General thermal camera/optics questions - Page 1
Forum section: electronics / general

author: polar
author_rank: Contributor
timestamp: April 17, 2019, 08:21:28 pm
content: Hi allI have several questions regarding thermal cameras/optics. I already checked Frasers "Free Thermography and camera use guides / books", but they don't explain technical stuff in too much detail. The Optris and Fluke guides were interesting, but rather than answers even more questions surfaced. Sometimes it is mentioned that while visible photography cannot be compared to thermal photography as it uses reflected light as opposed to emitted light for thermal photography. But this cant be the big difference, just take a visible picture of a LED in the dark an you also deal only with emitted light.Maybe some of you can explain to me often quoted facts, or point me to a good book about thermal camera/optics where I could find answers.a) NETD is linear proportional to the f-number of the lensb) out of focus pictures will show wrong temperatures (if completely out of focus this is clear, or if you have small hotspots, but as I understood this limitation is more general)c) sharp edges can exist between a black and a white line (max contrast in visible pictures) but sharpd edges cannot exist between a hot and a cold linewhen I used different focal lenght lenses (but same f number) it seems like the max/min temperatures shown change significantly. The min max areas are of course visible in both views, all in focus, the aeras were quite homogenous in temperature therefore most probably not and effect of hotspots the wide(r) angle lens could not resolve.thanks in advance

author: eKretz
author_rank: Frequent Contributor
timestamp: April 17, 2019, 10:05:24 pm
content: Differing lenses will almost certainly have different IR transmission percentages due to differing lens shape and thickness, as well as coatings. That will affect the indicated temperatures. You can't just change the lens without calibrating for it and expect accuracy.

author: bugi
author_rank: Regular Contributor
timestamp: April 18, 2019, 06:46:56 am
content: Quote from: polar on April 17, 2019, 08:21:28 pmI'm certainly total noob when it comes to thermal imaging, but I have seen in practice a very sharp edge between hot and cold "line" (and hot spots etc.). However, I might guess that what is meant here is that since heat tends to flow around (if not insulated very well), the differences smooth out already on the subject. But, one can have e.g. very hot object behind a block of ice, and that would give quite a sharp hot/cold edge as seen on camera, as long as the objects are not far apart in distance to keep both within the focused range. The remaining blurring might come from wavelengths/diffraction limits/lenses/effects on sensor, but those effects seem to be ignorable compared to things caused by heat spreading, short depth of focus, and relatively long exposure times (with moving objects/moving camera).

author: Bill W
author_rank: Super Contributor
timestamp: April 18, 2019, 10:28:36 pm
content: Taking in turn...a) NETD is linear proportional to the f-number of the lensNETD should be a function of the sensor alone, measured with an f/1 lens.However actual camera 'signal to noise' is absolutley a function of f number, but only due to reduced signal (thermal radiation)b) out of focus pictures will show wrong temperatures (if completely out of focus this is clear, or if you have small hotspots, but as I understood this limitation is more general)Depends on how close to the 'fuzzy' you are measuring, and how out of focus. A long way out you affect the optical system and the 'brightness' across the sensor which will have been calibrated in.c) sharp edges can exist between a black and a white line (max contrast in visible pictures) but sharpd edges cannot exist between a hot and a cold lineYou can to an extent, by cutting a hole in the hot plate to see the cold. However you still have conduction in the sensor, optical effects or not aligning to the pixels of the sensor.Generally thermal sensors are much lower pixel count and the lenses are being pushed to resolve well at pixel pitch. However a 'good' system could be over 90% of the maximum contrast for a 1 pixel line.Bill

author: polar
author_rank: Contributor
timestamp: April 20, 2019, 06:46:33 pm
content: thanks for all the answers, still not totally happy with them. But in the mean time I found a book that looks very promising: "Infrared Thermal Imaging: Fundamentals, Research and Applications" by Klaus-Peter Möllmann and Michael Vollmer. It covers theory, technical/engineering topics in depth as well as a lot of application examples including exotic applications like visualizing the standing waves in a microwave oven. Drawback: over 600 pages to read ... But I think it is really worth looking at if one is interested in thermal imaging, would not be surprised if a few people on this forum already have one.Now to the answers:a) When I first read that NETD proportional to f I thought the answer was simple: Compare it to an optical camera (where I feel more at home) with fixed shutter time: if I reduce the aperture the recorded signal goes down, the noise is the same, SNR goes down. So basically the same what Bill wrote. But the problem is that everywhere I read about NETD being dependent on the f number it was claimed that the relationship is linear. However, the light or in this case thermal radiation would be inverse proportional to f^2 not f. Therefore if NETD for f/1 is 50mK, for a f/1.4 lens it would be 100mK, not 70mK. Thats why I came to the conclusion that my simple comparison to the optical camera was wrong. So I still need a better explanation or somebody telling me that NETD is indeed proportional to f^2 and not fc) the statement was put like it is a fact based on physics (like: there are not magnetic monopoles), not technical limitations. But it was stated in a FLUKE document, so maybe more a technical fact.That different lenses will have an impact on the accuracy I would have expected. But I was surprised by the magnitude.

author: IwuzBornanerd
author_rank: Frequent Contributor
timestamp: April 20, 2019, 09:10:36 pm
content: Quote from: polar on April 20, 2019, 06:46:33 pmWould it be accurate to say that reason for the statement in "c" is the result of the fact that you will not find an infinite thermal gradient, including in the sensor as Bill W pointed out? You can have a row of white molecules right next to a row of black molecules but you are not likely to find a row of 100 degree molecules next to a row of 10 degree molecules. Even if you do cut a hole in a hot plate & view something cold through the hole I would expect that the edges of the hole would be cooler than the main body of the plate. How much probably depends on the thermal conductivity of the plate, and "etc.".If you use a palette designed to facilitate seeing that MDTD (that @Ultrapurple is looking for on another thread) and assign the colors to temperatures accordingly you can certainly see sharp edges between the small temperature bands in the non-infinite gradient. I attached a sample of such an image showing a "sandwich box" fresh from the freezer held in front of a warm body. Most people should be able to distinguish 0.2 degree Fahrenheit differences (and thus lots of noise) in this Seek Pro image. And there are some nice crisp lines, particularly on the lower left corner where the tab on the lid sticks out.For a fun example with steeper gradient, I attached an image of an old fashioned "carbon composition" resistor (quarter watt, 75 Ohms attached to 6V DC "wall wart" output). You can clearly see the effects of the non pixel alignment in the horizontal edges of the resistor. I found this gradient interesting as I was hoping, for other purposes, that the temperature of the thing would be more uniform...As for the SNR proportional to F stop, I would agree that it should be more like a square law thing as you stated, but since Bill W didn't go into the "how proportional" issue I suspect it may be more complicated than that. Perhaps due to different transmission through different size lenses ?

author: Max Planck
author_rank: Regular Contributor
timestamp: April 22, 2019, 03:53:48 pm
content: Quote from: polar on April 17, 2019, 08:21:28 pmIt is not. The equation to calculate the NETD value for a camera with a f/X lens based on the value for f/1 lens is:NETD(f/X)= NETD(f/1) / ((1/X)* (1/X))as long as we are talking about uncooled cameras.Quote from: polar on April 17, 2019, 08:21:28 pmDepending on the size of the object you want to image, distance from the objects and viewing angle, there could be several reasons:The temperature does not change abruptly (there is always a finite temperature gradient) whereas you can have a sharp color change on the same surface.The spatial resolution af the system (IFOV), i.e. the angle seen by a single pixel of the FPA through the lens. VIS cameras are usually having much higher resolutions.The third problem - if one hits it - is the diffraction limit. VIS systemes are using roughly the 0.4-1.1 um band whereas for a LWIR system we are talking 7-14 um. An order of magnitude difference.Please see the comparison:https://www.researchgate.net/profile/Chris_Ste-Croix/publication/235157386/figure/fig1/AS:649308953145347@1531818726542/Top-Nomenclature-for-various-parts-of-the-electromagnetic-spectrum-Bottom-A-picture_W640.jpgQuote from: polar on April 17, 2019, 08:21:28 pmBy the book, the IR camera should be calibrated for a given lens. For example the lens itself is a source of radiation, which could be different for different designs.Quote from: Bill W on April 18, 2019, 10:28:36 pmNot exactly. As per the ASTM standard, NETD is measured at the detector preamplifier output, thus taking into account the detector and preamplifier noise, neglecting the lens etc.The fact is, that today there is no universal NETD measurement standard that would be adapted to FPA based camera designs and the end user is interested in the performance of his entire camera, not just the detector or detectors in the FPA.The f/1 is a good practice - if we are talking uncooled - but not a must.Max

author: polar
author_rank: Contributor
timestamp: April 22, 2019, 06:16:00 pm
content: Quote from: Max Planck on April 22, 2019, 03:53:48 pmAh perfect, if it is proportional to f^2 then it makes a lot more sense to me, see above. I would never had asked the questionI am sure I read several times that NETD is directly proportional/dependent on the f-number of the lens. or l examples where NETD for f/1 was just multiplied by the f number of the lens and not the square of it. The drawback is: Would I have known beforehand that it is indeed ~f^2 I would probably have gone for TE-Q1 with the 13mm f/1 lens instead of the 6.8 f/1.3. But I plan anyways to get other lenses, so not lot of harm done

author: Max Planck
author_rank: Regular Contributor
timestamp: April 22, 2019, 09:43:51 pm
content: A brighter lens is one of the solutions. There are others (but not always possible):- taking advantage of the fact that we are talking about white noise and averaging some consecutive frames; static scene required- slowing down the camera (lowering the frame rate), which is realted to the redout process in microbolometric cameras

author: polar
author_rank: Contributor
timestamp: April 22, 2019, 10:09:31 pm
content: of course, on my dslr I just increase the exposure time, no so easy on the TE-Q1. It would be interesting to know what the manufacturers do in general when they limit a camera to 9Hz if there is also a 30Hz version (sadly not the case with the TE-Q1, but e.g. for the TE-M1). It would be nice if they would increase the readout time (or average 3 frames). But maybe they just skip 2/3 of the frames to output?

author: IwuzBornanerd
author_rank: Frequent Contributor
timestamp: April 23, 2019, 08:18:16 am
content: As an example, we know from @Mikeselectricstuff's teardown video of the original Seek Compact that the Seek sensor is clocked at around 4x the output fps. The fact that individual frames can have 4 images in them suggests that they average the "oversample" frames to yield the output frames. I have uploaded a sample showing 3 frames taken from a video showing a single finger stationary, starting to move, then "zipping" across the view.I don't recall if anyone has determined from a teardown that the Q1 sensor is not running at >30fps. Has someone?

author: Ultrapurple
author_rank: Super Contributor
timestamp: April 23, 2019, 09:46:28 am
content: Quote from: polar on April 22, 2019, 10:09:31 pmSomething like this has been discussed on theThermal Imaging Galleryrecently, comparing the 9Hz and 25Hz Therm-App cameras. Both seem to have pretty-much identical sensitivity, suggesting that there is no averaging or other special tricks going on with the slower frame rate version. It is possible to switch on multi-frame averaging in various of the driver software but this introduces the tell-tale multiple image artefact on moving objects.The 9Hz Therm-App cameras simply clock the ROIC slower. If you waggle the camera you get a significant 'rolling shutter' effect. Opgal said they had a choice between this and a fast single frame followed by a dwell time, leading to 'jerky' video, and they decided the wavy video was subjectively preferable. (Locking the ROIC readout rate to 9Hz in the firmware also means there is NO faster frame rate data floating around, reducing the chance of a 9Hz camera being soft-hacked to faster frame rates. The 9-to-25Hz hack requires physically replacing the firmware memory device).