A Uniformity Primer
For Cavity Blackbodies
Affect of Thermal Non-uniformities on Blackbody Radiant Output
The definition of a theoretical blackbody assumes that the emitting surface of the cavity is at a uniform fixed temperature (isothermal). In the real world, isothermal is a goal which is difficult to achieve except at the phase change temperature (usually the freezing point) of pure materials. All high temperature blackbodies rely on materials that have adequate structural strength at elevated temperature. However, these materials generally have less than ideal thermal conductivity. When this is combined with the need to provide a variable external heat source and non-uniform thermal losses (higher at the cavity opening), it is a design challenge to minimize temperature gradients in the body.
Fortunately, the very nature of a blackbody which provides multiple bounces to the energy before it escapes the cavity opening, tends to largely eliminate any thermal non-uniformities over the length of the cavity. This leveling effect is reduced near the front portion of the cavity and in the lead in tube due to the reduced number of bounces which occur in these areas. However, for the same reason, this area does not significantly contribute to the radiant output of the blackbody. Therefore reasonably large temperature differences in the cavity and the lead in tube have very little effect on total output.
has published uniformity data for it’s 25.4 mm diameter reference
variable temperature blackbody (VTBB) over the temperature range of 800 to 2700
°C. The NIST measurements of uniformity show a temperature distribution of
approximately 50 °C in the cavity. However, when measured over the central 10
mm, the cavity has a uniformity of ± 0.2 °C. NIST considers this
level of uniformity in the VTBB to be an acceptable reference standard. Even
with a 50 °C non-uniformity, NIST has calculated and assigned an uncertainty
budget of only 0.20 °C due to non-uniformity (out of a total error budget of
3.05 °C). It is clear that even fairly large temperature non-uniformity does
not significantly impair the usability of a blackbody when proper techniques are
utilized.
To fully appreciate the effects of non-uniform cavity temperature, one must first understand the relationship between effective emissivity and the body geometry and viewing angles. As explained in EOI ’s Emissivity Primer, surface emissivity and the viewing angle seen by the detector have large influence on the measured values. Measured temperature values must be corrected to account for the apparent fall off in reading as more of the cavity is viewed, as well as the cosine effect when viewing small portions of the cavity wall.
These physical effects explain why it is recommended that when the highest level of accuracy is required, an aperture be used to limit the viewed portion of the cavity to approximately one half of the cavity diameter. This recommendation is applicable for all styles of cavity sources.
A more rigorous description of the effects of viewing angles, etc on the measured output can be found in EOI ’s paper on the Radiometric Properties of Isothermal Diffuse Wall Cavities.
Uniformity Measurement Techniques for Cavity Blackbody’s
There is no approved or accepted "Standard Method". However, Both NIST and EOI measure cavity blackbody uniformity using similar non-contact radiometric methods. Non-contact methods minimize the uncertainty and non-uniformities that can be created by introducing a thermal load into the cavity. Contact methods rely on a consistent thermal interface and a consistent immersion depth of the thermocouple to get accurate measurements. This is difficult, if not impossible, to achieve using a contact sensor along the length of a tapered cavity. Contact methods also introduce significant local thermal loading into the cavity which can alter the readings in unpredictable ways.
An overview description of the NIST measurement techniques and a list of NIST references are attached.
EOI 4143 Null Balance Radiometer
The schematic overview of EOI’s measurement technique is shown here. The two methods are essentially equivalent, with the exception that EOI does not concern itself with calibration of lamps or similar sources. EOI tests against a broader range (five) of blackbody freezing points.
Freezing Point Blackbodies
The freezing point of pure metals are used to define the ITS-90 temperature scale. A freezing point blackbody uses these freeze materials to ensure a known, uniform temperature in the blackbody cavity. The EOI reverse cone cavity design provides an effective emissivity of 0.9997 ±0.0003. During the freeze the cavity approaches (±0.02 °C or less) the theoretical limits of iso-thermal radiation.
EOI’s freeze point blackbodies are literally the "gold standard" all other radiation sources can be measured against. EOI uses these freeze point blackbody’s as references for all thermometric and radiometric calibrations.
EOI Production Testing
The uniformity testing methods discussed above are time consuming and therefore expensive to perform. More economical "quick" checks are needed for production inspection. EOI employs three checks to ensure its blackbody’s meet it’s specifications.
Thermometric Calibration
Every EOI blackbody is calibrated thermometrically using NIST traceable standards. EOI’s working calibration thermocouples are calibrated against a special standards thermocouple which is in turn calibrated using EOI freeze point blackbody’s. EOI periodically does a radiometric calibration to compare the radiometric and thermometric calibrations are consistent.Quick Uniformity Scan
On a regular basis, a production blackbody is pulled and scanned for uniformity using a non-contact near IR optic probe with a wide degree field of view and a near IR detector. The probe is inserted into the cavity almost to the apex and the measured temperature is recorded along the length of the cavity. A typical curve is shown here.
Note that the fall off near the opening to the cavity is normal. This fall off is caused by a combination of the detector "seeing" the cooler lead in tube and the drop off in effective emissivity as the detector views the full cavity diameter. This is a graphic example of the effect field of view has on effective emissivity.
IR Image
The effective temperature profile of every EOI blackbody is recorded using an IR camera. The recorded image is reviewed to ensure the blackbody is within uniformity specification prior to shipment.. A copy of these records are kept in EOI’s internal quality records.
Special Testing and Calibrations
EOI offers several special calibration services for its customers. These special calibrations serve to reduce the error budget normally associated with using a blackbody.
Special Limits Thermometric Calibration
A direct transfer from freezing points covering the range from 156 °C (Indium) to 1085 °C (Copper) allows EOI to reduce the thermometric calibration error down to ±0.3 degrees from the normal ±1.6 degrees.Radiometric Calibration
EOI offers an optional radiometric calibration against up to five freezing point blackbodies as a service to our customers. These calibrations can be full spectrum or band pass limited depending on the customers requirements.For temperatures above 1085 °C up to 3000 °C, EOI uses a infrared radiation thermometer which is directly calibrated by NIST.
Uniformity Scans
EOI can provide two levels of radiometric scans. This first is the quick scan described above.EOI can also perform radiometric scans of the cavity using the EOI 4143 null balance radiometer. The EOI 4143 null balance radiometer focus on a small spot, which allows surface scans of the cavity walls.
If you have additional questions, please contact EOI.
