Handheld FLIR infrared cameras

Topic: Emissivity
Exploration of it's meaning in relation to thermal infrared inspections.  Tables attached at end of document.

Explain the hot spot?
    This can of suds is ice cold straight out of the fridge.  When scanned with an infrared camera you would expect the entire image to be relatively even in temperature and to appear "cold" in relation to the background.  Can you explain the apparent "hot" spot in the center of the can. Hint: it's not a fingerprint!

****To have Sierra Pacific Infrared Inspections arrange training for you or your firm contact:
(702) 369-3966

Live image reveals truth.
    The curtain is lifted and the truth revealed.  The paint on the outside of the can has been scratched off in a small area.  The bare aluminum has a different emissivity than the painted aluminum.   The camera can only allow for one emissivity setting at one time so to the detector the bare aluminum "images" hotter than the rest of the can.  Examples like this one show how emissivity can cause false readings in the field. 

The IR Papers
Emissivity

    Emissivity is a measure of the thermal emittance of a surface.  It is defined as the fraction of energy being emitted relative to that emitted by a thermally black surface (a black body).   A black body is a material that is a perfect emitter of heat energy in that it emits all energy it absorbs and has an emissivity value of 1.  In contrast a material with an emissivity value of 0 would be considered a perfect thermal mirror and imaging this material would result in readings of reflected energy only and not the actual material.  For example, if an object had the potential to emit 100 units of energy but only emits 90 units in the real world.  That object would have an emissivity value of 0.90.  In the real world there are no perfect "black bodys" and very few perfect infrared mirrors so most objects have an emissivity between 0 and 1.

    Emissivity is a variable that makes it very difficult to obtain exact temperature readings with an infrared camera or spot radiometer.  This is due to the fact that it is highly impractical to measure the emissivity of every object in your field of view.  For example, if you are scanning an electrical panel in a predictive maintenance application you will be imaging wires, fuses, nuts, bolts, and other materials all of which will have a different emissivity value.  So how do we obtain accurate reliable information? 

IR Papers Index
Update coming soon!

Basic Infrared Theory
How IR Works
Glossary of Infrared Terms

Issues for Thermographers
Emissivity

CCD image shows the variety of materials found in electrical panel.

    As stated before it is hard to determine the emissivity of all objects in your field of view.   It is also difficult to determine the emissivity of a single known material.   This is because emissivity is a measure of the "surface" emittance of an object.  The surface of objects (especially metals) changes with the passing of time.   For example, if you look at the table below you will see that corroded copper (E 0.78) has a significantly different emissivity value than shiny copper (E0.02).  This difference introduces a judgement call on the part of the thermographer.  How do you determine how corroded or shiny a piece of copper is?  The answer is you can come close but you cannot be exact.  Additionally, the numbers in the emissivity table are approximates or averages, in the real world values may be skewed.  So how do we manage emissivity.  It is possible to determine actual emissivity values for a material but it is not practical in the real world.  The process requires a spectrometer (expensive and usually not available) or dismantling the object and testing each piece.  Even if you determine an accurate E value, the next time you scan the item that value will have changed rendering your old value useless.  In most infrared applications exact temperature measurement is not necessary.  For example, if a circuit has a fault limit of 150° F and your instrument measures 100° F and the E value skews the temperature reading by 5°F  you are left with a +- 5°F variance which in this case is negligible.  Additionally, most thermal infrared applications rely on temperature difference (Delta T ) rather than exact temperature readings.  To use our previous example of the circuit we measured, there would most likely be more than one circuit next to each other.  If you use the same E value for both circuits they will both be skewed the same amount.  If the one circuit was reading 100°F (which we will assume is normal operating temperature) and the adjacent circuit reads 150°F we are left with a Delta T of 50°F which would indicate a problem and as you can see negates the emissivity problem.  E values become even less of a problem when trending an area over time.  If the same circuit with the reading of 100°F has a reading of 110°F the next time you scan it and a reading of 115°F the next time, with the same emissivity setting, we know a problem is developing regardless of the error introduced by emissivity.  

Thermal image of fuse clips shows large Delta T

    Dealing with emissivity is not as hard as it would seem.  The important things to remember are that exact temperature measurements are difficult to obtain (do not promise this to your clients unless you are sure you can back it up), temperature difference (Delta T) is more important than exact readings in most applications, and that trending an object can reveal problems regardless of E value error.  In the real world you pick an emissivity value that approximates the scene you are imaging and then you record it and maintain that same setting every time you scan that object.

    We hope this article answers your questions regarding emissivity.  A full explanation of emissivity is beyond the scope of this web site.  We have included a brief emissivity table below listing some of the more common materials you will encounter.  If you have any questions regarding this topic check out our thermal infrared forums or contact Sierra Pacific Infrared at 530-543-1186 or email us. 

ITAR Disclaimer

Most SPI night vision products are subject to the International Trade in Arms Regulations, administered by the US department of state.  NO EXPORT !

SPI Disclaimer

  SPI offers it's products for legal use only.  It is the sole responsibility of the purchaser to determine local, state, and federal regulations prior to purchase.  SPI will not be held responsible for any illegal use of it's products.  SPI reserves the right to refuse shipment to any person for any reason.  Specifications and prices listed in the SPI web site are subject to change without notice.  All images and brand names of products are either copyright SPI or are copyright or trademark of their respective companies.  SPI reserves all rights.   All other companies reserve their respective rights.  No part of this web site may be copied with out the express written consent of SPI and / or  the copyright / trademark owner.

(c) auw / spi 18972.2 /sP