Why all BST cores are 320x240? - Page 1
Forum section: electronics / general

author: lixixi
author_rank: Newbie
timestamp: February 06, 2019, 10:45:47 am
content: Hi all.I'm just curious, all BST cameras seems to have a 320x240 sensor, so why is that?Thank you.

author: Psi
author_rank: Super Contributor
timestamp: February 06, 2019, 10:56:22 am
content: BST?If you mean thermal camera sensors its probably ITAR rules.There are restrictions on the pixel size and framerate.

author: Fraser
author_rank: Super Contributor
timestamp: February 06, 2019, 01:20:11 pm
content: The simple answer is that the BST core was developed for military use using DoD funding. The military will have provided a specification that was to be met. Raytheon we’re heavily involved in the sensor array development and they produced a very capable 320 x 240 pixel core design that could be used in Military applications. This core design was also used in civilian applications such as fire fighting, law enforcement and maritime navigation cameras. It found its way into vehicle night vision due to its high resolution and 30fps frame rate.All was looking good for BST sensor development but then the Microbolometer sensors became adequately developed to compete with the BST staring arrays. Without going deep into the events that followed, the DoD cancelled funding of the BST sensor program and invested in the Microbolometer development programs instead. The Microbolometer was seen as the way forward in uncooled thermal imaging technology.Raytheon had seen the writing on the wall and changed over to Amorphous Silicon based Microbolometer production. The main Microbolometer technologies of that period were Vanadium Oxide and Amorphous Silicon. The VOx arrays produced superior imaging to the A-Si versions and ITAR effectively ignored A-Si for a while. As such A-Si based cores were easier to ship around the world. Fire fighting camera manufacturers recognised this advantage and started using Raytheon A-Si cores like the venerable 160 x 120 pixel 20fps Thermal Eye AS2000.What the US authorities had not recognised was the fast development of A-Si sensor arrays and video processing. The A-Si sensors were producing excellent images at high frame rates. Eventually this oversight was recognised and A-Si sensors were captured in ITAR regulations. At that point life got more complicated for companies using such A-Si technology for their internationally distributed products like fire fighting cameras.ULIS in France produce A-Si based Microbolometers and their location in Europe made purchase and deployment of the sensors within Europe far easier than from the USA. The rest is history.So BST sensor technology was originally a military funded project working to a specification. It became a heavily controlled civilian technology and, to a point, it still is as officially ITAR applies. I doubt anyone in the US authorities would get over excited over sales of BST based technology on eBay though !For information, ITAR was very strict on thermal imaging technology when BST was in common use. There was no reduction in the regulations for cameras running at less than 9fps at that time. That came later and created the low frame rate core market. ITAR looks at a combination of sensor technology, pixel count, effective resolution and frame rate. Any core that exceeds the permitted limits becomes heavily regulated under ITAR and the equivalent Dual Use Technolgy relations in the World.Was BST any good ? Yes it was an excellent technology but static cameras required a chopper wheel to create the required difference signal in the pixels. This chopper wheel suffers gyroscopic effects if the camera is moved quickly. Such an issue could likely have been resolved with further development. The sensor is also incapable of producing radiometric output and has a low dynamic range but this could possibly have been resolved through further development. In BST fire fighting cameras a separate Thermopile temperature sensor was used for temperature measurement. Raytek produce that IR temperature sensor technology to this day.Fraser(Transparency and Attribution - much of the above knowledge was provide by “Bill W” and I thank him for the insight into the history of BST technology.)

author: Vipitis
author_rank: Frequent Contributor
timestamp: February 06, 2019, 01:44:27 pm
content: I was thinking that wafer size decided by pixel pitch and yield gave this as best point. Maybe even trying to save money by using parts that already exist and the resolution/framerate was the highest bandwidth they cod handle. Might as well be size needed for optics. But as it sounds, a DoD development can cover a whole redesign with no old parts holding you back.E: words

author: Ultrapurple
author_rank: Super Contributor
timestamp: February 07, 2019, 09:00:15 am
content: Fraser is doubtless right about the 'specification' points but there is another, simpler answer.So-called'NTSC' (USA standard) analogue televisionused 525 lines, interlaced, at 30 frames per second. (Actually, 29.97 but I'll ignore that). The 525-line system comprised480 lines of active picture information; the remaining lines wereused for sync pulses.Those 480 active TV lines wereinterlaced, meaning eachfieldwas 240 lines. This requires 320 horizontal pixels on a 4:3 ratio screen to have equal horizontal and vertical resolution. That's basically where the 320x240 figure stems from.A 320x240 pixel image can be 'read out twice' to give an interlaced signal far more easily than making a 640x480 sensor, so that's where all the development effort went. Awell-focused, contrasty image doesn't need to be particularly high resolutionto still be very usable.(Compatibility with NTSC TV standards is also whyVGAis 640x480. There was a wealth of experience at making displays (eg TVs) for that resolution; producing anything to a different standard was costly).It's interesting to note that in625-line, 50Hz (PAL) countriesthe equivalent sums work out to576 active lines, so 384x288 sensors are the most convenient. That's almost certainly why the likes of Ulis developed sensors using that format. Converting between 240- and 288-line systems either requires a big border with no information or some kind of smearing that reduces the apparent resolution.Now we're largely away from the shackles of analogue TV systems having to maintain compatibility with black and white systems originating in the late 1930s, in theory we can have pictures (and sensors) of any format we please, although there are still some well-establishedscreenformats with their roots in computerland (1024x768, 1280x1024, 1600x1200, 1280x720, 1920x1080...) - but there's no reason why, if you have the cash, you can't have a triangular, circular, trapezoid or whatever display made for you, with more or less as many pixels as you want. And I'm sure that FLIR and their contemporaries would be delighted to relieve you of a (very) large wheelbarrow of cash to develop a custom sensor in the same format.The nearer to circular you make asensor, the nearer it is to using the maximum amount of thelens system's image circle. Back in the days of theArgus 1, which had a round imaging tube (apyroelectric vidicon), they made use of the maximum possible amount of the sensitive area by scanning the whole front (active) area and presenting the image as a circle within a large black border (an electronic mask for the no-genuine-information parts of the video signal). This also meant that the light from the lens was used as efficiently as possible. But circles are tricky to cut out of (silicon) wafers and/or wasteful of silicon real estate, so in practice the nearer tosquareyou get, the better. Display screens, too, are generally rectangular these days (the earliest CRTs were round) and an X-Y array is convenient to scan electronically. Today's 5:4 ratio sensors such as 640x512 or 1280x1024 are the closest we have to making optimum use of the lens image circle; electronic processing means it can be displayed in a number of ways. Widescreen 16:9 thermal imaging sensors seem to remain rare, perhaps because they make very poor use of the optics (unless there's ananamorphic elementin the lens, but that's another story).Perhaps one day widescreen thermal imaging will become the norm, especially if there's a revolution in sensitivity meaning that cheaper optical elements can be used. Moulded glass (of some sort) is very significantly cheaper than single point diamond turned single-crystal germanium but, as yet, nothing quite offers the same all-round performance, though chalcogenide glasses may be coming close. There are of course many other substances that could be used as LWIR lens elements (eg Kbr, KCl, TiBr-Til, ZnSe, ZnS to name a few) but I'm getting off-topic fast...

author: Fraser
author_rank: Super Contributor
timestamp: February 07, 2019, 09:29:12 am
content: An excellent explanation UltrapurpleI had not considered why the 320 x 240 pixel specification came about. Your explanation makes the reason very clear. Thank youFraser

author: Ben321
author_rank: Frequent Contributor
timestamp: February 11, 2019, 08:41:41 pm
content: What's a BST image sensor? I only have heard of microbolometer arrays (both VOx and amorphous Si), and single pixels sensors that use fast motorized scanning mirrors to sweep the sensing "beam" of the image sensor across the target to be imaged.

author: Fraser
author_rank: Super Contributor
timestamp: February 11, 2019, 09:06:38 pm
content: BST = Barium Strontium Titanate.An uncooled pyroelectric sensor array using BST pixels responds to changes in the thermal energy falling on the pixel. It is a thermal sensor technology that requires the pixels to be ‘reset’ before each read cycle. For this reason the cameras usually have a rotating chopper wheel in front of the sensor array.Raytheon produced BST cores under the “Thermal Eye” range. The Cadillac De-Ville car thermal camera uses such BST core. Many 1990’s fire fighting cameras also contain BST cores.Fraser

author: Bill W
author_rank: Super Contributor
timestamp: February 12, 2019, 01:51:14 pm
content: The DoD programme that gave us the first generation of commercial cores was for a rifle sight and, as was the way at the time, backed several manufacturers each of whom held rights to the necessary Honeywell patents.The Raytheon ASi (160x120 20Hz), the Raytheon BST (320 x 240 30Hz) and the original Lockheed VOx (320 x 240 30Hz) all came out of the same programme. The VOx was the performance winner so was under tighter control and went forward as the military option. None were ITAR though, all came under commerce department with rules based loosely around Wassenaar. as such ASi escaped for a while as Fraser noted, with the update that included ASi in controls also bringing in the 9Hz get-out.Bill

author: Fraser
author_rank: Super Contributor
timestamp: February 12, 2019, 11:26:00 pm
content: Bill,Thank you for the corrections. My memory is not what it used to be.Fraser

author: Bill W
author_rank: Super Contributor
timestamp: February 13, 2019, 01:08:52 am
content: As I guess the early Lockheed VOX might be news to some, here is a picture of it in the Argus3 configuration.Very few made, really only as demonstrators. Most Argus3 were BST and the rest ASi.As attached, the core had two pretty heavy duty processing boards as well as a detector board, plus a lot of wiring. Apart from the small board at the bottom and the chassis frame this is all the core and not Marconi/EEV partsBill

author: Bill W
author_rank: Super Contributor
timestamp: February 14, 2019, 12:43:44 pm
content: Quote from: lixixi on February 06, 2019, 10:45:47 amTo answer the original question, there was only ever one BST detector design produced, and it was 320 x 240.There were minor variations in the electronics over time, and many people used the core or parts of it in their own end products.Bill