Seek Thermal core measurement accuracy - what’s the story? - Page 1
Forum section: electronics / general

author: Fraser
author_rank: Super Contributor
timestamp: March 24, 2020, 12:59:44 pm
content: Now that I once again own a Seek Thermal product, I am taking more of an interest in the Seek cores that are currently shipping and thought I would start a discussion on measurement accuracy.First some background on microbolometer temperature measurement for those readers unaware of what is involved.A microbolometer is really just an array of temperature sensitive resistors. These may all have a slightly different response to the same amount of thermal energy and the response can differ at different temperatures that fall within the operational capabilities of the microbolometer. To deal with non uniformity in the microbolometer array, a NUC test and compensation table is created. This table takes account of the different pixel thermal responses within the array. NUC tables can be created for a single temperature range or for multiple ranges, depending upon the camera design. The result of applying an NUC table to the microbolometer array is a nice flat image with no visible Delta T indication across its surface when viewing a thermally flat scene, dead pixels are also dealt with during the NUC table creation but that is not relevant here.So now we have a microbolometer that the camera system ‘knows’ and can correct any pixel response offsets that exist. There is a further correction applied to the output of the microbolometer in the form of Flat Field Correction. This may be considered a ‘touch-up’ or fine tuning function that helps to maintain the cameras flat field response during use when pixels begin to drift. Have I mentioned how badly behaved the little microbolometer pixels can be ? no ? Well they can beSo what can disrupt the harmony of a well corrected and ‘levelled’ microbolometer imaging array whilst in use ?Well the pixels will naturally drift by a small amount when operating and there can be minor self heating due to the pulsed read current applied to them (this should be minimal however). The other issue is die temperature. If the microbolometer die is not temperature stabilised it operates in a thermal equilibrium mode, the temperature of which is dictated by the ambient temperature around it, self heating and the thermal energy entering the lens system that illuminates it. It is a fact of life that as the die temperature drifts, the pixels can drift at different rates or to different extents. The NUC was carried out at a set ambient temperature, it is not normally Dynamic in its response to ambient temperature change. For this reason the FFC is used to flatten the arrays output at regular intervals to keep everything pretty at the camera output. The temperature of the FFC flag is also known by the camera so a temperature calibration can also be carried out to correctly compensate for changes in ambient temperature. The microbolometer array also has temperature sensors on it so the system can monitor changes there as well. There are modern thermal cameras that do not use a mechanical FFC correction process but they are not really relevant to this discussion as the Seek cores use a mechanical FFC flag.So in précis, the NUC calibration captures non uniformity in the pixels at a certain die temperature. The regular FFC event keeps the pixels response relatively ‘flat’ as the die temperature changes or pixels naturally drift. The ‘knowns’ with regard to temperature are that of the air around the core, the microbolometer die, the FFC flag and the lens system. In some simple systems, a temperature sensor on the cameras chassis is used as the reference for ambient, FFC flag and lens temperature. This can mean that the FFC flag temperature is, in reality slightly different due to its physical location within the camera, internal convection air currents etc.OK, so now we have got some of the calibration and compensation theory out of the way, what about temperature measurement ?As you likely know, a microbolometer pixel may be used as a relatively accurate temperature measurement sensor. To be such a radiometric measurement sensor it needs to be characterised so that it’s response to thermal energy may be known in order to calculate radiance and then temperature.It is important to characterise the pixel response to thermal energy. This is normally done at a set ambient temperature for a stable die temperature running in thermal equilibrium or temperature controlled.The pixel under test is pulse biased to avoid self heating and its output monitored as it is sequentially exposed to known thermal energy in small increments. An Energy in Vs signal Output response plot will result for that pixel, but that pixel alone ! In an FPA there are a lot of pixels ! The output from each pixel will need to be plotted at each Energy point and a thermal response ‘map’ created. If it is found that the Energy response plot is pretty much the same across all pixels in the array, a generic response plot may be employed to production cores. Note that I am deliberately ignoring the blind pixels that also form part of a microbolometers measurement system.Calibration is expensive and can be time consuming. For this reason it is not uncommon for a camera to undergo a single point or dual point calibration. As stated above, if a particular microbolometer FPA series exhibits a relatively even response to varying energy levels across its pixels, an energy vs output plot can be used to predict the output from the pixels when measuring the energy in a scene presented to it. This plot needs references however. Without such it is just a floating response plot. The temperature calibration process provides one or two reference points for application of the response plot. A two point calibration is preferred. This may be an ambient and an ambient plus 30C BlackBody calibration that ‘anchors’ the response plot to known thermal stimulus at a known ambient and die temperature. In some cases the calibration points can be much further apart, at say +10C and close to the maximum temperature capability of the cameras current range. If more than one temperature range is available on the camera, the calibration will need to be repeated as there are changes to the Microbolometer biases. After a 2 point calibration with known die temperature, the cameras radiometric measurement system can measure the radiance at any point in its range coverage and convert this to whatever thermal units are desired. There will, of course, be the potential for error when relying upon a generic calibration plot. To improve accuracy more calibration points can be used to fine tune the energy vs pixel output table for a specific camera system. More calibration points improve accuracy but are expensive in terms of time.You will recall that I mentioned that the temperature of the cameras interior, FPA die, FFC shutter and lens is often monitored by the camera system. This is to improve measurement accuracy as all of these temperatures can impact upon the measurement. The Ambient temperature can effect the core wholistically as thermal sensitivities can exist in many components that support the FPA and measurement system. The die temperature is an essential data point for the measurement process. The FFC flag ‘blinds’ the microbolometer to provide a ‘flat field’ reference scene against which to correct pixel outputs. That flag will be at a certain temperature and if this is measured, the value can be used to provide fine calibration to the measurement system. Lens temperature can effect measurement accuracy so is also considered in many radiometric camera systems. For a camera that does not use a temperature stabilised microbolometer it is desirable to place the whole camera in an environmental chamber and capture the error introduced to measurements by changes in Ambient temperature. The results can be captured by the measurement system in the form of an ambient temperature offset table.What else effects the radiometric measurement accuracy ?Well let us not forget emissivity, distance to target (atmospheric influences) and lens characteristics. These can be accounted for in many radiometric camera systems.The end result of all this is a temperature measurement accuracy of +/-2C or 2%, whichever is greater, for many radiometric cameras. We must remember that this is a ‘best case’ scenario tolerance ! Errors can creep into a measurement to create increased tolerance. For this reason it is always wise to use another, more accurate, measurement technology when great accuracy is required. A simple example would be not knowing the actual emissivity of a surface and choosing a figure that ‘looks right’ . Sadly visual inspection of a surface is not normally a reliable measure of actual emissivity. To create a known emissivity it can be necessary to apply a material of known emissivity specification to the target. Such is not always possible however. The use of an accurate contact temperature sensor can provide another measurement data point for the thermographer.So to the topic of this Post. Where does the Seek Thermal range of cores come into this discussion ?Well Seek Thermal provide a radiometric measurement accuracy specification for the PRO core of +/-5C or 5%, whichever is greater. This does, at first glance, look high for a modern thermal camera core. The question is..... why is the Seek Thermal core unable to provide the usual +/-2C or 2% accuracy specification commonly seen in other cameras specifications ?Discuss ?Fraser

author: Fraser
author_rank: Super Contributor
timestamp: March 24, 2020, 03:06:01 pm
content: OK, so let’s just do a quick hypothetical analysis of the Seek Thermal Pro core as a ‘straw man’.........1. Is the 12um VOx microbolomter significantly different in behaviour to its bigger brothers of 17um, 25um and 35um ? No, not in terms of its needs and physical responses to stimulus.2. Does the Seek Thermal Pro microbolometer use an NUC table ? Yes it has to use such a table or equivalent to create a reasonably flat field for a pleasing display and user experience. It also requires an NUC table or equivalent to make measurements. Is the NUC table the cause of the wider temperature tolerance. I think it unlikely as it is ‘just’ a pixel output variance flattening process.3. Does the Seek Thermal Pro microbolometer use an FFC flag ? Yes it has a flat field correction flag built into the cores optical block. Is it adequate ? That is open to question as early Seek cores suffered from a temperature gradient across the scene. The FFC shutter is made of plastic and does not have direct temperature monitoring. Whilst adequate for FFC it is not known how effective it is as a known temperature reference as the position of the FFC flag environs temperature sensor, if such exists, is not known. Could this cause a wider temperature tolerance ? Possibly as the FFC flag temperature normally forms part of the radiometric calibration. The emissivity of the FFC flag is another consideration.4. Die Temperature - does the Seek Thermal employ temperature stabilisation ? No, the microbolometer die operates in a temperature equilibrium, uncontrolled mode and will vary with changes in ambient temperature. The die will have a temperature sensor built into it that keeps the host system advised of the current die temperature. Will die temperature be monitored and compensated for in measurements ? Yes. Such is a requirement for accurate temperature measurement. Could the die temperature tolerance contribute to the temperature tolerance of the Pro core ? Yes it is possible for an error in measuring die temperature or appropriate compensation to cause increased measurement errors. If the die sensor has a poor tolerance figure it will transfer that tolerance to the whole system.5. Lens temperature - does the Seek Thermal Pro core employ direct lens temperature monitoring ? Not in my view no. If the metal lens barrel is temperature monitored it might provide an indication of lens temperature but such is open to error. Could this be the reason for the wide tolerance specification ? In my opinion no. The lens will be at a similar temperature to the ambient temperature within the core assembly and would not create such a tolerance in measurements, especially as Chalcogenide IR glass is used for the lens.6. Ambient temperature - does the Seek Thermal Pro core monitor the ambient temperature around it ? I can only say such is wise and I hope it does. It is true that the microbolometer die temperature is directly monitored but the ambient temperature sensor is useful to predict thermal influence on the die, other components in the core design and, where no separate sensor is used, the temperature of the FFC flag and lens. Could the ambient temperature sensor cause the wide tolerance of the Pro core measurements ? Yes it could, if it were of poor accuracy and tolerance. The Ambient temperature sensor, if used for FFC flag temperature, will directly influence camera calibration carried out during an FFC event. If not used for FFC flag temperature monitoring an error I the ambient temperature sensor can influence camera temperature compensation systems and introduce some error into the system. The magnitude of such an error is not known.7. Design - can the physical design and layout of the Seek Thermal Pro core influence its measurement tolerance ? Yes it is possible to compromise the measurement accuracy of the core by poor placement of any temperature sensors and thermal creep in the PCB to which the microbolometer is attached. Such can cause localised heating of the die and push the temperature compensation system to its limits. The enclosure into which the core is installed can also influence how warm it will get in operation and the likely ambient Delta T across the core PCB. It is advantageous to not expose the core to heat from other systems within the casing and to avoid a large Delta T across the cores chassis. Could such a design issue effect the tolerance specification of the core ? Possibly, as the temperature compensation systems may not be able to cope or the core temperature may move too far away from its calibration temperature and errors in the generic ambient temperature compensation table become an issue.Well that will do for nowFraser

author: Fraser
author_rank: Super Contributor
timestamp: March 24, 2020, 03:31:21 pm
content: Core calibration ........This is a total unknown where the Seek Thermal Pro is concerned. I have no insight into how the Seek Thermal cores are calibrated. What I can say is that in mass production, calibration can be a relatively significant cost due to the time it can take to do properly. It is an area where costs can be saved by a ‘good enough’ policy ! Such a policy may involve using generic temperature compensation tables created from testing multiple core samples and accepting that any single core will not perfectly match the generic table. Production tolerances can lead to a wider than normal core temperature measurement tolerance. All microbolometer cameras will need at least a basic NUC so this element of ‘calibration’ cannot easily be avoided as each microbolometer die is unique in terms of pixel output levelling. A poorly executed NUC can lead to more work for the FFC system to correct later. Errors in the NUC and FFC systems normally lead to visible mottling or defects in the image being produced, especially when viewing a low Delta T scene.Temperature calibration of the ambient, die, FFC flag and lens sensor systems is an area open to cost cutting that may impact upon overall accuracy of a measuring system. Such may not be apparent in non radiometric measurement roles but does introduce increased measurement error tolerance into the systems specification.Basically, if you cut corners on calibration you can introduce an increased measurement error into the system. Such an error may be hard to quantify so a significant tolerance has to be stated in the specification to avoid customer complaints and returns for repair or calibration.Fraser

author: bap2703
author_rank: Regular Contributor
timestamp: March 24, 2020, 04:56:10 pm
content: 8. Yield. It would be that most devices are actually better than +/-5C or 5%, but in order to have greater number of devices to sell the specification is a bit relaxed.That just transforms the question to "why would the yield be lower at seek than at other manufacturers?".By the way I never understood how a percentage applies to a temperature in degrees. Does it mean that at 0°C you get 0°C from the % part ?I bet you do not go back to Kelvin and calculate 5%*273.15=13.66 as that would be huge.

author: Fraser
author_rank: Super Contributor
timestamp: March 24, 2020, 10:07:19 pm
content: Bap2703,My understanding of the specification statement “+/-2C or 2%, whichever is greater” is that the error in a measurement will potentially be a minimum of 2C but can be as much as 2% of a reading if the result is greater than 2C. That is to say a measurement of 0C could have a true target temperature of up to -2C to +2C but a measurement of 200C could have a true target temperature of +196C to +204C.It should be noted that where high quality radiometric cameras are involved, the specification is believable and worst case scenario. The specification does not mean the camera will definitely present an error of 2C or greater ! Industrial Thermal cameras that I have tested against a calibrated BlackBody have often provided radiometric measurements within 0.2C at a 50C target temperature rather than the acceptable 2C error.So for the Seek Thermal Pro core the error present in measurements could be as follows as a worse case scenario........Potentially a 5C error in temperature measurement between the cores minimum temperature measurement capability and +100C. Above 100C the 5% tolerance comes into play so a 200C measurement could have an error of +/- 10C and a measurement of 300C could have an error of +/- 15C.Thermal cameras can have multiple tolerance specifications that apply to differing sections of their temperature measurement coverage.Fraser

author: IwuzBornanerd
author_rank: Frequent Contributor
timestamp: March 25, 2020, 03:18:16 am
content: Quote from: bap2703 on March 24, 2020, 04:56:10 pmI wouldn't bet too much on that not being the correct interpretation! Marketeers hope that people interpret things the way they like them to be.I vote for at least one of the factors to be the use of only one set of NUC values. You should take the Reveal outside in the chilly air & capture a couple tiffs for examination & comparison with tiffs captured inside at normal room temperatures. I expect a difference in noise between the 2 sets. While outside, set the thing down on a surface so your hand isn't keeping it warm. I think the Seek app tries to track out the difference so I would assume the Reveal does that too, but it may be that neither is good at it.Capturing tiffs will make it possible to examine them with whatever color span is desired & we will be able to see noise regardless of the span in vogue at time of capture.

author: Fraser
author_rank: Super Contributor
timestamp: March 25, 2020, 12:35:32 pm
content: Just for interest...... the attached image is of a FLIR calibration check and recalibration cell. Such calibration is multi temperature point and costs thousands of US Dollars to have done to your camera. Budget core designers and manufacturers have to find ways to avoid such expensive calibration procedures. Accuracy of measurement can suffer as a result but for those who need great accuracy there are more advanced cameras, albeit at much higher cost.An example of calibration costs may be found here. It includes a 14 point check on the camera but the cost is painfull on the pocket. Note: 1 Saudi Riyal = US$0.27http://wesams.com/wp-content/uploads/2016/04/Calibration-for-FLIR-Infrared-Cameras.pdfCalibration of a FLIR iX or Ex camera is around US$1300 !I am very pleased to now have several high quality calibration Blackbodies of my ownFraser

author: IwuzBornanerd
author_rank: Frequent Contributor
timestamp: March 28, 2020, 01:58:32 am
content: Another factor influencing the Seek accuracy spec. might be that some units can't sense the full specified temperature range. See my post from a few minutes ago on the Seek Compact Pro Radiometric JPEGs thread.