Is it possible to get good MWIR images even with a bad cooler? - Page 1
Forum section: electronics / general

author: Ben321
author_rank: Frequent Contributor
timestamp: November 12, 2019, 09:46:40 am
content: I know if the sterling cooler goes bad, you won't normally see any image at all on your MWIR imager. However, I was wondering if there's a way to compensate. As the temperature goes up, the averages signal level will go up, and so will the noise level. I think it might be possible to compensate for both. Here's my theory. First, you could cool it somewhat with a peltier thermoelectric coolor. That would help a bit. Then to get the analog signal level back within the voltage range of the ADC (analog to digital converter), you could use a potentiometer in conjunction with a regulated voltage source to set an offset voltage that would be combined with the MWIR focal plane array's signal via an op-amp configured as a summing amplifier (doesn't boost any signal strength by multiplication, but rather adds 2 signals together). Since the average voltage from the MWIR sensor array would be higher than it should be to be within the ADC voltage range, you would need a negative voltage. You could modify the camera's power supply provide an additional output with that negative voltage for this purpose (assuming the camera's power supply normally only has positive voltage output).So once you get the signal within the ADC range, how do you handle the increased noise? Simple, just do it on the computer. Write a program that averages 100 frames from the camera to remove the noise. At 30fps that would take just over 3 seconds to capture 100 frames. Obviously this would reduce the temporal resolution of the imager, but it would allow it to get some decent still-images.At least that's my theory. Can somebody else here with some experience in the field let me know if this would even be possible? Or is there some other factor I'm missing in my theory?

author: Vipitis
author_rank: Frequent Contributor
timestamp: November 12, 2019, 10:42:42 am
content: the problem won't be noise, it will be contrast. There are peltier cooled MWIR cameras as well as uncooled MWIR cameras. But they aren't meant for scenes -40 to +100C more like 400C and above.

author: Fraser
author_rank: Super Contributor
timestamp: November 12, 2019, 11:41:28 am
content: Ben321,You really do need to research thermal sensor technology a little more. You will find such research very rewarding, I certainly did.There is an important word in cooled thermal detectors ........ bandgapThere is an excellent wiki article on the common MCT detector material and it’s use in imaging. I strongly suggest you read it and other easily located articles on cooled detector theory and use. It is fascinating stuff and quickly clarifies why cooling is needed.https://en.m.wikipedia.org/wiki/Mercury_cadmium_tellurideA detector designed for use at an operating temperature of 77K will generate so much self produced noise at room temperature that it will be effectively blind. It is not the correct terminology but the sensor becomes effectively saturated so cannot provide a useable signal for you to extract from the noise. Some specially doped MCT detectors operate at a higher temperature and use Thermoelectric cooling to reduce their noise production. These were used in the AGEMA Thermovision 450 and 470 cameras. Noise performance is not as good as the cryo cooled version but does compete well against the microbolometer in terms of noise from the sensor array.Regarding your idea ......... study of a cooled FPA thermal camera design will show you that the FPA resides inside a Vacuum Dewar and the Stirling cooler is connected to the FPA using a cold finger. Good luck adding a Peltier cooler to that design. The Thermoelectric cooled versions are a different construction and use a very specialist Peltier stack with carefully matched stages, all running within a vacuum to improve efficiency and prevent ice formation.As I have said before...... do your research, Google the topics and read books. That is the way to fully understand the topic rather than getting snippets from forum members like me. We cannot condense the contents of a book into a few lines of text for you without losing a lot of detail and context. I find this technology fascinating but if there is one thing I have learnt, it is that thermal imaging technology is still very young and there is much to be learnt about the challenges involved in designing a system that performs and the physics that are at play in all types of imaging systemFraser

author: Fraser
author_rank: Super Contributor
timestamp: November 13, 2019, 12:20:12 pm
content: A little off topic but may be of interest to others.The earlier technologies used to create thermal images included the use of a single pixel detector that was cryogenically cooled to 77K. This single pixel was illuminated by a scanning mirror and/or prism. The scanning section of the camera was mechanical in nature and produced the required faster scan for television image production. The single pixel detector responded to the changing input and the thermal image was created by combining the scanning raster with the associated dynamic level coming out of the detector.The detector was a semiconductor device that was photon sensitive. It was cooled to 77K because it’s self generated noise at ambient would swamp any incoming signal from the scanner. The Liquid Nitrogen cryo-cooling requirement made these cameras user unfriendly until Stirling Coolers were used for the task. Examples of the scanning type thermal cameras include the AGEMA Thermovision 880 and 870. These were originally cooled by Liquid Nitrogen but later options and retrofits included a Stirling Cycle cooler. Inframetrics also produced scanning type thermal cameras with examples being the 500 and 600 series of Liquid Nitrogen cooled units. The later 700 series offered a Stirling Cycle Cooler in place of the Liquid NitrogenThe cooled scanning type thermal cameras prevailed in the market until cooled FPA staring arrays we’re adequately developed to replace the scanning assembly and single pixel detector. Scanning type cameras continued to be used in science applications for some years afterwards though. Replacing such hardware was not cheap as nothing was transferable to the new FPA based camera technology. When you have spent $200K on a scanning cameras setup, you need to get your value out of it before disposalThe AGEMA Thermovision 550 was the first Agema Stirling Cycle cooled cooled Staring array model to market. It was shaped like a large palmcorder and produced excellent low noise images. There were also the Thermovision 450 and 470 cameras. These used a modified MCT detector design to enable operation at only -70C. Such a temperature could be achieved using its Peltier Thermoelectric cooler stack. The camera was a development of the older 800 series mechanics and gave additional life to the scanning based technology. The end product was large and bulky, like am OLD full size VHS or Betamax tape based shoulder mounted video camera. Compared to the new Thermovision 550, it was a monster.These scanning cameras were so expensive that they were often well cared for and used in relatively clean environments. For that reason it is possible to find examples in very good condition and at low prices on the secondary market. Many potential buyers would be put off by the requirement for Liquid Nitrogen in the early models or a potentially dead Stirling Cooler in the later units.The scanning cooled cameras were available in both MWIR and LWIR. There were even dual band 500 and 600 series cameras available from Inframetrics. They were effectively two detectors sharing a single optical scanning block and lens. The optics had to be dual band or wide band for these cameras. Normally a camera was either MW or LW and the optics were often matched to the operating band. This means that a LWIR cooled scanning camera could, in theory provide optical parts fir a microbolometer LWIR camera. I have used this fact to good effect in the past. However, instead of salvaging optics from an obsolete LWIR scanning camera, it always struck me as a novel project to retrofit such a camera with a modern low resolution microbolometer or uncooled semiconductor detector. In the case of the microbolometer the ‘data’ output from one or more pixels would be processed to either be compatible with the original cooled detectors output or a new signal processing and raster scan video creation electronics package would be grafted onto the cameras scanning mechacon. Such a project would not make commercial sense and would be very much a project for a hobbyist or thermal camera tech with too much time on their handsIt should also be noted that the scanning cameras operate at quite high frame rates so the sensor or detector must be able to match the response time of the original cooled detector.For this reason I never took the project forward. Some forum member may have more enthusiasm for such an electro mechanical camera project though. For me it would only be a “I did it because I can” type of project and I just do not have the motivation or time at the moment. Scanning type cameras are common on eBay and often come with all the accessories, lenses and controller etc.This would not be a beginners project !Fraser

author: Fraser
author_rank: Super Contributor
timestamp: November 13, 2019, 12:49:03 pm
content: Just thinking about the speed of the sensor response needed to replace a cooled detector in a scanning camera, I think finding such an uncooled replacement would be very challenging and likely expensive. That pixel is ‘working’ very hard To create the image.For the common 128 x 128 pixel image found in scanning cameras that is 16384 spot measurements by the detector pixel for each frame. The duration of a frame in a 30fps camera is 0.0333’ Seconds. Each measurement of the pixels must be completed, including any overheads, in 0.00000203 seconds. 1/0.00000203 = Approx 492,000 measurements per second.To calculate it a simpler way, but with less period detail, 16384 pixels points (1 frame) x 30 FPS = approx 492K measurements per secondA common 256 x 256 scanning camera resolution makes the situation even more demanding !65536 pixels (1 frame) x 30 FPS = 1966080 (approx 1.97 Million) measurements per second. Sensor pixel response time becomes critical at these sample rates.This is where the staring FPA really delivers in our modern cameras. The original single pixel cameras worked that poor pixel very hard indeed. Some cameras used multi pixel detector arrays but the scanning rate remains high and the measurement frequency equally so.Fraser

author: Ultrapurple
author_rank: Super Contributor
timestamp: November 13, 2019, 03:05:19 pm
content: FraserAre you sure all the single pixel cameras operated at 30Hz (25Hz) frame rate? I would have expected them to scan at a physically lower rate, largely for the reasons you describe, and then run frame rate conversion in dedicated electronic hardware.I seem to recall that some of the earliest TI cameras had imaging rates in the order of an hour per frame, presumably due to bandwidth limitations on the pixel. ("The first American thermographic cameras developed were infrared line scanners. This was created by the US military and Texas Instruments in 1947" - Wikipedia).Based on your figures, a 128 x 128 image with a true (mechanical) frame rate of 7.5Hz (30/4) requires a bandwidth of "only" 125kHz from the pixel. Still a lot, I grant you.I believe some cameras used more than one pixel (up to aboutwith relatively wide 'vertical' spaces between them and used optical tricks to use the pixels to scan (in this case) 1/8 of the lines each, with the picture either put back together in semiconductor memory or, for some hand-held night sight devices, (in this case) 8 similarly-spaced LEDs modulated directly by the pixel outputs.Given even a 4-pixel arrangement the bandwidth requirement of a 128x128 x 7.5Hz imager reduces to about 30kHz.Of course, I could be talking complete balderdash but someone will probably put me right.

author: Fraser
author_rank: Super Contributor
timestamp: November 13, 2019, 03:35:03 pm
content: Just some specs I could place my hands on.....AGEMA THV880 LW MCTField rate 25HzLine rate 2500HzLines 280Samples per line 175AGEMA THV900 LW MCTField rate 15Hz/30HzLine rate 2500HzLines 136Samples per Line 272AGEMA THV1000 LW MCT (5 SPRITE detectors read in Parallel)Field rate 30Hz (NTSC) / 25Hz (PAL)Line rate 3000HzLines 400 (NTSC) / 445 (PAL) - 5 lines read in parallel by the 5 SPRITE detectors.Samples per line 798Fraser

author: Ben321
author_rank: Frequent Contributor
timestamp: November 23, 2019, 04:48:37 am
content: Quote from: Fraser on November 12, 2019, 11:41:28 amCan't you compensate for that saturated signal by bringing it out of saturation? I'm thinking of using an op-amp based voltage subtractor to pull the signal back down into the usable range (the range used by the analog to digital converter). That might work, right? I don't have such an MWIR camera yet, but I want to know if such is possible, before I consider spending money on even a used MWIR camera. Cheapest MWIR cameras will be old ones, and old ones will be based on liquid nitrogen, but I don't have access to LN2, so my project might be the only way to get even a low quality image (as opposed to no image at all) out of one. But I don't want to waste money if such project would be 100% guarantied to be an utter failure.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 23, 2019, 05:29:44 am
content: Quote from: Ben321 on November 23, 2019, 04:48:37 amFrasers advice to do some learning is good. But your instincts are not totally wrong. For a lot of reasons that will become clear if you do the research the detector performance on these cameras drops rapidly as temperature rises. But an InSb camera designed to work at liquid nitrogen temperatures (77 K) can still provide some measureable performance at temperatures in the 100 K range. If you have access to the bias supplies (sometimes a software function) you can extract this performance. You might be able to extract some hours of operation from a failing Sterling cooler by cooling the warm end, which should also cool the cold end and twiddling the biases on the array. Be aware that uninformed twiddling of the biases can also destroy the array.There is much to recommend against this path. A failing Stirling cooler is just that - a failing cooler. It may provide 100 hours at the current level of performance, or 100 minutes, or 100 seconds. If you don't know the failure mechanism you have no way of guessing which answer is correct. If you cannot get the temperature to the nominal operating temperature the performance will be severely degraded. The high temperature I mentioned might be associated with more than an order of magnitude loss in NEdT. Fixed pattern noise will also be far less stable and difficult to correct.There are good books on infrared imaging. For less than the cost of one the used cameras you are considering you can buy one and learn why these things are true. Or you can likely find one in a university library nearby and possibly on line somewhere. It will take time and energy to read and understand, but it will reduce the total time and energy you spend getting to an image.An older book is Barnes, Infrared System Engineering. It should be widely available (at least as such specialty books are concerned). The technology is now vastly outdated but it is an easier read than some of the more modern texts and the fundamental principals are all there.A little googling will identify more modern resources. While many tricks and details are closely held national or corporate secrets all of the key elements of these technologies are widely published in the public domain.

author: Fraser
author_rank: Super Contributor
timestamp: November 23, 2019, 10:24:33 am
content: Ben321,My advice on an affordable MWIR cooled camera would be to search for an Agema Thermovision THV 450 or THV 470. That camera uses a long lived Peltier stack as the cooler and the technology within the camera is such that you have a good chance of being able to maintain and repair it if required. The camera is self contained and so relatively portable without training cables and controllers etc. Performance is far better than what you could dream of producing using your signal extraction method, even if you could get such to actually work. I honestly believe your approach to extracting the signal from the noise ‘saturated’ detector is a non starter. To extract a signal, it must be present in some form to start with. I do not think such a signal will be present in a detector designed for operation at 77K, operating at close to ambient, viewing an ambient temperature scene. The money woulld be better spent on a Agema THV4xx series camera.The THV4xx series cameras are available in MWIR and LWIR versions. Buy the one you need and make sure that the lenses are correct for the camera body as in Industry where both MWIR and LWIR camera versions are used, lenses can get mixed up between cameras, especially during disposal. The band is written on the lens ID ring so easy to see.As to price, I saw a THV470 sell a year ago for £10 as it was collection only and that puts off buyers in some cases. The camera is bulky, old fashioned and of limited appeal to many who are looking for a thermal camera. Prices are low except where dealers and hopeful sellers are involved. Do not over pay as these cameras are museum pieces and may need some TLC to get them working. I paid £75 (delivered) for a THV470, complete, in its transit case, with all accessories. Make sure the camera includes the lens as that sometimes get sold separately or is lost.Do your research on the a Agema Thermovision 470, I think it might appeal to you. There is a lady reviving such a camera on YouTube who seemed impressed with it and what it can do.https://youtu.be/1u4E0NK8_lIThere is also an EEVBlog forum thread on Agema 470 .....https://www.eevblog.com/forum/thermal-imaging/agema-thermovision-470/Fraser

author: Ben321
author_rank: Frequent Contributor
timestamp: November 24, 2019, 12:54:26 am
content: Quote from: Fraser on November 23, 2019, 10:24:33 amJust watched the video. That looks like one of the 1980s visible-light camcorders. The thing takes a separate power unit or external battery pack. Even the view finder is a CRT. Thing looks so old like it might break at any moment.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 24, 2019, 02:14:45 am
content: Ben321, you will be buying this, so your judgement remains supreme. But if you want help you should specify all your requirements up front.Fraser gave you excellent advice on a dirt cheap, repairable by normal humans thermal camera.Now it turns out that you have additional requirements. It must look modern. It must be compact. Must be one unit.That is a totally different animal and will come at a totally different price point. The units Fraser suggested are not attractive to many current buyers - which is one of the big reasons they are cheap. What is really important to you.

author: Fraser
author_rank: Super Contributor
timestamp: November 24, 2019, 11:45:51 am
content: Ben321,Looks can be deceptive.There is nothing wrong with CRT EVF’s. Good resolution and excellent contrast. The technology remains reliable and easy to maintain.These AGEMA products were built like tanks and intended to last. The cost of these cameras was so high (think $120K+ In the 1990’s) that there was room in the BoM to use excellent engineering within them. AGEMA were one of the best commercial manufacturers of thermal imaging equipment in the World. This is why FLIR bought them, along with Inframetrics.Your thread topic asks if it is possible to use a cooled MWIR camera with a bad cooler. Having just read your comments on the THV470, I am left wondering what cooled camera you have in mind. Such cameras remain quite specialist and many on the used market will be quite elderly, but well built. A failed cooler is often the reason for disposal of such expensive assets. The AGEMA THV550 is a favourite of mine. It is palmcorder shaped but larger. Even when faulty, the 550 can cost a lot of money. The Inframetrics PM280 is sometimes seen used but can be expensive. It is very compact for a cooled camera of the era. If you are looking for a more modern cooled camera like the SC4000 that I bought for less than £1K, good luck, that is a very rare event. Modern cooled cameras have longer lived coolers and remain very expensive acquisitions. As such they are often worth sending for repair. My SC4000 cost its original owner £150K plus the cost of lenses and accessories ! A cooler rebuild is less than £10K.