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The
type of calibration source used in source-based calibration and calibration verification
of Infrared Radiation Thermometers, Optical Pyrometers, Infrared line-measuring
thermometers and Area-measuring thermometers or Quantitative Thermal Imagers are
often called "Blackbodies". While this is a misnomer, the name is commonly
used in literature and technical meetings, even in the titles of papers in refereed
journals.
Suffice it to say there are no real blackbodies in captivity, and the technically correct name for devices on the market is "Blackbody Simulator". This term is used in some technical literature. The dual names for the same item causes some confusion in an already confusing technology. That's a result of the lack of understanding, education and standards that pervades this technology. People tend to believe that they understand the meaning for a word or term used. Unfortunately, lacking a defined or agreed terminolgy, no two understandings are quite the same. This adds to the confusion.
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The facts are that commercial blackbodies come
in a wide array of sizes and capabilities. They approximate the true, ideal blackbody
by having high emissivities. The higher the emissivity, usually the higher the
cost. There are typically four parameters, in addition to temperature capabilities,
that the user and purchaser of blackbodies often seek. They are:
1.
High emissivity, often measured in 9's (two nines means an emissivity of
0.99x, three nines means emissivity 0.999x etc. where x is an integer less than
9). Note that an emissivity error can be translated readily into a radiance error
in calibration. For instance, a two nines blackbody is systematically different
from the ideal blackbody by about 1% in radiance. Given
that the usual minimum ratio of calibration source uncertainty to that of the
item under calibration is 1:4, then a two nines source, assuming no temperature
gradient errors, would be used to calibrate an instrument with a typical uncertainty
specification that was >±4% in radiance. If the instrument was an 8 to
14 micron waveband calibrated at about 500 °C, the typical temperature bias
due to calibration would be about +14 °C, since the unit reads lower than
true and is adjusted, usually to the "true" from the cavity reference
contact sensor (thermocouple). Whereas a 0.9 micron, narrow waveband unit calibrated
at 1000 °C would have a calibration biasof only about +4 °C on the same
"quality" blackbody. (Ref: Table 5.21-DeWitt & Nutter)Some metrologists favor a 1:10 ratio for calibration source uncertainy
to final uncertainty. That doesn't change the bias, but reduces the uncertainty
by the RSS method.The table below compares them.
| Blackbody
emissivity | Radiance
error | Temp.
Bias. at 1 micron at 1000°C (4:1) |
Temp.
Bias at 8-14 micron at 1000°C (4:1) | |
0.99 |
0.01
(1.0%) | +4°C |
+14°C | | 0.999 |
0.001(0.1%) |
+0.4°C |
+1.4 °C | |
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2. Grayness, or the uniformity of its spectral emissivity with wavelength.
This is an essential feature of a useful blackbody simulator so it can be used
with many different waveband and two color or ratio thermometers. Lacking certification
of a high quality of greyness, a blackbody furnace should be gray to within the
allowable errors required in use, when either a two color, ratio thermometer is
to be calibrated or verified versus a standard single waveband thermometer. Also
if the grayness properties are known, they should be specified by, for example,
a spectral emissivity table and/or curve of the furnace at equilibrium.
3.
Entrance aperture diameter, usually the larger the better, so as to accomodate
instruments having large target or spot diameters and to correct for or quantify
the size of source effect of an instrument under test. Often the latter is needed
to develop or practice a calibration procedure that accounts for off-target thermal
radiation. One can always reduce an aperture size easily with variable aperature
plates or fixed aperture inserts. It is impossible to increase the size of the
aperture, once a furnace is constructed.
4.
Response time is the time spent waiting for the source to stabilize at
a new temperature, after a change in setting, prior to testing. Many high temperature
(i.e. >200 °C) furnaces have relatively long response times. Of course,
they are sometimes used by hurried workers before stabilization is reached, often
without an understanding or measure of the source's internal temperature gradient.
This type of use negates the first requirement for high emissivity, since a furnace
temperature gradient has been shown to be just about the equivalent of a similar
percentage reduction in emissivity value.
Blackbody
Calibration Furnace Vendors
With
all those facts in mind here's a short list of some of the more well known makers
of blackbodies for use with radiation thermometers and thermal imagers. We will
update it as we learn about other vendors and changes in the offering by these
organizations.
- Chino America (
Chino Works-Japan)(USA)
A comprehensive line of precision products supporting
Radiation Thermometers, starting from a high quality furnace incorporating up
to four different fixed reference freezing point cells, to 50 mm aperture cylindrical
and conical cavity furnaces covering from 50°C to 3000°C.
CI-Systems(USA)
A range of furnaces with apertures up to 25 mm covering the range 50-1200 °C.
Ircon,
Inc.(USA)
A small product line aimed mostly at supporting their Infrared
Thermometer products.
Land
Instruments International (UK)
A line of precision sources and transfer
standard devices for radiometric calibration from -40°C to 1600°C.
Mikron
Infrared, Inc.(USA)
Huge product line covering nearly every aspect of
the market beginning with primary standard freezing point sources and special
transfer radiation thermometers.
Thermo
Gauge Instruments, Inc.
A line of unique, rapid response very high
emissivity (>0.995)graphite tube furnaces covering 300 to 3000°C.
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