Introduction to Black Body Emission

Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body held at constant, uniform temperature.

Ordinary grade black bodies can be used to make routine measurements based on black body emission approximating devices. The 62500 Large Area High Temperature Black Body enables producing unparalleled amounts of highly predictable flux from the infrared to ultraviolet. The fixed point black bodies allow achieving unprecedented temperature repeatability and stability at nearly unity emissivity levels.

The Variable Temperature Black Body can achieve temperature reproducibility on a 20mK scale. All of these black bodies are operable in a vacuum environment to achieve additional stability. Having knowledge on emissivity, temperature and cavity non-perturbing emitting aperture size allows full definition of the emission characteristics of a black body. Oriel Instruments also supplies precision apertures for radiating area definition and Transfer Standard Pyrometers for temperature determination/setting.

Black Body Cavities

Black body cavities enable establishing near equilibrium radiation field conditions to approximate Planck's Law black body. Figure 1 depicts the schematic of 62500 High Temperature Black Body, showing an elongated cavity with a conical bottom. A long cavity of this design along with a limited aperture at the exit can show emissivity 0.99 or better.

Schematic of 62500 High Temperature Black Body

Figure 1. Schematic of 62500 High Temperature Black Body

One may not get perfect results using a perfect black body. Accuracy of a measurement relies on all the components employed and a range of error sources that can affect their purpose. These systemic errors can be reduced with transfer standard pyrometers, water cooled apertures, and precision apertures. Experimental performance can be stabilized using vacuum environment.

The emitted power and the total radiation absorbed by a black body from the environment can be predicted using the Stefan-Boltzmann Law. The T4 dependence ensures that the absorbed power is very less than the emitted power for high temperature black bodies. Since heat loses not just take place through the output aperture, it takes 6-7kW, and 350A, to maintain the black body operating temperature above its temperature range.

Temperature stability and accuracy is key parameter for a black body to function properly. Figure 2 illustrates the changing rate of spectral radiant exitance with temperature, (dMeλ/dT)/Meλ. High temperature black bodies have weak temperature dependence, whereas higher temperature stability and accuracy is important for low temperature ones to show useful performance.

Temperature Sensitivity of Spectral Exitance at Various Black Body Temperatures

Figure 2. Temperature Sensitivity of Spectral Exitance at Various Black Body Temperatures

The 62500 Black Body

The 62500 Black Body is a unique High Temperature Black Body, which is based on a space age pyrolytic graphite radiator. High operating temperatures necessitates operating the radiator in an oxygen free atmosphere. The entire system, including the external aperture and power leads that provide the black body with the power supply, needs to be water cooled because up to 7kW of power is needed to maintain the radiator at its optimum operating temperature. Standard plant water may also be used. The service life of the radiator is 500hr at 2500K and 50hr at 2900K optimum operating temperature.

The specifications of the 62500 Black Body are as follows:

Black Body
Temperature range : 1400-2900K
Cavity diameter : 22mm
Cavity length : 130mm
Cavity output opening : 14mm
Maximum aperture diameter : 8mm
Spatial radiance uniformity : 0.2% @ 500nm
Spatial radiance uniformity :
    2500 K
     2900 K
:
:
500hr
50hr
Maximum current : 350A
Maximum voltage : 20V
Argon gas flow (or vacuum) : 40l/hr
Cooling water : 180l/hr
Power Supply
Maximum output : 400A, 30V
Input : 460/240V, 3 phase, 60Hz
Cooling water : 6 lpm min
Communication : IEEE-488
Size (approx.) : 5.3 H x 2.6 W x 2 D ft. (1.6 x 0.8 x 0.6m)
Weight (approx.) : 990lbs (450kg)

Radiance Stability

The integrated optical feedback system ensures radiance stability because electrical means are not practical at these temperatures. A detector package samples rear flowing radiation, which controls the output of the power supply via a computer-based control system.

Absolute Temperature

Oriel Instruments offers an extremely reproducible, high emissivity, large area, homogenous source with abundant radiance from the ultraviolet to far infrared. The system can be set depending on the absolute accuracy requirements of customers using different methods. Optical temperature determination methods can only be used at the extremes of temperature range. Precision transfer pyrometers calibrated at NIST or at a commercial calibration laboratory can be employed for temperature setting of the system.

A more accurate approach involves a multiple wavelengths based comparison to a temperature standard. Oriel Instruments helps customers in the selection of an appropriate method suitable for their measurement requirements. Under the best of conditions and with extreme care, temperature determination can be performed with accuracies close to 2K. This needs access to a High Accuracy Cryogenic Radiometer and tunable laser transfer instruments. Accuracy that can be achieved with more routine methods is in the range of 4-10K.

About Oriel Instruments

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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