EEVblog Electronics Community Forum
Products => Thermal Imaging => Topic started by: Lambda on February 16, 2021, 04:00:01 pm
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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
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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.
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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
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The chopper wheel has the nice extra to provide a reference temperature, so ideally there is a temperature sensor for the wheel too.
In 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
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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
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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...
That 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
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Thank you Bill, for this complement.
I forgot the requirement to synchronize the periodic blinding process with the readout, indeed!
Stéphane
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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.
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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
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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.