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 emiddivity 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). BOTTOM
LINE:
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 or FREE at The Mikron Instrument
Company web site (after filling out a simple form.)
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.
Trail Tracks:
E-Trail Stop 4 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! | 