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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.


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

 
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.

  1. 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.

  2. CI-Systems(USA)
    A range of furnaces with apertures up to 25 mm covering the range 50-1200 °C.

  3. Ircon, Inc.(USA)
    A small product line aimed mostly at supporting their Infrared Thermometer products.

  4. Land Instruments International (UK)
    A line of precision sources and transfer standard devices for radiometric calibration from -40°C to 1600°C.

  5. 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.

  6. Thermo Gauge Instruments, Inc.
    A line of unique, rapid response very high emissivity (>0.995)graphite tube furnaces covering 300 to 3000C.


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