Thermal Expert Q1 - Temperature deviation some time after calibration - Page 1
Forum section: electronics / general

author: morningglory747
author_rank: Contributor
timestamp: August 09, 2019, 03:09:29 pm
content: Hello,I attached to this post two pictures taken with the Thermal Expert Q1. I've had the machine for about a year. The first picture (IMG_20190607_150438) was taken instants after a calibration on a flat wooden surface. The second picture (IMG_20190607_150707_IR) was taken a bit more than 2 minutes after that calibration on that same wooden surface. The location on the wooden surface is not precisely the same on both pictures, but I would assume a uniform temperature distribution across the plank. I took 2-3 pictures of random objects in between those two times. I must note that I also leave the camera 2-3 minutes opened before taking any pictures with it to make sure that temperatures are somewhat stable. The pictures were taken outside, under sunny skies. Previous posts on the impact of heat and solar radiation makes me wonder if that could be the issue.What I find peculiar is how the distribution of temperature deviated from the first to second picture. Because what this leads me to believe is that the objects that I photographed in between those two moment have a positive bias in the center and a negative one near the edges. I should also add that i've seen this deviation occure in many other sets of pictures, but not all with the same intensity, nor with the same pattern distribution.Unless I've screwed up my method, I wonder if this sort of deviation from the initial calibration is something that's been observed and documented before in other thermal cameras, but I don't how to look for these studies/documents because I can't find the proper term that defines this deviation. Would anybody know if this deviation is related to the internal procedures of the camera and perhaps even know what is usually done to account for this issue?Thank you!

author: bap2703
author_rank: Regular Contributor
timestamp: August 09, 2019, 05:49:54 pm
content: That surely looks like a plank distribution...Seriously that could be microbolometer chip temperature variation which is not uniform across the chip.Or lens temperature variation maybe ?

author: Bill W
author_rank: Super Contributor
timestamp: August 10, 2019, 11:35:23 pm
content: When the image is rescaled to the 384x288 detector size, it looks circular. That suggests lens related, sensor related would usually be 4:3 ratio or offset to where the higher dissipation is.The 'lens related' could however be simply the gain map, as illumination falls off to the edges.The less easily explained is that the edges are colder, usually goes the other way.I'd guess that a temperature sensor got hot faster than the system itself, and in correcting for that the camera has overdone it.For example:Measures 2° hotterActual is only 1° hotter so sensor output is only shifted by 1° of ambient effectsCamera processing subtracts a 2° shift, not 1°.More lens effects at edges, so subtracts more=> Cold cornersBill

author: morningglory747
author_rank: Contributor
timestamp: August 12, 2019, 01:34:25 pm
content: Hello Bill,It amazes me that such considerations exist in thermal imaging. It is truly a much more complicated process than what we can initially expect!I must note that in some of my experiments it does happen that the edges are hotter than the center.What do you think could cause a hotter temperature sensor? Would solar radiation or simply being turned on for too long could produce this?I'm interested in knowing in more details the mechanics behind this lens effect, but i'm not sure how to start looking for this. What sort of key words do you think I should be looking for?Lastly, I'm thinking of a way to correct this problem. I assume that the deviation occurs linearly over time and that it wasn't something that happens instantly. There would be a different coefficient for each pixel. What I would do is use the picture post-calibration as the initial state and subtract each pixel from both pictures to know the temperature deviation. From the subtraction I could get the rate of temperature change per second and then apply this correction for every picture taken in between. What would be your thoughts about this approach?Cheers!

author: Bill W
author_rank: Super Contributor
timestamp: August 12, 2019, 04:33:26 pm
content: The temperature sensor is (probably) a thermistor or LM45 type device, so is at one point. It is used (probably) to compensate all sorts of things like lens temperature, flag temperature (if there is one).Even if detector temperature is measured on die, it is still not 'on pixel'.With a camera dissipating around 1W unevenly distributed there will be temperature gradients from turn on until everything evens out again. You could try running for 20 minutes and then do a restart - is that then stable for longer ?While I have not used this camera, it is fairly typical that lens/detector temperature change of 1° moves the detector signal by 4 times the movement from a scene change of 1°.The lens effect is mainly illumination:Actual vignetting is rare in modern lenses with only a couple of elements.https://www.edmundoptics.com/resources/application-notes/imaging/sensor-relative-illumination-roll-off-and-vignetting/Note how bad it gets for low f numbers and wide angle lenses - exactly what you get in a thermal camera.The inverse of the illumination is 'baked in' to the camera calibration so you see an even image of an even scene, but that means any overall shift in detector output gets multiplied by the illumination loss.regardsBill