Infrared Training Center

Wednesday, February 8, 2012

Thermographic Measurement Techniques - Measuring Emissivity

Introduction
An infrared camera measures and images the emitted infrared radiation from an object. The fact that radiation is a function of object surface temperature makes it possible for the camera to calculate and display this temperature.

However, the radiation measured by the camera does not only depend on the temperature of the object but is also a function of the emissivity. Radiation also originates from the surroundings and is reflected in the object. The radiation from the object and the reflected radiation will also be influenced by the absorption of the atmosphere.

To measure temperature accurately, it is therefore necessary to compensate for the effects of a number of different radiation sources. This is done on-line automatically by the camera. The following object parameters must, however, be supplied for the camera:

  • The emissivity of the object
  • The reflected apparent temperature
  • The distance between the object and the camera
  • The relative humidity
  • Temperature of the atmosphere

Emissivity
The most important object parameter to set correctly is the emissivity which, in short, is a measure of how much radiation is emitted from the object, compared to that from a perfect blackbody of the same temperature.

Normally, object materials and surface treatments exhibit emissivity ranging from approximately 0.1 to 0.95. A highly polished (mirror) surface falls below 0.1, while an oxidized or painted surface has a higher emissivity. Oil-based paint, regardless of color in the visible spectrum, has an emissivity over 0.9 in the infrared. Human skin exhibits an emissivity 0.97 to 0.98.

Non-oxidized metals represent an extreme case of perfect opacity and high reflexivity, which does not vary greatly with wavelength. Consequently, the emissivity of metals is low – only increasing with temperature. For non-metals, emissivity tends to be high, and decreases with temperature.

Finding the emissivity of a sample
Step 1: Determining reflected apparent temperature
Use the following method to determine reflected apparent temperature:

  1. Crumble up a large piece of aluminum foil.
  2. Uncrumble the aluminum foil and attach it to a piece of cardboard of the same size.
  3. Put the piece of cardboard in front of the object you want to measure. Make sure that the side with aluminum foil points to the camera and is at the same angle and orientation as your object.
  4. Set the emissivity to 1.0.
  5. Measure the apparent temperature of the aluminum foil and write it down.
  Thermographic Measurement Techniques, emissivity T reflected measurement_Page_4_Image_0001  
 

Figure 1. Measuring the apparent temperature of the aluminum foil. (1 = Source of the Reflections)

 

Step 2: Determining the emissivity

  1. Select a place to put the sample.
  2. Determine and set reflected apparent temperature according to the previous procedure.
  3. Put a piece of electrical tape with known high emissivity on the sample.
  4. Heat the sample at least 20 C (36 F) above room temperature (the higher the better). Heating must be reasonably even.
  5. Focus and auto-adjust the camera, and freeze the image.
  6. Adjust Level and Span for best image brightness and contrast.
  7. Set emissivity to that of the tape (usually 0.95).
  8. Measure the temperature of the tape using one of the following measurement functions:
    - Isotherm (helps you to determine both the temperature and how evenly you have heated the sample)
    - Spot (simpler)
    - Box Avg (good for surfaces with varying emissivity).
  9. Write down the temperature.
  10. Move your measurement function to the sample surface.
  11. Change the emissivity setting until you read the same temperature as your previous measurement.
  12. Write down the emissivity.

Note:

  • Avoid forced convection
  • Look for a thermally stable surrounding that will not generate spot reflections
  • Use high quality tape that you know is not transparent, and has a high emissivity you are certain of
  • This method assumes that the temperature of your tape and the sample surface are the same. If they are not, your emissivity measurement will be wrong.

51 comments:

  1. This might be a silly question but here goes.
    Were in the northwest and the temperature is 30 degrees, no wind (covered area, tarp overhead) and a humidity level of 48.9.
    Were checking a new substation installation, the substation has been at about 90 percent simulated load for three hours. Buss doors have been opened for us to safely take pictures. They were open during the entire warm-up period.
    Copper bussing is covered by a thick insulating material and were interested in all the tie points in the cubicles. As the copper bussing is usually polished will the insulation material affect this? Camera was set for .79 emissivity.
    Now the question: As were standing outside the cubicle in 30 degree weather that’s what I set the camera (T400) for. Interior temperature will of course be much higher due the load on the bussing. So for the most accurate read was I correct?
    Any suggestions?

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    Replies
    1. Gary has provide some good points but I think there are some other aspects that should also be considered. I think the questioner was raising the point as to which temperature value to use for setting Trefl, either, that associated with internal ambient reflections, external ambient reflections or perhaps a mixture of the two!
      Well the only way of measuring this would be to the crinkly foil method, but as Gary rightly points out this should never be attempted when there are exposed live surfaces, and if you were to kill the supply you would need to perform the crinkly measurement quickly before internal reflected temperatures diminished too much.

      My suggestion would be to kill supplies and apply high emissivity paint to all busbar connections just enough to take a reliable measurement from.
      The benefits of this approach is as follows:

      1) For high emissivity surfaces (>0.9), Trefl settings have a negligible effect on surface temperature measurement accuracy, particularly as I suspect you can tolerate up to a 10% discrepancy in accuracy for this application.
      2) Also the high emissivity paint would serve to provide reference points for your measurements for now and future inspections.

      One other point to consider is that in addition to determining the operating temperature of the bus system, it is equally important to identify variations in temperature at each joint and comparing same phase joint temperatures. Bus joint temperatures should generally be the same, any differences, may be indicative of a higher resistance joint. The effects of such defects are catastrophic when these systems are subjected to short circuit events.

      the other point to remember is that internal temperatures will be considerably higher once the substation panels have been replaced.

      Hope this helps, and good luck!

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    2. Hi! I want to determine emissivity of ice so I can't use the above mentioned method "Step 2: Determining the emissivity" because i can't heat it up by 20 degree celsius. so what should i do?

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    3. An emissivity measurement requires a temperature difference between the target surface and the reflected apparent temperature. In most cases it is easiest to raise the target surface temperature. However, with ice we have the possibility of melting!

      So in this case I would suggest to lower the reflected apparent temperature. Take the ice target outside on a clear crisp night without cloud cover. A clear night has a much lower reflected temperature than the air, so now the ice will be at a higher temperature than the reflected temperature; and the emissivity measurement can be made.

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    4. Hi Gary! thank you so much for your support.
      now i can create difference between object temperature and ambient temperature by minimizing the reflection.
      is the other procedure same?
      are the external window temperature and atmospheric temperature same?

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    5. Hi Umer,

      The external optics parameters are to be used only if you are viewing your target through a window of some sort. For your application, I doubt that is the case, so set the transmission at 100% (or 1.0). Leave the window temperature at the default parameters.

      The atmospheric temperature accesses a model to correct for IR transmission loss between the target surface and the infrared camera. The inputs are distance, atmospheric temperature, and relative humidity. To measure the emissivity, I would set the distance to 0 and leave the other parameters at their default values.

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    6. Hi Gary! Thank you for your continuous support and sharing free webinar with me.
      1. I tried to create artificial night by surrounding the camera and object with a thick black cloth. But ambient temperature is still near the room temperature and it is much more than actual temperature of ice.

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    7. The reason for doing this on a clear night is
      1. Eliminate solar reflection and solar gain
      2. Provide a very cold background (T reflection) at a temperature MUCH COLDER than your target.

      A black cloth does not provide the COLD background temperature, and will not work.

      You should really get formal training in IR measurement techniques and science, as these measurements are all about the physics of radiation.

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    8. Hi Gary! I agree with you! Can you suggest me paid or free online course or training that can help me!
      I watched the webinar about emissivity measurement in Plane English that you recommended me.

      Best Regards

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    9. I would suggest these two courses as starters:
      1. WEB-TH10 Introduction to Level I;
      http://irtraining.inquisiqr4.com/catalog/course.asp?id=8817&cid=1528

      2. WEB-TH40 - Reflected Apparent Temperature Made Simple;
      http://irtraining.inquisiqr4.com/catalog/course.asp?id=14040&cid=1528

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  2. 1. As the copper bussing is usually polished will the insulation material affect this?

    Big time! Always use the emissivity of the surface you are viewing and measuring; insulation emissivity for the insulation, copper emissivity for the copper.

    2. As were standing outside the cubicle in 30 degree weather that’s what I set the camera (T400) for. Interior temperature will of course be much higher due the load on the bussing. So for the most accurate read was I correct?

    Use aluminum foil in the same plane and orientation of your target in order to measure reflected temperature. Using ambient air temperature as reflected temperature can lead to major errors. BUT REMEMBER SAFETY FIRST. DO NOT USE ALUMINUM FOIL NEAR ENERGIZED ELECTRICAL SYSTEMS.

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  3. i am new to all this but could he have put the foil on securely before energizing then take the readings of the foil , remove foil then energize and take another reading, i am curious on which other way you could do this

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    Replies
    1. NO, ABSOLUTELY NOT! SAFETY PROTOCOLS PROHIBIT THIS. THAT IS A GOOD WAY TO GET YOURSELF KILLED. IF YOU DON'T UNDERSTAND ELECTRICAL DISTRIBUTION SYSTEMS, YOU SHOULD NOT BE INSPECTING THEM.

      An easy way to get the reflected temperature is to hold the aluminum at arm's length in front of you in the same orientation as the electrical equipment BUT OBSERVING SAFETY DISTANCE PROTOCOLS.

      These techniques and more are all discussed in our training courses.

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  4. Sounds like a dangerous practise all-round. With the bussdoors being left open any subsequent arcflash wouldn't care whether it was a real or simulated load. You're also working in the open, a bird flying in the cubicle could cause an arcflash.

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  5. And.. when the temperature of the object is above 500ºC, how do you measure emissivity?

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    Replies
    1. One way that works well is to drill a hole in the material that is at least 7 times deeper than the diameter. If the object has a homogeneous temperature all the way through, the radiation exiting from this hole will have an effective emissivity extremely close to 1.0. Use this hole as your reference instead of electrical tape.

      Just make sure that your IR camera can optically resolve the hole for measurement purposes.

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    2. Thx Gary for the answer about emissivity measurement in hot objects.

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  6. Hi Gary, I'm not very familiar with this method. What is meant by "source of reflections"?

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    Replies
    1. When dealing with thermal imaging, we have to understand that surface in our environment is emitting "infrared light" based on the object's temperature and it efficiency factor (emissivity).

      Therefore, when an infrared camera is pointed at an object, what the camera "sees" is infrared energy emitted by the object based on its temperature and emissivity and also the infrared energy reflected by the surroundings off the object (source of reflections).

      We need to measure and enter the apparent temperature of the surroundings temperature, so the camera can properly evaluate its data and compute the temperature of the surface we are actually interested in.

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    2. Thanks Gary. I'm using the FLIR thermal imaging scanner, but I'm not sure if I'm using it properly to determine the emissivity of some samples. The name "reflection source" suggests that it's a source point, but from your description, it seems to be everything surrounding the sample. So can I know the incident angle between my reflection source and my sample? Why must the reflection source and viewing incident angles be the same? How do I make sure that they are the same?

      For one of my samples, once I measure the tape's temperature in #8 of Step 2, I can't bring my sample's temperature down to match that in the tape by adjusting the emissivity; not even when I reach emissivity = 1.00. Does that mean that my sample has an emissivity higher than that of the tape?

      Two more questions: What's the purpose of taking a thermal image ("freeze the image") in #5 of Step 2? Is the device on the right hand side of Figure 1 the thermal imaging scanner?

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  7. Hi Gary,

    When dealing with metallic targets (i.e. emissivity < .2) what methods are available for getting accurate temperature readings? Could you still use some sort of blackbody-like reference?

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    Replies
    1. When dealing with very low emissivities, the best and easiest approach is to raise the emissivity of the metal via a coating of paint or other high emissivity substance.

      In some cases, drilling a hole into the material that is at least 7 times deeper than the diameter can provide a near unity emissivity as a reference (if the material is uniformly heated all the way through). Be aware of spot size considerations when using this method though.

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    2. Thanks Gary! Can you go into a little bit more detail about how/why the hole would work? I assume it would simulate a blackbody, in which case could you just use a high emissivity coating in one small (hole sized region)of the low emissivity surface as reference rather than a drilled hole?

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  8. Hi, I hope you guys are still reading the comments on this topic.
    I would like to determine the emissivity of reptiles, preferrably of live animals of course.
    First of all I assume the emissivity will change once the animal is dead and second of course i do not want to sacrifice an animal for that.
    Does anyone have a suggestion?
    Thanks a lot!
    S.

    ReplyDelete
    Replies
    1. Measuring emissivity of an animal's skin, coats, etc. is always a challenge. Since reptiles are ectotherms, we face an even bigger obstacle. Here is a method I would consider on a live animal.

      1. Place a high emissivity material on the animals skin, e.g. Liquid Paper or similar (research toxicity first of course).
      2. Place the animal in a warm environment (out of direct light).
      3. Move the animal to a uniform cold environment (at least 20 C colder) and immediately capture a thermogram. Measure the reflected temperature of the environment before exposing the animal.
      4. From the thermogram and the emissivity reference (Liquid Paper), you will be able to determine the emissivity of the skin.

      One other thing, I am not sure the emissivity of the skin would change that much if the animal is recently deceased, but this needs to be researched.

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    2. One other thing, the animal must stabilize in the warmer environment before exposure to the cold environment.

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    3. Thank you Gary, that sounds great! I will try that. Cheers, S.

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  9. Another question: if I measure surface temperature with a hand-held infrared thermometer (Lafayette TRP-39; sensitivity: 0.1°C; precision: ± 2%) and simultaneously with a thermo-camera (FLIR
    145 B-200; sensitivity: 0.08°C; precision ± 2%; IR image resolution 200x150 pixels) and I get higher values with the camera the higher the surface AND ambient temperature, which measurement is probably more accurate and where lies the problem? Thanks a lot! S.

    ReplyDelete
    Replies
    1. Let both pieces of equipment stabilize in an operating condition in the environment for 30 minutes before taking critical measurements.
      Make sure both have the same emissivity setting and that the spot being measured are both larger than the minimum spot size that can be measured.

      The results should be similar, but I would trust the imager (not having any experience with that TRP-39).

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    2. Great, thanks Gary!

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  10. Hi Gary,

    I would like to know of an alternative method of measuring emissivity.
    I usually capture high temperature vessels with refractory inside and depending what im shooting, the temperatures can be well above 300 - 400 degrees Celsius.
    Alot of the time when I get onto a refinery or petrochemical plant to take infrared scans I am not able to measure the reflected apparent temperatures because I simply cannot reach the object, and therefore sticking insulation (electrical) tape onto it would be a greater challenge. My Thermography Teacher mentioned that I could use a contact thermometer to gauge the difference in temperatures and then adjust my emissivity like that? Is this an accurate way of measuring emissivity? He also mentioned that when measuring high temperature equipment that reflected apparent temperature would not be a major concern. I recently did a scan for sasol and found two major problem areas on their riser line, but the temperatures do not tell the correct story as if the shell was 500 degrees celsius then the paint/coating in that specific area would have peeled/burnt off and it is still fine so I am going to need to go back and re scan with a different emissivity (the correct one this time hopefully) I used an emissivity of 0.84 and It could be between 0.7 to 0.9 in my experience but i need to be 100 % certain before submitting my findings and potentially shutting down a portion of the refinery!!!!
    Please could you advise and Im in dire need of help.

    Kind regards,
    Shaun

    ReplyDelete
    Replies
    1. If you trust your contact thermometer, then that is a good way to calculate the emissivity.

      If your target emissivity is fairly high, and the reflected temperature is low, then accuracy on the reflected temperature is not as critical. However if the surface emissivity is low OR its reflecting something hot, a correct reflected temperature is a must.

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  11. If we know the physical temperature and apparent temperature then how can we calculate the emissivity?

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    Replies
    1. The easiest way is to use your camera or IR software to determine this. Any other way becomes more complicated.

      The basic formula used in cameras is:

      S = e x f(Target Temp) + (1 - e) x f(Reflected Temp)

      Where:
      S = IR radiance received by the camera
      e = target emissivity
      f(T) = the calibration curve of your camera for blackbodies vs temperature

      If you know all the other factors, then solve for e.

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    2. I didn't understand the term f(T), what does it mean? Kindly educate me about the calibration curve?

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    3. A calibration curve is generated for an infrared camera by pointing the camera at blackbody sources at various temperatures, and then plotting the camera's signal strength (S) vs temperature. The curve then represents a function of Blackbody temperature f (T).

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    4. From where I can have the calibration curve, or I should draw it by my self?! if so how I am going to know the signal strength of the camera?

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    5. I would contact the manufacturer of your infrared camera for that information.

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  12. Hi Gary,

    Why do we set the emissivity to 1 when measuring the reflected temperature of the aluminum foil?

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    Replies
    1. I realized that is a dumb question.

      We are assuming the foil is a black body when setting the camera to an emissivity of 1. However, the foil is essentially a perfect reflector, so it really isn't emitting anything.

      If the foil was perfectly encased in a black box, and the foil is not emitting anything, the camera (with an E=1) would not detect any heat from the foil, assuming the foil is at some high temperature. The foil could possibly be emitting a very little amount of heat, but the camera would barely pick it up. BUT, we know the foil is still at a higher temperature.

      Therefore, we would have to lower the E value of the camera to compensate for this drop in emissivity of the foil to try and calculate an accurate temperature.

      Now, the foil in your instructions is not in a black box. It is getting reflected light from all over the place, the main source being the object you want to measure.

      By setting the camera to E=1, and knowing the foil does not really emit anything, the camera will still see the reflected temperature and will NOT compensate by increasing the temperature, since it is assuming the foil is a perfect black body. The camera is assuming that the reflected heat/light is the emitted heat/light by setting E=1. Therefore, you know that the only temperature you are seeing is the reflected temperature.

      Ta Da!

      Now, this is assuming the camera is calibrated properly using a blackbody calibrator. Is this correct?

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  13. Apparent temperature measurement explained above is same as ambient temperature setting in the thermal imager?

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    Replies
    1. Hi Ravi,

      Some cameras refer to the reflected temperature as ambient or background temperature (see your operator's manual to be sure). In this case, the reflected temperature you measure is entered as the ambient temperature in your camera.

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  14. Hi, Sorry but looks like I am getting confused on different terms here. What is reflected temp, is it the same temp, that we see in camera? what is apparent temperature? and what is background temp. ?

    I am trying to find an answer of background temp. If I am doing Building thermography outside building, what would be my background temp ? Can i use Air temp with air velocity meter as my background temp. ??

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  15. Apparent Temperature: When an infrared measurement device emissivity setting is set to 1.0, the resulting temperature is called an apparent temperature. Apparent temperatures are readings NOT compensated for emissivity, and reflected radiation.

    Reflected temperature is the average apparent temperature of all radiation sources impinging on your target and reflected back to the infrared camera. Reflected radiation contributes no information about the actual temperature of your target, and must be subtracted from the total IR signal coming from your target (performed in your camera or IR software). Background temperature is another term for Reflected temperature.

    Reflected temperature is NOT air temperature. The easiest way to measure this for building analysis is to place a piece of aluminum foil next to a wall or surface of your building, and measure its apparent temperature.

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  16. HI
    I HAVE A QUESTION,AM USING FLIR INSTRUMENT I DON'T KNOW HOW TO PUT REFLECTED TEMPERATURE AND DISTANCE BETWEEN THE CAM AND OBJECT.

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    Replies
    1. You need to locate the "Parameters" menu on your camera. It is accessed differently on different cameras. You can download a manual for your particular camera at http://flir.custhelp.com/

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  17. Hi!
    in Infrared Thermography, before performing temperature measurement of different objects we have to enter IRObject Parameters(ambient temperature, distance of object, emissivity etc). but mostly in an IR image we have more than one objects with different emissivities, so the entered emissivity value should relate to which object?

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    Replies
    1. I am trying to measure the emissivity of ice by comparing with some standard emissivity object. But I am not able to enter the emissivity of the standard i.e. painted stainless steel thermos and Similarly i can't change the emissivity of ice to ahieve the same temperature.
      can anyone guide me how i can perform this?
      Best Regards
      Umer Sohail

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    2. The emissivity should be for the particular object surface you want to measure. Some infrared cameras and software allow a separate emissivity to be entered for each measurement tool on the image.

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    3. To learn more about emissivity and measurement techniques, take this short free online course,
      WEB-TH55 - Emissivity Explained in Plain English
      http://irtraining.inquisiqr4.com/catalog/course.asp?id=51715&cid=1528

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