How do I know what emissivity factor to use for IR thermometer? - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: September 12, 2019, 10:49:16 am
content: IR - reflective objects 'emit' some combination of their own heat and reflected heat. Given the reflected heat depends on surroundings (as well as reflectivity), surely the emissivity factor (typically assumed to be 0.95) does not add enough information to enable the determination of the 'target' surface's temperature?That is, it seems to me thatboththe reflectivity of the test objectandthe temperature its neighbouring objects are all combined? However, the contribution from adjacent objects is over-estimated.So, 'emissivity' tells us to what extent we are seeing reflected IR, but we do not know how big the incoming/reflected IR contribution is.So, if emissivity tells us "well, 40% of your signal is from surroundings", then that still leaves the question of how hot the surroundings are, 40% of a big surrounding signal would swamp the reading (?)I'd imagine a spectrum of IR, but no way to pick out the 'desired' object's contribution.Seehttps://thermometer.co.uk/content/111-emissivity-infrared-primerfor a brief discussion on this subject.

author: Fraser
author_rank: Super Contributor
timestamp: September 12, 2019, 12:07:21 pm
content: Using a thermal imaging camera can provide an excellent insight into non contact temperature measurement as you can often see the effect of different emissivity and reflected energy. If you get the opportunity, have a play with a thermal camera and you will quickly realise how challenging accurate radiometric imaging can be when dealing with some target surfaces. The problem with IR thermometers is that you cannot see what is being presented to the unit by the thermal scene.There are some basic ground rules that should be followed when using an IR thermometer and I list a few here......1. Test an IR thermometer using the melting ICE and boiling water test source technique to establish whether it is anywhere near accurate to start with. You can also use an accurate contact thermometer on a surface with good emissivity and compare the surface temperature with that measured by the IR thermometer reading. This can be very useful as it can also reveal an emissivity differential between the target surface and the IR Thermometer setting.2. Always make a visual assessment of the area that is to be measured with the IR Thermometer. Is the area uncluttered, of basically one material and with no sources of large amounts of thermal energy near to it ? If the surface is multi faceted in terms of materials that have different emissivity values, you will only get an average reading of those that fall within the field of view. Inaccuracy will result. If the surface is a single type of material with an emissivity of around the same as the setting on the IR thermometer, you get a measurement of the surface that fills the unit’s field of view. If required, move the IR Thermometer closer to the target to ensure that the FOV is filled by the area of interest. I cannot emphasise enough, with a single pixel IR thermometer, the area to be measured must fill the thermometers field of view or else you will measure an average of the target and its surroundings as well !3. Emissivity value matching on the IR Thermometer to that of the target is very important for accurate measurements. If the Emissivity on the IR thermometer cannot be changed, the emissivity of the target needs to be modified or some maths applied to compensate for any disparity after measurement. A common way to modify a targets emissivity is to apply 3M pvc electrical insulation tape to it. This has a known emissivity of around 0.96 and works well for bringing a surfaces emissivity to a known value. Alternative modifications can involve the use of spray paints, powders or even special wax crayons. There are many emissivity tables for various different target materials on the internet and in most thermal camera user manuals. Highly reflective surfaces like polished aluminium are a nightmare to work with when trying to measure the surface temperature. The Emissivity is very low indeed and reflectivity is very high. When using a thermal camera against such a target, the user often sees their own reflected heat coming back from the surface rather than the true surface temperature. In such scenarios it is best to either modify the surface emissivity and reflectivity with PVC tape etc or remove all sources of thermal energy radiation from around the target. This includes the user and even the camera itself can become a source of thermal energy that must be considered. Measuring low emissivity/high reflectivity surfaces can be very challenging and thermographers learn many techniques to achieve accurate measurements of such surfaces. An IR Thermometer is often hand held and the thermal radiation from the users hand can cause reflected energy issues when working with some surfaces. This needs to be carefully considered by the user.4. Remember that a thermal camera or IR Thermometer is often specified as having an accuracy of +-2C or 2% of range. This is important. Thermography is not always the best temperature measurement technique where great measurement accuracy is required. Beyond the basic accuracy tolerance of the measurement instrument there is also atmospheric influences over greater distances, emissivity mismatch and the effects of ambient temperature and local thermal emitters to be considered. Thermography, done properly, is not a simple task. It requires skill and understanding of the issues and principles involved.5. Wherever possible, a non contact temperature measurement should be validated using accurate contact measurement techniques. In the case of a thermal camera, it may highlight a component on a PCB that is getting very hot and needs further investigation. The next step might be to measure the components temperature using a contact sensor such as an RTD or Thermocouple probe. This provides confidence in the diagnosis and measurements obtained. It is not always possible to accurately set an emissivity for an area of interest so an error in non contact measurements can occur. Think about your average ceramic Resistor from the 1960’s...... ceramic tube with a nichrome winding and a Matt green ceramic paste coating. Without some tests it could be hard to predict the resistors surface emissivity. It will be high for sure, but how high ?In some cases absolute accuracy is not required though. In such cases, near enough is good enough.Finally, remember what I said about the target needing to fill the field of view of the IR thermometer. There is only one pixel in those instruments and it needs to be fully illuminated by the area of interest or area temperature averaging will result.Hope this helpsFraser

author: Fraser
author_rank: Super Contributor
timestamp: September 12, 2019, 12:12:43 pm
content: Here is a link to a comprehensive table of Emissivity for various materialshttp://www-eng.lbl.gov/~dw/projects/DW4229_LHC_detector_analysis/calculations/emissivity2.pdfFraser

author: mzzj
author_rank: Super Contributor
timestamp: September 12, 2019, 12:16:49 pm
content: Welcome to my life.Some better infrared thermometers and most infrared cameras allow you to set the background temperature. Measuring wrinkled aluminium foil gives good questimate of background temperature in the interesting spectral range.Sometimes there are large surprises like objects outdoor "seeing" the cloudless blue sky can have effective background temperature below 0°C even if outdoor temps are +20.Special industrial IR thermometers have sometimes separate input for background temp measurement. Just imagine brand spanking new shiny galvanized steel strip with 10% emissivity in a furnace that varies in temperature range between +50 and +600°CBlack* matte spray paint and black* matte electrical tape are one of the best tools to "fix" the emissivity if they just survive the environment.*) almost any color or even most translucent clear coats have close to 0.95 emissivity in 4-20µm spectral range but black is usually best bet without surprises.

author: Fraser
author_rank: Super Contributor
timestamp: September 12, 2019, 12:17:34 pm
content: More emissivity table links ...... there are many found easily by Google.http://www.vanderbilt.eu/docs/Trebla_Services_Emissie_Tabel.pdfhttp://support.fluke.com/find-sales/Download/Asset/3038318_6251_ENG_A_W.PDFhttps://www.thermoworks.com/emissivity_tablehttps://thermometer.co.uk/img/documents/emissivity_table.pdfhttps://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html

author: Fraser
author_rank: Super Contributor
timestamp: September 12, 2019, 12:27:59 pm
content: I almost forgot ....... some materials undergo emissivity value changes as their temperature changes ! Life is never simple is itFraser

author: mzzj
author_rank: Super Contributor
timestamp: September 12, 2019, 12:28:20 pm
content: Quote from: Fraser on September 12, 2019, 12:07:21 pmWord of warning: Contact or surface temperature probes are equally easily misleading way to measure temperatures.

author: amspire
author_rank: Super Contributor
timestamp: September 12, 2019, 12:29:47 pm
content: When testing temperature, I often just put some thin paper tape, plastic tape or most acrylic paints on the object I want to measure. Maybe not pure white paint. This way, you get the 0.95 emissivity, regardless of the object's surface.

author: mzzj
author_rank: Super Contributor
timestamp: September 12, 2019, 12:33:02 pm
content: Quote from: Fraser on September 12, 2019, 12:27:59 pmAnd too often emissivity tables forget to tell on what wavelenght range they are valid.One can ass-u-me its long-range (ie 4-20µm) if not specified and the numbers are not from short-wavelength IR-thermometer manual.

author: Fraser
author_rank: Super Contributor
timestamp: September 12, 2019, 12:34:47 pm
content: Flir’s guide.....https://www.flir.co.uk/discover/rd-science/use-low-cost-materials-to-increase-target-emissivity/

author: mzzj
author_rank: Super Contributor
timestamp: September 12, 2019, 12:35:35 pm
content: Quote from: amspire on September 12, 2019, 12:29:47 pmThis is also my first suggestion if possible in any conceivable way.Sometimes doesn't work for obvious reasons like red hot steel strip traveling at 100km/h past the measurement point.

author: Fraser
author_rank: Super Contributor
timestamp: September 12, 2019, 12:42:11 pm
content: I bought an Exergen D501 RS (remote sensor) IR thermometer a few years back. A very nice piece of kit and very accurate in my experienceI will be honest and say it’s ability to operate without the need for an emissivity setting did make me wonder about its viability, but it does seem to work very well in my use.https://www.wolfautomation.com/media/pdf/irscanner/exergen/dseries/exergen-microscanner-dseries-brochure.pdfFraser

author: mzzj
author_rank: Super Contributor
timestamp: September 12, 2019, 04:53:50 pm
content: Quote from: Fraser on September 12, 2019, 12:42:11 pmIdea seems similar to Gold cup pyrometers(ir thermometers)https://www.ametek-land.com/products/industry-application-specific/gold-cup-reference-pyrometer-for-reformer-tubes

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 12, 2019, 08:19:35 pm
content: I put some black acrylic paint on most things I need to measure. Otherwise, I use a thermocouple and set the IR to read equal to that. I never trust the thing to be better than a few degrees. Generally works well enough for anything I do.

author: 741
author_rank: Frequent Contributor
timestamp: September 17, 2019, 09:08:07 am
content: The "Gold Cup" page says "A hemispherical reflector produces a measurement area which is emissivity independent – ideal for tubes of unknown emissivity. The reflector edge prevents reflected radiation entering the area"Is the idea that the only IR 'flux' seen is that which is 'directly in front of' the detector. (The IR sensor sitting in a 'radio telescope' arrrangement)? If so, that seems easy to implement, though perhaps expensive to produce the hemisphere. I'm unsure how visible smoothness relates to IR optics. There will be some reflection from the sensor itself, but that can be calibrated out I guess.Can a cheap 'IR gun' be usefully enhanced with something similar and home-made? A problem is the IR sensor would need to look back into the reflector.

author: Berni
author_rank: Super Contributor
timestamp: September 17, 2019, 09:53:33 am
content: Simple. You cover the object in a material with known good emissivity.I like to use Kapton aka polyamide tape for it. It acts pretty close to a black body despite looking glossy for visible light. Its more convenient to use than paint/permanent marker and can be easily removed after use. Also Kapton has a rather wide temperature range so you can use it on stuff that gets really hot, certainty hot enough to cover the usable range of inexpensive thermal cameras or IR thermometers.As a rule of thumb, most things that are metallic or shiny will be problematic, but not always. For example your usual bedroom mirror can be measured just fine with an IR thermometer because glass acts close to a black body at room temp.