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Note: The E-trail has needed updating for some time. We are getting to it now, so please be patient if you see pages with different formats. They'll all look great soon and there will be more stops along the E-Trail for you to explore!
The Emissivity Trail: Stop 3

Know when an emissivity correction is needed.
Better yet, know why!

That sounds better

Everyone knows that you have to correct an IR Thermometer reading for emissivity, or else it's no good, don't they?

(Shhh! Don't tell anyone, but knowing when and why helps takes some of the mystery out of it.)

An Infrared Radiation Thermometer measurement with an emissivity correction is almost always required when one meets two simple conditions:

a) the object of interest is expected to be significantly hotter than its surroundings (and there's no other source of IR radiation which can reflect off the object into the Thermometer, like sunlight, arc lamp or quartz lamp radiation etc.) and,
b) when you are reasonably confident that you know the value of the spectral emissivity of the object (of course within the response waveband of the Thermometer).

Fail to meet either of those conditions and you are better off using the spectral radiance temperature, that is, the reading you get with the emissivity correction set at 1.00.

At least with an emissivity setting of 1.00 there's a chance one can have reproducible measurement conditions when it comes time to try to repeat a set up or operating condition, even if the numbers sound kind of weird or impossible.

The conditions which may not be so repeatable are those where the object is in surroundings that are hotter than it is and one does not seperately measure the surroundings temperature and correct the Radiation Thermometer readings for the effect of reflected thermal radiation. The way that gets done is a bit tricky and not the subject of this stop along the trail. Just be aware that the errors due to reflected thermal radiation from surroundings are significant and should be avoided like the plague whenever possible.

Except, of course where the surroundings are at about the same temperature as the object, discussed in E-trail2.

One way to insure that the surroundings or any extraneous source of reflected radiation is insignificant, if they are present, is to cast a "cool" shadow onto the surface where the actual measurement takes place. As long as it does not disturb the thermal state of the object, such a shadow then enables one to make the emissivity correction and measure the temperature of the object better than any other method around.

A noted worker in the field of industrial radiation thermometery, Dr. Tohru Iuchi, briefly describes the "Cooled Shielding" method in Reference 1 (Chapter entitled "Recent Advances and Research Activities in Japan" References to Read) and in much more detail in his original paper, with J. Ohno and R. Kusaka, in the 1976 Transactions of The Iron and Steel Institute of Japan, Volume 16, page 195. He also describes a method of verifying the readings on moving steel strip with a contact thermocouple.

(Much to our disappointment, we were never been able to make the contact thermocouple method work, too much friction. But the cooled shield works quite well as long as one knows the object's spectral emissivity . There are other ways to verify the reading of an IR Thermometer, something that's not always done because it seems too hard or otherwise impossible. It often calls for ingenuity).

Iuchi's solution for steel strip covers a most common case, but is not the only way to beat reflected radiation. Steel strip is a diffuse reflector and it takes some extreme techniques to cast a "cool" shadow. Specular reflecting objects also abound in industry and science.

It's a lot easier to place a "cool" shadow onto a specular object. One example that has been used numerous times is the case of sheet glass in a tempering preheat furnace. Sheet glass is a spectacularly specular surface and its emissivity is easily estimated from its index of refraction, n, using the ever popular Fresnel equations for reflection and transmission from an air-glass interface

(Refer to any basics physics text on optics for the Fresnel equations e.g. Jenkins & White, "Fundamentals of Optics" or Born & Wolf "Principles of Optics" or the Wikipedia or Scienceworld or Hyperphysics at Georgia State on the Web. Once you have the reflection coefficient, R, one can estimate the emission coefficient from the fact that it is 1-R when T=0. BTW, emissivity or emittance is really the emission coefficient of a material and it is equal in magnitude to the absorption coefficient -absorbtance or absorbtivity. Be sure to use the correct spectral region for your instrument and object; not all index of refraction curves are readily available for all materials at all wavelengths throughout the infrared).

One approach is to select the optics of the Thermometer so that it sees itself by reflection from the glass surface, i.e. the thermometer can not see any source of extra radiation, such as furnace heating elements, by reflection from the glass surface(s).

IR Thermometer temperature measurements can be "a piece of cake" (assuming you've got a "good" instrument-we'll talk about what makes a "good" instrument later) when all you have to deal with is measurements of objects in cooler surroundings whose spectral emissivity you know.

Some of the nastier measurement conditions arise when the objects are in hotter surroundings. If at all possible, try to avoid dealing directly with the reflected radiation by shadowing it out or shielding the measurement spot from the extra radiation. If the surface is diffuse-like a sheet of paper (it does not show the reflection of your face-that's not evil, it's diffuse) the cooled spot will need to be much larger than in the case of a specular or mirror-like surface.

Again, if this is not making sense, try another mantra: E-trail mantra #3.

Repeat 200 times between dinner and bedtime for one week:

E-missivity. When you use it, stay away from Reflectivity!

Got it?

Good, but not great.

You really need to know more than a mantra or three if you are a serious user or specifier of infrared radiation thermometers. Try looking at some of the references. The one refered to above is still in print at ASTM near Philadelphia for a relative pittance, especially when you realize all the distilled wisdom contained therein. (It contains some interesting ways to evaluate the optical quality of an IR Thermometer in a chapter by R. Barber and M. Brown on the calibration of Radiation Thermometers).

The last topic relates very strongly to the "goodness' of an instrument as further described in an ASTM standard, ASTM E-1256 available for purchase from ASTM.

Some thought-invoking questions for this major stop on the trail.

1) If you use a 2-Color, or Ratio Radiation Thermometer, and you aim it at an object in cool surroundings, what e-slope or non-grayness adjustment will give the correct temperature readings?

2) How can you be sure you are right?

HINT: There's more than one way to find a non-grayness value.

A what?

What the heck is non-grayness or e-slope, anyway?

More questions.

Got answers? Let us know with easy feedback at the special site "Spectral Emissivity & Emittance".

Trail Tracks:

E-Trail Archives at www.SpectralEmissivity.com or, the trail next followed.

E-Trail Stop 2 or, the trail's last "unholy" stop.

E-Trail Stop 1 The first stop-on The E-Trail

E-Trail Start, Where you learn about thinking spectrally!

References to Read or, the best trail to follow.

P.S. Interested in a listing on our vendor directories. It's self service at www.TempSensor.net, click here.




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