What exactly is meant by a differential temperature thermal imager? - Page 1
Forum section: electronics / general

author: Ben321
author_rank: Frequent Contributor
timestamp: April 22, 2021, 01:00:07 am
content: I know that the cooled thermal imagers (such as HgCdTe) are considered absolute temperature thermal imagers, while the more common uncooled VOx thermal imagers are considered differential temperature thermal imagers. However, differential means a difference between 2 things. What is the temperature difference that is being measured with such thermal imagers? Both types of thermal imagers appear to be designed to give actual temperature (not temperature difference) readouts. I mean on my FLIR One, I have a temparature scale displayed on the side of the image, that I can use to compare the color or brightness of the pixel with what point on the scale it is to get a rough estimate of the temperature (not a difference between that pixel some other temperature, such as another pixel on the screen). Likewise, if I turn on spot temperature mode, I will get the exact temperature of the center pixel on the device (not a difference between that pixel and some other temperature, such as another pixel on the screen). So can someone here explain exactly what temperature difference is being measured?

author: Spirit532
author_rank: Frequent Contributor
timestamp: April 22, 2021, 04:10:30 am
content: Cooled imagers are chilled to a point where basically nothing emits any significant amount of thermal radiation(77K-ish and below). There is also little to no self-heating.This, essentially, means that you can almost directly convert the incident photon energy to the object's temperature(assuming our lens/housing/etc are made out of I-VI unobtainium supercoldium).With uncooled detectors, in simple terms, you're measuring the difference between the sensor's temperature and objects. You have to take into account self-heating and other things.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 22, 2021, 04:38:48 am
content: There may be somewhat different usages of these terms by different companies, schools and regions.The definition I would use is slightly different than the prior poster's, but shares much of the same thought process.A differential imager makes no pretense of knowing the absolute temperature of any scene element, but provides an image where brightness difference between display pixels (or data in memory storage) is at least approximately proportional to the difference in temperature of the corresponding scene elements. Some differential sensors may be gain calibrated so that over a specified temperature range the difference can be scaled in temperature, but still make no claim to represent absolute temperature of the scene element.Calibration to provide absolute temperatures is much simpler for cooled sensors. As Spirit says it mostly boils down to knowing the photons to brightness scale factor. (Mostly. There are more and more details the more accurately you need the results and the broader the temperature range over which you need them.) Calibration is possible for uncooled sensors, but there are many more variables resulting in much lower precision for a given amount of effort. This concept also gets into definitions. One of the techniques to help achieve some level of calibration is to stabilize the temperature of the array. Is a stabilized temperature array considered uncooled? Depends on who you ask.

author: Ultrapurple
author_rank: Super Contributor
timestamp: April 22, 2021, 08:25:42 am
content: Quote from: Spirit532 on April 22, 2021, 04:10:30 amDamn - we've just scrapped nearly half a ton of pure transparent Supercoldium because no-one was interested in buying it.

author: Max Planck
author_rank: Regular Contributor
timestamp: April 22, 2021, 08:30:40 am
content: Quote from: Ben321 on April 22, 2021, 01:00:07 amI think this is about quantum vs thermal detectors and their principle of operation.Max

author: electr_peter
author_rank: Unknown
timestamp: April 22, 2021, 09:05:04 am
content: Task of general thermal imager is to provide image that:represents thermal gradient of the subject (main goals - high sensitivity, high linearity, etc.) ANDrepresents actual temperature (goals - high accuracy)Absolute temperature is generally of less importance in thermal cameras than thermal gradient. Thus "differential" in "differential temperature thermal imager" means that camera has limited ability to provide accurate temperature data. Camera may not even show temperature units and just say that white parts are hotter than black parts.You can think in terms voltage measurement with ADC and reference. ADC gives relative (differential) value, reference gives absolute value reference, both are needed.To get accurate voltage levelget ADC value (which is relative to whatever reference used) - this can be converted to % of ADC full range, say 50%get or know value of reference used (say 2.048V)multiply ADC value by reference value to get final value (50% * 2.048V = 1.024V)If reference is not stable, drifting, then absolute value is not very accurate. But overall relative ADC values give you relative change.Not all thermal cameras have a good reference, so to speak.

author: Bill W
author_rank: Super Contributor
timestamp: April 22, 2021, 10:08:21 am
content: Quote from: Ben321 on April 22, 2021, 01:00:07 amThe flag temperature (or the calibration state in shutterless).The basic temperature measurement logic in an uncooled goes like this:here is a flagthe flag is 31°Cremember this datahere is no flagall differences in data are the differences of the scene from 31°C (subject to the gain per pixel and lots of secondary stuff)Bill

author: Lambda
author_rank: Regular Contributor
timestamp: April 22, 2021, 11:01:56 am
content: It is a very interesting thread! Thank you to both of you for that!One question:If the reference temperature is given by the temperature of the shutter flag itself, and knowing that it is generally better to leave the sensor itself in peace and working at its normal working temperature (factory defined ?):Would it not be interesting to try to cool down (uniformly) ONLY the shutter flag by itself (again, without acting on the temperature of the sensor, and therefore without trying to cool said sensor down)?It would be therefore an increasing of the resulting "delta T" or better said, "delta signal" delivered by each pixel of the sensor? Such increasing resulting in a better sensitivity??Regards.Stéphane

author: Bill W
author_rank: Super Contributor
timestamp: April 22, 2021, 12:49:20 pm
content: Hi Stephane,These days most sensors simply run at ambient, they do not contain a pletier or even a simple heater. The bonus of the ambient flag is that it also cancels out the lens temperature (if you can assume it is the same).With your suggested operation the calculation becomes:Here is a flagThe flag is 0°C (maybef, it will have some reflections from the sensor temperature if not emissivity 0.99999999)Here is no flagThe energy on each pixel is from the scene plus some the lens metalwork / scatter / germanium (non)transmission.The end picture noise is still some proportion of the calculated scene differences so a large step from the flag is no help.Bill

author: Lambda
author_rank: Regular Contributor
timestamp: April 22, 2021, 01:25:40 pm
content: Ok Bill, i got it.I forgot indeed the influence, in term of signal participation, ofQuote from: Bill W on April 22, 2021, 12:49:20 pm... Meaning, more generally, if i understand well, any kind of materials or elements belonging to the camera itself and which would be permanently in the neighborhood, or "visible" from the FPA of the sensor.Stéphane

author: Fraser
author_rank: Super Contributor
timestamp: April 22, 2021, 01:46:08 pm
content: Slightly off topic but in the same realm.........I got a bit of a surprise when I bought my FLIR SC4000 science camera. It had been calibrated for a specific use scenario.... imaging heating elements THROUGH glass and ceramic using special filters. That meant all of it’s calibrated ranges were set up for high temperatures.I used the camera at room temperature and selected a suitable temperature range..... the measurement was presented to me as “Counts” and not Celsius or other temperature units. That had me scratching my head. The reason was simple..... the camera contained no calibration tables for the temperature span I was using so presented the RAW data to me. That RAW data is radiance counts of the ADC. To have the camera present the counts as a temperature unit you have to carry out the calibration process that is available to the user. This applies a Radiance Count to Temperature unit conversion plot that is matched to the cameras count output with Blackbody temperature references. The conversion from ADC radiance counts to temperature units is then done my the camera and you get your calibrated temperature readings. A thermal camera works in Radiance counts from the ADC and not directly in any temperature unit. Units of temperature are purely a result of the application of count to temperature conversion curves. This is why a perfectly accurate thermal imaging system can have its accuracy degraded by poor application and calibration of the count to temperature unit curves.‘Housekeeping’ tasks within a thermal imaging camera may use the RAW ADC Radiance counts or the calibrated temperature scale depending upon the systems design. In general it would be better to work with the RAW ADC counts and only convert to temperature units where required for users understanding.All of the above is usually invisible to the user as it firms part of the systems calibration. It is just that my Science camera provides the professional or scientific community with the ability to calibrate the camera as they wish using the RAW counts data that is normally hidden from the end user.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: April 22, 2021, 01:52:46 pm
content: Then there is‘lock-in’Blackbody (BB) referenced thermography where the camera uses a very accurate BB temperature reference in the cameras FOV as a means to‘lock-in’correct its measurement calibration tables and provide a more accurate temperature measurement. The camera or associated software recognises the temperature differential between true and measured temperatures and makes the required calibration corrections. This is why many fever detection camera systems use‘lock-in’BB referenced thermography temperature references ... it is to improve their measurement accuracy to that required for medical use.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: April 22, 2021, 02:27:12 pm
content: Regarding the built in ‘calibration’ / FFC shutters.....The FLIR SC4000 and Amber Radiance 1 cooled cameras used a pair of calibration ‘flags’ in the 2 point self calibration mode of operation. The ‘flag’ temperature is set by the cameras control system using Peltier elements mounted on the flag. The temperature of the FFC flag in a thermal camera does assist the camera in checking its own measurement calibration but it occurs in front of the sensor array and not in front of the lens block. It knows the temperature of the FFC flag by using either a direct contact sensor on the flag or the temperature of the environs in which the flag resides. The quality of the FFC flag temperature measurement system can have a direct effect on the accuracy of the camera systems temperature measurements as an error can be transferred to the measurement offset and correction system. Some cheaper thermal camera cores use nothing sophisticated as the FFC flag temperature sensor. It is often a Silicon diode or thermistor that feeds an ADC to create a flag temperature value that is incorporated in flag ADC output value calibration tables.

author: Max Planck
author_rank: Regular Contributor
timestamp: April 22, 2021, 02:59:11 pm
content: Quote from: Fraser on April 22, 2021, 01:52:46 pmLock-in thermography is something different. It is a technique used for example in NDT (non-destructive testing). The principle of lock-in thermography is based on the application of a periodic input energy wave (i.e. thermal emitter, ultrasound, microwave, eddy current, flash lamp) to the surface of the object being examined and analyzing the resulting local temperatures on the surface of the object.You could call it synchronous thermography as the temperature measurements are synchronized with the heating wave.Max

author: Fraser
author_rank: Super Contributor
timestamp: April 22, 2021, 03:04:45 pm
content: Ah thanks Max, I have done the ‘lock-in’ thermography that you describe and misused the term in my comment. Thanks for clearing that upIn my world it was also considered ‘active thermography’ as opposed to ‘passive thermography’Fraser

author: Max Planck
author_rank: Regular Contributor
timestamp: April 22, 2021, 03:37:07 pm
content: To by precise, lock-in is a type of active thermography. Active thermography doesn't have to be lock-in.By the way, your SC4000 camera should have an input/output for lock-in, probably labeled lock-in or sync.Max

author: IR_Geek
author_rank: Regular Contributor
timestamp: April 22, 2021, 06:43:01 pm
content: All IR 'cameras' produce a current or voltage at the single pixel detector level. Assuming you are not measuring that directly (think pour filled single pixel sensors with a pre-amp), then the signal is converted to a digital 'count'. These count values will vary depending on how large of an A/D converter is used. Usually it's 8bit (0-255), 12 bit (0-4095), or 14 bit (0-16383). Then if the system is a thermography type the counts are converted via an internal calibration into temperatures. Then the user only see's temperature being output. However, most high end cameras will dump the RAW count values into a collection computer. Items like bad pixel, gain, and offset tables can be applied along with a radiance (or temp) calibration. Sometimes those corrections can stored in the camera head.Uncooled vs. cooled:uncooled generally fall into bolometer (resistance changes when exposed to IR that is then sampled electronically) or pyroelectric (similar to bolometer but has to have a chopper to induce the changes in signal)Cooled: generally operates under photovoltaic or photoconductive principle ... i.e a photon is absorbed into a material inducing a signal. For IR, the material must be cooled to get below ambient noise in the bulk material.That is quick and dirty as there are entire books dedicated to specific types. Antoni Rogalski has done several wonderful open papers on the topic and a MASSIVE well documented book called "Infrared Detectors". It's not for the faint of heart, but has many excellent diagrams.

author: Ben321
author_rank: Frequent Contributor
timestamp: April 24, 2021, 07:08:16 pm
content: Quote from: Fraser on April 22, 2021, 01:46:08 pmWhat is, meant by "counts" anyway? Counts would make sense if a microbolometer array contained high-sensitivity detectors, capable of detecting single LWIR photons, so the "counts" number for a given pixel represented the actual number of photons that struck that pixel on the sensor during the acquisition of that image. However, from my understanding, uncooled cameras (and I assume the one you are talking about is an uncooled camera, as these are the most common type of thermal imagers currently being made now), are actually the differential type (looking at 2 different pixels on the image gives you an idea of which pixel is from a hotter object than the other, but not their actual temperatures until some computations have been performed).

author: Fraser
author_rank: Super Contributor
timestamp: April 24, 2021, 07:46:08 pm
content: IR_Geek answered this question, above.‘Counts’ refers to the conversion from the analogue domain to the digital domain using an ADC. The count output of the camera represents the value coming out of the ADC.8 bits = 0 to 255 counts12 bits = 0 to 4095 counts14 bits = 0 to 16383 counts

author: Ben321
author_rank: Frequent Contributor
timestamp: April 24, 2021, 07:53:47 pm
content: @Fraser where did you get that FLIR SC4000? Those things when new cost like $100,000 or something, and I don't think most people here could afford that. Did you actually find one on eBay for a reasonable price?

author: Fraser
author_rank: Super Contributor
timestamp: April 24, 2021, 07:54:48 pm
content: Regarding uncooled microbolometer construction, it is worth mentioning that the microbolometer contains both ‘active’ and ‘blind’ pixels. The ‘blind’ pixels are not illuminated by the systems optics so they provide a ‘baseline’ against which to compare the ‘active’ pixels output. This is basic microbolometer design stuff.

author: Fraser
author_rank: Super Contributor
timestamp: April 24, 2021, 08:03:37 pm
content: I still own my FLIR SC4000 ... it is an amazing piece of kitMy SC4000 was originally purchased by a UK company for imaging heating elements through a glass or ceramic sheet. It cost over $100K as it is the special order 1.5um to 5um (SWIR/MWIR) model.Such cameras should not normally be sold on public auction sites as they are still heavily controlled but they have appeared on eBay in the USA. To ship one across borders would require an export licence. My camera was bought on eBay UK as the result of the original owners going into liquidation (bankruptcy). I got very lucky as it appears no one else knew its capabilities or value

author: IR_Geek
author_rank: Regular Contributor
timestamp: April 24, 2021, 08:17:12 pm
content: Something to keep in mind about uncooled cameras is that they have a fixed "time constant". This is fixed at design and cannot be changed. Although I have seen 'games' played to slide up and down the AD conversion by changing a bias voltage to simulate an integration time. Basically it allowed shifting your dynamic range to see hotter or colder scenes.A cooled system works on an integration time and depending complexity of system can be changed by the user. Want to look as something hot without saturating? Decrease the integration time to optimize for the scene content. For something cold? Increase the integration time.Visible digital camera systems can operate like this, but unless they are scientific the usual way is to change the collecting aperture (i.e. F/#). open up the aperture for more light, reduce it for less light.Basically they are all analogous to how long you want to keep the sensor exposed before sampling the scene.

author: Bill W
author_rank: Super Contributor
timestamp: April 24, 2021, 09:38:38 pm
content: Quote from: IR_Geek on April 24, 2021, 08:17:12 pmThe terms are not the same, but uncooled sensors do have an 'integration time' variable that acts the same as a gain control. It is how long you connect the variable resistor (pixel) to the integrating amplifier, so is a proportion of a TV line time.Equally some biases will vary the gain, and for very high temperatures (>1000°C) the designer might need to use both controls for the best signal to noise (even then neTD goes up to 1°K or more on a "50mK" sensor).What they do not have is a pixel exposure time (so consider like motion blur) and pixels reach equilibrium temperature / resistance in 2ms or 10ms or whatever depending on the structure.Bill

author: Ben321
author_rank: Frequent Contributor
timestamp: April 24, 2021, 11:40:48 pm
content: Quote from: Fraser on April 24, 2021, 08:03:37 pmEven the people at the company selling off their equipment didn't know what it was worth?

author: Max Planck
author_rank: Regular Contributor
timestamp: April 25, 2021, 08:01:16 am
content: https://www.hamamatsu.com/resources/pdf/ssd/e07_handbook_Thermal_detectors.pdfSee Fig. 2.4. A microbolometer redout circuit. There are two resistors, a reference one and a measurement one (microbolometer). The scene temperature is measured based on these two elements resistance (temperature) difference. Thus, a differential thermal imager.In quantum detectors, simplifying, there is direct photon counting. Thus, absolute measurement.Max

author: Lambda
author_rank: Regular Contributor
timestamp: April 26, 2021, 09:32:40 am
content: Hello.Thank you Max Planck for this document!I was always asking myself what concretely was this "reference resistor" and how it was electronically implemented for obtaining the final differential output signal.Best regards.Stéphane

author: Bill W
author_rank: Super Contributor
timestamp: April 26, 2021, 10:07:16 am
content: Probably worth adding that there can be 'reference pixels'. The resistor noted above is setting up the DC conditions of the integrating amplifier, removing or supplying the 'non signal' current in the active pixels.ULIS (now Lynred) have a similar explanation in their early published papers, search for 'ULIS readout circuit':https://www.researchgate.net/figure/Pixel-Readout-Architecture_fig6_242268489and similarSome detector designs had 'blind' or reference pixels, which were otherwise normal pixels but thermally connected to the substrate and therefore did not respond to the scene.Bill

author: Fraser
author_rank: Super Contributor
timestamp: April 26, 2021, 10:16:43 am
content: It is interesting that the reference resistance is shown as a fixed value in that document. It was my understanding that in a thermal imaging camera microbolometer, there are active and blind pixels that are effectively identical but the blind pixels are not exposed to the incoming energy through the microbolomter window. The blind pixels are exposed to the same internal temperature changes in the microbolometer die as the active pixels so there is effectively a built in correction for die temperature changeThe differential output is effectively that between two pixels of the same characteristics.... one seeing only the inside of the microbolometer and the other seeing the incoming energy from the optical block. It is a clever self compensating system to address the influence of changing microbolometer die temperature.So in the diagram in that reference document, I would have shown both the ‘reference’ and active pixel resistance as variable and with no external excitation they should have resistance that closely tracks each other.In the real World there are not blind pixels for each active pixel. There are a number of blind pixels on the microbolometer die that act as the blind pixel references for many active pixels.It is some time since I studied microbolometer design and my memory is not what it used to be. I think I have captured the key points in this post however. It is important to understand why such a differential pixel reading scheme was needed.... we are dealing with what are effectively high sensitivity thermistors and changes in the temperature around them or induced within them due to the readout current will effectively skew measurement accuracy. The readout current is usually short duration pulses to manage self heating but the die temperature may be influenced by the ROIC beneath it or external temperature change. Even a microbolometer that is stabilised by a TEC will suffer some die temperature variation. Small thermistors are used to measure the die temperature for the TEC but the use of blind pixels and differential pixel output compensates for much of the die temperature changes that are unavoidable. In non stabilised microbolometer systems this countermeasure to die temperature change could be seen as essential. It is all about taming the undesirable thermal behaviour of the microbolometer.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: April 26, 2021, 10:17:20 am
content: Bill, you beat me to itFraser

author: Bill W
author_rank: Super Contributor
timestamp: April 26, 2021, 05:05:50 pm
content: Quote from: Fraser on April 26, 2021, 10:16:43 amThe so-called 'reference' is a fixed resistor, and is part of the column amplifier circuit, so there are only 160/320/384 etc of them.Another ULIS paper figure attached from researchgate.net.The 'blind' bolometer Rb is to provide the constant current ib to counteract the current ip for the 'typical' pixel at ambient.Therefore only deviations from ip get integrated in the amplifier and appear as signal. Even then those deviations in ip are mostly due to differences in pixel fixed resistance, not so much is actual thermal signal.Bill