One of the earliest devices employed to measure radiant energy was the thermopile. A circular and linear type of thermopile were introduced by W. W. Coblentz who subsequently arranged with the Eppley Laboratory for their commercial manufacture and improvements. In principle, they are have been referred to as “compensated thermopiles”, which applies to their thermal compensation where the total area (mass) of the shielded cold receivers is equal to that of the single hot receiver.
Eppley now manufactures wire-wound thermopile detectors that are essentially sensitivity-independent over wide intensity ranges. Originally designed for installation in rocket, satellite and spacecraft vehicles, these sensors have successfully withstood the mechanical shocks encountered in launching.
Eppley manufactures four models (E6, G3, H6, J3) that are calibrated in accordance with customer requirements. Low intensity calibrations (40-80 µW/cm-2, 3 points) are undertaken by exposure of the thermopile to a NIST furnished carbon-filament secondary standard of total irradiance, in accordance with NIST procedures. High (10-100 mW/cm-2, 3 pts) and Very High (100-250 mW/cm-2, 3 pts) calibrations are undertaken by exposure to a tungsten source; the radiation reference is the World Radiometric Reference Scale with the sun as a source.
Thermopiles can be mounted in three different case designs: air, vacuum, and fluid (e.g. water) cooled. Various window materials to minimize the effects of air convection are available. These windows are normally mounted in metal slides or inserts, each of which is interchangeable.
Air and vacuum cases are limited to exposure to a maximum flux density of 100 mW/cm-2 for lengthy periods and 300 mW/cm-2 for short periods (approx. 30 seconds). Water jacketed cases are limited to a maximum flux density of 2 W/cm-2. Both of these values are for conditions of continuous operation. When using a water-jacketed case, care should be taken to keep the coolant temperature close to +25°C, the approximate temperature used during calibration. One-quarter inch (1/4”) I.D. high-pressure tubing is recommended to connect the coolant to the thermopile case.
All standard cases are constructed of stainless steel and are supplied with copper binding posts and slide holders for easy insertion of a removable window mounted on a slide. The two etched lines on either side of the case indicate the position of the thermopile receiver surface inside the case.
If a special design is required, non-standard thermopiles can be constructed. Please contact the Eppley Laboratory with a detailed statement of requirements. The following information should be included: Nature and approximate intensity of the source; Distance between the source and the thermopile; Wavelength range of radiation to be measured; desired shape and size of the receiver; and Desired sensitivity, responses time and resistance.
| Thermopile Type | Aperture Size | Sensitivity µV/µW cm-2 | Impedance Ohms | Response Time (1/e) |
| E6 | 3/8" | 0.10 | 200 | 1.0 |
| G3 | 1/4" | 0.10 | 800 | 0.8 |
| H6 | 9/16" X 1/8" | 0.05 | 300 | 1.0 |
| J3 | 1/8" | 0.02 | 300 | 0.5 |