The future of cooled MWIR cameras? - Page 1
Forum section: electronics / general

author: cejoba
author_rank: Regular Contributor
timestamp: September 27, 2022, 01:54:10 pm
content: Hi, I'm just thinking.With most devices goes to "solid state", what will happen to the cooled MWIR cameras, with heavy, expensive, short-life cooler?Will they continue to develop because of the advantages, or don't need cooler in the future, or be replaced by uncooled ones?Anyone want to talk about this?

author: ArsenioDev
author_rank: Frequent Contributor
timestamp: September 27, 2022, 02:15:58 pm
content: Thing with MWIR is the requirement for cooling is a quantum well limitation of the MCT/InSb sensor materials, note response graphs of cooled vs uncooled material on this responsivity graph.You'd have to discover some wacky magic material to get true uncooled and then evolve ROIC and process over several gens to get high fidelity and linearity across an array.

author: Fraser
author_rank: Super Contributor
timestamp: September 27, 2022, 02:41:56 pm
content: Cejoba,Cooled MWIR and Uncooled LWIR both have their advantages and disadvantages. Traditionally a cooled MWIR or LWIR FPA has been used for the best quality low noise imagery with excellent NeTD figures. In science and Military applications the cooled technologies are still very much alive and kicking. Cooled cameras high sensitivity permits higher F number lenses and so smaller lenses for long distance work. There is, of course, the need for a MWIR capability and this currently sits in the cooled FPA realm.With regard to cooler life, the technology has improved greatly and the old 2000 hour life of a Stirling Coller has extended to 10,000+ hours for a rotary cooler and 100,000+ Hours for linear coolers. Cooler lifespan is not the issue that it once was and the benefits of the cooled camera can outweigh the inconvenience of cool-down times, power consumption, bulk and service life.You will note that we do not see cooled thermal cameras in the consumer marketplace. There is good reason for this. Cooled cameras are very expensive compared to uncooled microbolometer types. The consumer market is well served by the uncooled technology and there is little appetite amongst manufacturers to try to market expensive cooled solutions against inexpensive uncooled models. The cooled cameras remain the preserve of the “Professional” users, where they provide excellent, if expensive thermal imaging. If you need low noise MWIR imaging, you have little choice but to buy an expensive cooled camera. LWIR serves most needs in the consumer/Prosumer market though. It is in Science and the Military that MWIR has its main customer base.I have noted ever improving NeTD figures stated for uncooled Microbolometers and this has interested me. It is not uncommon to see an NeTD of 35mK stated for a modern microbolometer. It is clear that microbolometer noise management has been an area of significant development over the years. I still think cooled cameras will produce a better low noise image however.So, will we see the expensive, bulky and finite service life Stirling Cooled MWIR cameras disappear and go the way of BST and PZT FPA based technologies ? Nope, there is no better choice of technology for more demanding science and military applications. That said, they are already scarce outside Science Labs, Industry and the Military and that will likely remain the case. Used units can be amazing…… if they still work ! I wrote a piece on buying used cooled cameras some time ago. I will add a link in a minute.I own a few cooled MWIR cameras that are still happily working and producing lovely imagery. I love that technology and it was in the mid 1990’s with an AGEMA THV550 handheld cooled thermal camera that I discovered how great a cooled FPA camera can be. A few years back I won an auction for a very nice THV550 with an end price of 99 pence ! I was amazed when I received the camera and found it to be in perfect working order. Sometimes it is better to be born lucky than richhttps://www.eevblog.com/forum/thermal-imaging/a-stirling-cooled-qvga-thermal-camera-for-99-pence-!-yep-no-kidding/Fraser

author: Fraser
author_rank: Super Contributor
timestamp: September 27, 2022, 02:45:13 pm
content: The link to my post discussing the purchase of used cooled cameras…..https://www.eevblog.com/forum/thermal-imaging/thermal-camera-purchasing-the-perils-of-buying-a-cooled-camera/Fraser

author: Fraser
author_rank: Super Contributor
timestamp: September 27, 2022, 02:49:08 pm
content: While we are discussing cooled cameras, let us not forget that the market for Stirling coolers is still very healthy. Such coolers are to be found in low noise RF amplification systems and the toys that NASA likes sending into space on voyages of discovery ! Cooled semiconductors are still required in specialist applications. The coolers were never exactly a cheap item on the BoM and that has not changed…. They have just been developed to have a longer, more reliable lifeI wonder if Voyager 1 and 2 contained Stirling Collers and if so, are they still working ?https://eng.ox.ac.uk/cryogenics/research/cryocoolers-for-space-applications/Fraser

author: Fraser
author_rank: Super Contributor
timestamp: September 27, 2022, 03:29:07 pm
content: As a side story about the users of cooled thermal cameras………I was fortunate enough to win an auction for a FLIR SC4000 cooled MWIR camera that was in great working condition and had low hours on its Linear Stirling Cooler. That camera had been disposed of when the company that owned it had gone out of business.I did a lot of research on the cameras previous owners and even identified when the company went out of business and why. The camera had only been purchased a year before the company closed, hence the low hours. It had been used in the R&D department to develop specialist heaters that sit behind glass.The MWIR SC4000 was recommended to the company by FLIR because it could see through the glass and correctly image the heating element. A LWIR camera cannot do this as the glass blocks the image of the heater that sits behind it. A special “Hot Glass” filter was supplied with the camera and the SC4000 selected for the task was a special “broad spectrum” version that covers MWIR plus part of the SWIR bands. The camera easily imaged the heating element that was positioned behind the glass plate and served its owners well. I have sample images from the R&D testing and the clarity with which the spiral heating elements was imaged is excellent. The R&D team were designing new, complex, heating element assemblies and needed to check for “hot spots” on the elements that would lead to its early failure through localised overheating.The company that bought a £150K FLIR SC4000 thermal camera was civilian. This was not some specialist lab or military establishment. They were effectively forced to buy such an advanced camera because of the need to image through hot glass with high clarity and there were very few choices of suitable thermal camera for that task. Sometimes a specialist need requires an equally specialist solution. The industry in which the original owner worked was a multi Million Pound one where the release of a sub standard product could bring ruin to the factory. The cost of the camera was therefore justified as it was part of a critical testing regime at the R&D stages of product development. The company had to apply for special permission to purchase the SC4000 broad spectrum camera as it was/is the direct product of a US Military development program with FLIR. Such specialist kit is still required and may be purchased by those who satisfy the relevant background and usage checks. They also need to be able to afford the high cost as well though !Fraser

author: cejoba
author_rank: Regular Contributor
timestamp: September 28, 2022, 01:19:35 pm
content: Thanks a lot guys!It's interesting to read these!

author: IR_Geek
author_rank: Regular Contributor
timestamp: September 29, 2022, 12:14:27 am
content: Fraser is nailing it on the benefits of a cooled MW. If size, weight, power, and cost are not super critical, then it's really hard to beat a cooled MW. Even more so if you are in a signal rich scene. To get similar resolution at useful longer ranges the size of an optic for an uncooled sensor destroys any benefit that a small uncooled camera core brings to the table.

author: Bill W
author_rank: Super Contributor
timestamp: September 29, 2022, 10:03:38 am
content: there was an old rule of thumb that if the lens cost more than the camera, you had bought the wrong camera !A typical MWIR lens is f/4, not the f/1 of a LWIR uncooled. They are working on 1/16th of the input energy, and more so due to Planck curve intensity if looking at an ambient scene.Note that NeTD of MWIR are often quoted at f/4 not f/1.That far higher basic detector performance that allows an f/4 optic has saved 93% of the optics volume (weight, cost) and also increased depth of field significantly.Bill

author: cejoba
author_rank: Regular Contributor
timestamp: November 14, 2022, 01:07:28 pm
content: I just have a new question that may not worth a new thread, so I just posting here:Why almost all (except some MCT) cooled thermal cameras are working at 77 K?

author: Fraser
author_rank: Super Contributor
timestamp: November 14, 2022, 04:16:42 pm
content: 77K (-196C) is the thermal detector temperature achieved in a Liquid Nitrogen cooled thermal camera. Early cameras often used Liquid Nitrogen before the Stirling Coolers or ‘Hot’ sensors appeared in cameras. ‘Hot’ sensors often operated at -70 Degrees Celsius.Why Cool to such a low temperature ? The semiconductor thermal detector creates internal noise at a room temperature of, say, +20C. This internal noise saturates the device to the point that it becomes blind to incoming thermal energy. To reduce the internal noise of the detector, a cooling system is employed that takes the detector down to -196 Degrees Celsius. At this very cold temperature the internal noise of the detector is so low as to make the detector very sensitive to incoming thermal energy and we then have a useful sensor for a thermal imaging camera. Early designs cooled a single pixel detector but later designs cooled an FPA.‘Hot’ thermal detector designs use a different material for the semiconductor and operate adequately at -70 Degrees Celsius. That said, they remain higher noise than those detectors operating at -196 Degrees Celsius (77K)As to why 77K was used when Stirling Coolers were introduced in place of Liquid Nitrogen …. I honestly do not know. It appears the designers of the camera/detector systems took 77K as a good temperature at which to operate known semiconductor mixes that operate well with Liquid Nitrogen temperatures.Fraser

author: cejoba
author_rank: Regular Contributor
timestamp: November 18, 2022, 01:00:03 pm
content: Quote from: Fraser on November 14, 2022, 04:16:42 pmThank you.I also had that guess, many designs have historical reasons like this.

author: Fraser
author_rank: Super Contributor
timestamp: November 18, 2022, 02:03:55 pm
content: The ‘HOT’ cooled thermal sensors are quite interesting technology and worth researching if you find time. The challenge facing designers was how to cool a sensor pixel or FPA to -70C. This temperature is low, but achievable using semiconductor based cooling technologies. The Peltier element was perfect for the task. It offers a solid state cooling technology that is well understood and offers a long operational life if the drive circuit is correctly designed. Now using a Peltier module to produce a ~90 Degrees Celsius temperature differential is not simple. Those familiar with the operation of Peltier modules will know that you cannot simply stack a few modules together to increase the temperature differential capability. A Peltier module creates a temperature differential across itself to create a cool side and a hot side. The hot side is often connected to a heatsink to remove the heat and permit the cool side to reach low temperatures. This is how Thermo-Electric fridges work. A single Peltier module cannot create the required 90C temperature differential though. I have already said that stacking modules will not work… why ? Well the first module in the stack produces heat energy on its hot plate that is a combination of the Peltier effect and self heating. The self heating effect is significant so a stack of three or four modules all producing self generated heat will destroy the coolers efficiency and performance. The solution is a ‘pyramid stack’ with the first module being smaller than the second, which, in turn, is smaller than the third and so on. What this pyramid design does is keep the previous Peltier modules self heating at a manageable level and creating the required temperature differential across the stack by using more stages of appropriate dimensions to manage the self heating issues. To increase the performance of the Peltier cooler stack, it resides in the vacuum of the thermal sensor or FPA that is being cooled. This vacuum reduces the thermal load on the Peltier stack and, importantly, stops the issues associated with icing of the cold finger. Operating at -70C in a normal atmosphere would cause the formation of frost, ice and water. None of which are a good idea inside a camera. The down side of incorporating a Peltier stack cooler in the sensor/FPA vacuum module is the increased size of the module and the potential for Peltier stack out-gasing poisoning the vacuum within the sensor/FPA module. In my personal experience, I have not seen performance degradation through Vacuum poisoning in these types of cooled cameras and some date back to the early 1990’s. Another issue with using a Peltier cooler stack is the total current draw of the Peltier stack. This technology is not known for its low power consumption. Appropriate sizing and management of the Peltier cooling stack makes this technology suitable for portable cooled camera applications running on a standard camcorder battery pack.I attach some pictures of Peltier cooled sensors. I also include a few pictures of the Peltier cooled SPRITE strip thermal sensor from the AGEMA THV470 thermal camera.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: November 18, 2022, 02:36:07 pm
content: A YouTube video showing someone building a 3 layer Peltier stack. This demonstration shows the principle of pyramid stacking and the issue of frost formation on the cold face of the Peltier stack when operated in open atmosphere. The experiment reaches a cold side temperature of around -60 Degrees Celsius in atmosphere with no thermal load applied.https://youtu.be/D50O0oRjsLEFraser