electronical alternative to the chopper wheel used in BST sensor ? - Page 1
Forum section: electronics / general

author: Lambda
author_rank: Regular Contributor
timestamp: February 16, 2021, 04:00:01 pm
content: Hello.One question came to me when considering the BST core principle, requiring chopping wheel for allowing to generate periodic variation of charges at the surface the pyroelectric pixels (if i do not mistake my self), and rendering the system able do detect not only moving subject but also static one.Would it be possible, instead of chopping the optical signal by using the wheel, to chop electrically each pixel from the sensor ?More precisely, would it be a possibility to periodically "ground" each pixel for draining their build charges acquired during the previous exposure, on a periodic basis, with a given duty cycle (exposure with charges building -> grounding with charges draining -> exposure with charges building -> etc...), which could be eventually adjusted for modifying the resulting output signal intensity delivered by the sensor (could be useful for some AGC control, optimizing dynamic range...).Perhaps silly/naive idea, but with the wish to get rid of the chopper wheel, weak part from the principle....In advance, thank you for your input.Best regards.Stéphane

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2021, 04:05:05 pm
content: Electrical chopping after the sensor does not work. The problem is that the sensor itself is kind of AC only and so one needs chopping before the sensor. The chopper wheel has the nice extra to provide a reference temperature, so ideally there is a temperature sensor for the wheel too.

author: Bill W
author_rank: Super Contributor
timestamp: February 16, 2021, 04:32:07 pm
content: The problem with the suggestion is that it is the temperature of the pixel that has to change to generate a signal. The charge gets drained away when the pixel is read out.You'd need a switchable thermal short circuit.A few people spent a LOT of time and effort trying to make a solid state chopper (think about a kind of LCD shutter made of germanium) for pyroelectrics and failed.Bill

author: Bill W
author_rank: Super Contributor
timestamp: February 16, 2021, 04:35:40 pm
content: Quote from: Kleinstein on February 16, 2021, 04:05:05 pmIn theory yes, but in practise the Raytheon chopper is clear / diffuse not clear / solid. That was I believe to assist in dynamic range.You could also (in theory - I never tried) operate the 'flag' in an ASi / VOx bolometer at field rate by making it a wheel and so remove the picture freeze effects at the expense of some added noise.Bill

author: Lambda
author_rank: Regular Contributor
timestamp: February 16, 2021, 05:36:38 pm
content: Thank you to both of you for your teaching!I easily imagine that brillant people were and are still working on it.... It is just some ideas more dedicated to help me to build my almost inexistant culture in this domain..Would it be possible, to imagine otherwise, a periodically polarizable "LCD" screen to be settled in front of the sensor and which would be easily switchable between opaque (for LWIR) / black state and transparent state (for LWIR), with the desired duty cycle? i read in the past some references dealing with "LCD" screen transparent to LWIR...Thank you for.Stéphane

author: Bill W
author_rank: Super Contributor
timestamp: February 16, 2021, 06:19:24 pm
content: Quote from: Lambda on February 16, 2021, 05:36:38 pmThat was roughly the idea apart from one missing feature.The switching between opaque and transparent has to follow the readout scan up the sensor, so giving the same (and maybe maximised) time for each pixel to change temperature.An LCD shutter would need to be multiple lines.That is why a 'normal' chopper wheel is that odd spiral shape. You can plot it out simply on a spreadsheet, but gets more complicated once the motor axis is offset to get the 'curve' to sit horizontally.Bill

author: Lambda
author_rank: Regular Contributor
timestamp: February 17, 2021, 08:06:10 am
content: Thank you Bill, for this complement.I forgot the requirement to synchronize the periodic blinding process with the readout, indeed!Stéphane

author: Kleinstein
author_rank: Super Contributor
timestamp: February 17, 2021, 08:29:29 pm
content: LCD shutters are usually at best 50% transparent, as they use polarization. Usually the liquid crstalls are some organic material and chances are they would absrob IR radiation pretty goood.I would more like consider a DLP device and thus a reflective switch - though here the angles are limited.

author: Lambda
author_rank: Regular Contributor
timestamp: February 18, 2021, 09:21:11 am
content: Thank you Kleinstein.Just for your last sentence... What is a "DLP device" (my English is a bit weak and acronyms are like a jungle for me, time to time...) ?Best regards.Stéphane

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2021, 09:55:05 am
content: With DLP I meant those arrays of micro-mechanical mirrors used in modern video projectors. The more I think about it, the more I think this would not work so well, as the mirrors only work with a relatively small angle and thus a relatively small aperture for the optics in front. It is also a rather expensive technology. So better start with a proper DC capable IR sensor in the first place.