Influence of the rpm value of a chopper wheel on sensitivity of BST sensor? - Page 1
Forum section: electronics / general

author: Lambda
author_rank: Regular Contributor
timestamp: April 30, 2021, 02:18:54 pm
content: Hello.When considering the working principle of BST sensor, requiring to use a chopper wheel or generally speaking an optical chopper, i was reminding a small experiment i did during my studies:I was simply using a PIRE sensor, therefore based on the using of pyroelectric material building charges when exposed to IR light, and usually used with Fresnel lens, for simulating a kind of optical chopping when IR source is in movement....For my little experiment, i took only the sensor naked, without any lens, and i wired its three terminals to a DC power supply, ground and the exit was loaded with a loading resistive charge (between dozen of kOhms, and 100 kOhms, ) and then the signal was picked up at the entrance a simple oscilloscope.The sensor was attached to a simple tripod and a LWIR source (small halogen lamp with a filter (passband around 6-7 um from memory)) was settled at one meter from the sensor,in addition, a motor with a simple chopper made from a cardbox disc with a slit section was settled in front of the sensor for chopping the light coming from the source.It was working as expected:- no chopper: as a signal picked by the scope: just a short dirty pulse when the LWIR source was switched on, then after this transitory signal, nothing except a weak background noise. And then, a new pulse when the LWIR source was switched off.- with chopper: as a signal picked by the scope: a nice periodic kind of pulses signal. stable, the frequency being in accordance with the rpm delivered by the chopper.What was interesting is that:When i was playing with the rpm value of my chopper, with the LWIR source running in a stable manner (supplied V and I being constant), after a warming step, i noticed that i obtained a maximum amplitude of signal for a given value of rpm... Chopper to slow, the signal was collapsing, Chopper to fast, the same.There was a value of rpm "not toot low, not too high" where the repetitive signal was with a maximum amplitude.From this (old) memory I was therefore wandering if it would not be interesting to try to render variable the rpm of the chopper wheel used in system using BST sensor, like for the Argus 2/3 or other thermal camera using BST sensor, in general?This, in the purpose to increase the sensitivity of the camera? increasing its dynamic range when controlling, in real time with a feedback loop, the rpm of the chopper wheel? by playing perhaps between 7 or 9 Hz and 30 or 40 Hz (juste an arbitrary example)?Just an idea...Best regards.Stéphane

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: April 30, 2021, 02:31:12 pm
content: Optical choppers are often used to modulate a light source by a controlled frequency and the detected intensity of the modulated light is then demodulated (e.g., a phase locked loop, phase sensitive detector, lock-in amplifier etc). This allows weak signals of interest to be detected in the presence of noise. My suspicion is that the processing electronics probably have a bandpass filter and by operating at the extremes of rpm, you are simply seeing the limited bandwidth.Just speculation based on similar stuff I've done.https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=287#01692D5A-0F29-4021-9444-237E7B94C7FF-287

author: Bill W
author_rank: Super Contributor
timestamp: April 30, 2021, 04:30:20 pm
content: The effect will mainly depend on the thermal time constant of the sensor material / pixel.Due to the relative ease / sense in running at TV video rates, the BST sensor structure will have been optimised for being read out around every 16ms.Your single pyro cell could easily have had a time constant in numbers of seconds. At a guess the 'slow speed' fall off was charge leakage into the load resistor ?Bill

author: dmendesf
author_rank: Frequent Contributor
timestamp: April 30, 2021, 11:54:42 pm
content: Best way to do it would be to use a lock-in amplifier (angular velocity of wheel locked to integration time of amplifier). This is done in physics experiments to study mechanical waves induced by light (forgot the correct name for that).

author: dmendesf
author_rank: Frequent Contributor
timestamp: May 01, 2021, 12:10:36 am
content: Photoacoustic spectroscopyhttps://en.m.wikipedia.org/wiki/Photoacoustic_spectroscopy

author: Lambda
author_rank: Regular Contributor
timestamp: May 01, 2021, 12:57:41 am
content: Great inputs from both of you! Thank you!Yes Bill, it is true that i did not consider the "RC" relaxation time in my little experiment. I should have played with different values of charge resistor... Big chance i would have obtained a maximized signal for different values of rpm....Best regards.Stéphane