VoX vs BST history... - Page 1
Forum section: electronics / general

author: vucaxi
author_rank: Newbie
timestamp: April 24, 2019, 02:22:55 pm
content: Hello.Why did US government chose VoX over BST? And what if the history changes, If BST got the chance to live, will they finally be able to measure temperature by the sensor itself, and reduce the halo effect?Thanks.

author: Hyper_Spectral
author_rank: Regular Contributor
timestamp: April 25, 2019, 12:26:20 pm
content: I would imagine resolution, for the most part.

author: Vipitis
author_rank: Frequent Contributor
timestamp: April 25, 2019, 02:11:49 pm
content: Silent operation, no moving parts

author: Bill W
author_rank: Super Contributor
timestamp: April 25, 2019, 05:02:47 pm
content: Actually neither of those.The early BST and VOx were both 320x240, and the VOx still needed a shutter. So it was 2 motors for BST (Chopper / iris) and one, or a solenoid, for VOx.Pro BSTLower powerFor an OEM easier to useEasier to make (VOx is a pig, needs it's own foundry)The 'halo' was liked by many30 % cheaperPro VOxDirect temperature measurement / calibrated image dataSmaller frontal area (flag vs chopper wheel)No iris - VOx was carefully specified as no CONTINUOUS moving partsBetter (on paper) NeTD so could make very good imagers if used wellBill

author: Hyper_Spectral
author_rank: Regular Contributor
timestamp: April 25, 2019, 06:08:04 pm
content: Thanks for that information. I figured VOx looked better for high resolution cores at the time and that's why they chose it but maybe they thought high resolution would be just as difficult for either type?

author: Bill W
author_rank: Super Contributor
timestamp: April 25, 2019, 08:52:20 pm
content: I think BST would have been a lot more difficult to get smaller pixels, which is really what was needed to go to higher resolutions.VOX and ASi are both there now, but there was a possible thin film ferroelectric detector being floated by Raytheon as a successor to BSTI think it would be possible to calibrate a BST sensor, but it would need a solid chopper wheel of known temperature, and that would lose dynamic range or cost MDTD if expected to look at much over 50°CBill

author: yucel
author_rank: Newbie
timestamp: April 27, 2019, 02:31:52 pm
content: Quote from: Bill W on April 25, 2019, 05:02:47 pmMy friend'd BST camera don't have an iris, and since VoX is getting shutterless recently, why wouldn't BST become chopperless if development continued?

author: Fraser
author_rank: Super Contributor
timestamp: April 27, 2019, 03:08:14 pm
content: Yucel,You are talking about two very different technologies.Microbolometers use a FFC flag for flat field correction to counter pixel drift in the offset tables. Microbolometer cameras that operate without an FFC flag rely on image processing algorithms and generic pixel drift compensation tables. Such flagless are OK for pretty picture applications but poor for thermography radiometric measurements.The BST semiconductor sensor array does not use the chopper wheel for flat field correction. The sensor pixels output change of charge state only and have to be ‘reset’ ready for each frame capture. Without the chopper wheel the BST sensor will only register thermal targets that move in the scene or show a panning image if the camera is panned to create change in the scene presented to the sensor pixels. If you remove the chopper wheel a BST camera that suddenly has its lens cap removed will initially show a thermal scene and then the image will fade to grey with no thermal scene evident. If a person walks across the scene then they will be visible as they are changing the scene that the pixels see. This physics driven behaviour was actually used by IRISYS to create clever thermal camera people counting and tracking devices. The camera has no chopper wheel and only registers warm targets that move in its field of view. Static warm targets are ignored at the scene collection level so no need for image processing based static object removalSo a Microbolometer flag is a form of image quality and calibration tool, whereas the chopper in a BST is an image creation tool required by the physics under which the sensor pixels operateIt might have been possible to create a more compact solution to the rotary chopper wheel for BST sensors if development had continued. DLP mirror chips come to mind with their miniature moving mirrors or maybe some Thermally transparent film like LCD technology that could have its pixels switched to thermally opaque under the control of the ROIC ? There are always solutions to problems, it just takes money and motivation. The military had both and chose microbolometers, hence why we are now using very refined versions of this technology.You need to reverse the equation really.... with modern microbolometers why would a developer want to go back to BST and invest in its further development. If it offers no serious advantage over the Microbolometer it is dead technology. Just look at Betamax vs VHS..... VHS won on cost grounds and in that particular case was an inferior technology !The way forwards is likely the development of an even better technology than that which has been tried previously. Not looking back on the old tech, but looking at new materials or approaches to thermal imaging sensors. Technology like laser frequency mixing or some crystal based pixel technology. There are currently pixel size limitations due to the wavelengths being used. Maybe this can be overcome whilst maintaining collection efficiency ? The military will be the driving force behind any leaps forward in this technology and sadly we could find ourselves back in the 1970’s in terms of the technology being heavily controlled whilst it provides a tactical advantage on the battlefield.Fraser