameras were based upon cooled detectors and scanning mirror or prism optical blocks that created a thermal scene using a raster scan system. These mechanical scanning type cameras were born out of necessity as the cooled detector was often a single pixel and not the array type sensors that we see in modern cameras. With just on pixel to work with, the designers had to present the thermal scene to the pixel in a serial mode of operation with the scene broken down into horizontal scanned lines and a complete field created by vertical movement of the scanned line across the thermal scene. A reverse operation using the scan coils of a CRT monitor could recreate the thermal image on a display for the user. With further development the scanner signal became digitized by a ADC and digital signal processing was possible. As stated, the scanning assembly was mechanical in nature and could comprise two spinning Germanium prisms or a combination of tilting and spinning mirrors. Galvanometers and high quality motors were used in these scanning systems and these needed to be carefully synchronised to create the desired raster feed to the detector pixel. These cameras were complex precision electro-mechanical assemblies and this meant that they were relatively bulky, heavy and expensive to manufacture.
Once the thermal scene signal had been captured using the optics and scanning assembly, the detectors signal was passed to amplifiers and then electronic processing stages, either analogue or digital in nature. The detector pixel presented its own challenges to designers however. The detector was made using specialist semiconducting materials but these were effectively blind at ambient temperatures due to their own internal thermal noise. In order to use the detectors for thermal imaging, they had to be cooled to cryogenic temperatures of 77K. This was achieved using Liquid Nitrogen that was contained within the camera in a Dewar that had an integrated detector pixel. At 77K (-196C) the detector pixel was very low noise and was capable of good quality thermal imaging. The drawback to this system was the need for Liquid Nitrogen. Liquid Nitrogen is not the most user friendly substance, it limited the continuous running time without refilling the Dewar and the camera could only be used in a roughly horizontal plane and definitely not invented ! Transporting Liquid Nitrogen also presented logistical challenges for portable use. The use of Liquid Nitrogen was acceptable for laboratory use but a less cumbersome cooling method was needed for portable use. Development work lead to the introduction of the miniature Stirling Cycle mechanical cooler. This is basically a miniature refrigeration heat pump that can achieve a cold finger temperature of 77K or lower. The cooler uses a Helium gas fill to achieve this level of performance. The Stirling Cooler was a godsend to Thermographers working in mobile situations but it sadly had its own drawbacks. The precision cooler had a life of around 2000 hours before it needed a complete rebuild and re-gas. This was an expensive operation as the detector pixel was integrated into the cooler assembly so a simple “cooler swap” was not possible and the camera had to undergo calibration if the cooler/detector was changed. The intrinsic cost of the cooler+detector was also high and this lead to scary service costs for owners of the equipment. The high servicing costs of the sterling Coolers drove development of cheaper cooling alternatives. Some thermal camera manufacturers had opted to use pressurised gas cooling, such as Hughes and their Argon gas based detector cooler. This used the expansion of gas passing through a heat exchanger to create the cooling effect for the detector. It was easier than using Liquid Notrogen, but still not ideal !
Development surrounding the Peltier Element technology discovered that a Peltier ‘stack’ could produce temperatures of -70C reasonably easily. The ‘stack’ was pyramid shaped and comprised several Peltier elements stacked one on top of the other. I will not go into the physics here but you cannot just stack Peltier Elements of the same size and achieve -70C at the detector. The elements must start small and increase in dimensions with each subsequent layer. Such a cooler has limited cooling power at -70C so, like with the Stirling Cooler, the Detector pixel resides inside a vacuum with the cooling source. This lowered the thermal load on the cooler to acceptable levels. Now the sharp minded will note that the TEC only achieves -70C and not -196C so how could this work in a thermal imaging system ? Well new material doping mixes were developed for the detectors to achieve a Band-gap that would operate as a thermal detector at ‘only’ -70C the noise level coming out of the detector was higher than that of a detector operating at -196C (77K) but was acceptable.
It is this Peltier Element Stack based cooling technology that resides within the THV470.
To Be Continued ..........
Fraser:
Ok, next installment.... the THV470 camera......
This needs a little history to explain its existence and design.
The THV470 has a brother called the THV450 which looks the same except it does not have a floppy disk drive. That is because it is basically a cost reduced version of the THV470. The cost reductions include the loss of the floppy disk drive and a fixed mount lens rather than a removable lens. There are many areas of the THV450 that are the same as the THV470 however so this teardown will be useful to owners of the THV450 as well.
The THV450 and 470 are both built around a common scanner block that is found in several cameras. Let me expand upon this.
In the beginning there was the THV880 scanner type thermal imaging camera. It used Liquid Nitrogen for its cooling but there was a later upgrade to Stirling Cooler that a user could purchase. The "head" assembly comprised a relatively large and heavy metal 'box' casing that contains a combination of fixed and moving mirrors that pass their output to the cooled detector via some Germanium relay lenses and any filters that are fitted. The signal from the detector is amplified and sent down an umbilical cable to the desktop controller for 'processing' and image recreation. The controller came in several versions to suit the different needs of the user. They ranged from simple analogue systems to relatively sophisticated computer based image analysers such as the BRUTE system. The mid point was a controller that digitised the analogue signal from the detector pixel in order to provide basic measurement and analysis function for the user.
A THV870 scanner head was developed that used the Peltier Element TEC system to cool a 'HOT' detector pixel that could function well at only -70C. This THV870 head looked the same as the THV880 Liquid Nitrogen cooled head and contained the same mechanical scanning engine as the 880 model. The 870 differed at the point where the image signal reached the detector. The 'HOT' detector was integrated into a vacuum package with the Peltier cooler and this was thermally bonded to a forced air cooled heatsink. The Controllers were very similar to those of the THV880 with only minor changes needed to the design.
A drawback to the THV880 and 870 was the lack of portability. In order to make the system portable, a bulky cart was needed to hold the controller, its power source and the monitor. AGEMA addressed this matter by building a Frankenstein THV870 system that integrated the original THV870 scanning head into a common chassis with the electronics normally found in the separate bench mounted controller. The resulting bulky 'all in one' camera solution was turned into a shoulder mounted portable thermal imaging camera with the addition of a large battery belt and electronic viewfinder. Portable it was, Pretty it was not ! Once the side covers of the THV470 were removed, its ancestry was clear to see. A clever piece of engineering but you cannot escape the fact that it is basically a THV870 with the controller PCB's bolted on the rear ! It worked though and it did provide a portable, if still bulky, solution for thermal imaging whilst mobile.
At this point I should state that the AGEMA scanning thermal camera line-up of the period can get a little confusing. The same scanner head can be seen with different model numbers as they formed part of specific 'systems' that AGEMA were marketing. This why the THV487 scanning head has the appearance of a THV870 ! AGEMA were pairing scanning heads with different controller systems and branding the system as a Thermovision THVxxx even though parts of the system were available under their own, different, model numbers. As highlighted by the THV450/470 differences, some systems were designed to be more versatile than others or capable of radiometric measurements when others were not. Lots of marketing at work here ! I should also state that the scanning cameras heads were often available in MWIR and LWIR to suit the users needs. The version is identified on the ID plate and lens annotation.
OK, so now we know the background to the THV450/470 FrankenCamera ;D
I will generate some posts that show the cameras detailed in the above comments.
Fraser
To be Continued........
Fraser:
The vernerable Agema Thermovision THV880 scanning thermal imager head unit including a cutaway view of the image path to the Liquid Nitrogen cooled detector.
I also include pictures of the associated THV800 standard controller unit.
Fraser
Fraser:
The Peltier TEC cooled Agema Thermovision THV870 camera and controller
(Some images borrowed from : http://wll.kr/bbs/board.php?bo_table=06&wr_id=795 )
Fraser
Fraser:
AGEMA Thermovision 450 shoulder mounted camera with non removable lens and no floppy disk drive.
Fraser
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