A special-purpose solar simulator includes (1) a tungsten lamp that serves as a gray-body radiator with a temperature of 3,200 K and (2) a mosaic of filters such that the filtered lamp output has the same normalized spectral irradiance as that of sunlight outside the atmosphere of the Earth. This solar simulator is intended for use as the illuminator in a portable instrument that measures solar absorptances and total emittances of samples of materials.
The extra-atmospheric sunlight spectrum [also known as the airmass zero (AM0) spectrum] in question is the one described in standard E490-73a of the American Society for Testing and Materials (ASTM). The present solar simulator was developed because previously developed solar simulators are unsuitable for a variety of reasons: some do not cover the wavelength range (300 to 2,800 nm) required in the design specification for the instrument, some are too power-hungry and/or not durable enough for inclusion in a portable instrument, and some deviate too much from the ASTM standard. The mosaic of filters includes commercially available filters plus a keystone filter developed specifically for this application.
The mosaic was designed by use of a mathematical model based on the transmittance characteristics of the filters. The model was evaluated against optical “bread board” measurements. Areas, overlaps, and other design parameters were determined for the finished filter configuration.
A prototype instrument was built for proof of design. All design specifications relating to the filter were met: The filtered lamp output covers the spectral range from 300 to 2,800 and agrees more than 95 percent with the ASTM standard sun. Solar-absorptance measurements taken by the instrument are characterized by errors with magnitudes of less than 3 percent (that is, within a range of ±3 percent) of total absorptance.
The major benefit afforded by the design of this solar simulator is portability arising from low power consumption (relative to other solar simulators) and robustness. In addition, the compactness of the solar simulator (relative to other solar simulators) made it possible to integrate both the measurement of solar absorptance and the measurement of emittance into a single instrument package, whereas it would otherwise have been necessary to package them as separate instruments. Other benefits afforded by the design of the overall instrument are that a measurement can be made in <1 minute, and the instrument is more economical than are spectroradiometric systems.
The original intended use of the instrument is in measuring the radiative properties of components of the International Space Station. The instrument could also be used to measure the radiative properties (especially with respect to thermal radiation) of coatings and other materials, particularly in the aerospace industry.
This work was done by David G. Crandall and John S. Harchanko of AZ Technology, Inc., for Kennedy Space Center. For more information, contact the Kennedy Commercial Technology Office at 321-867-6224. KSC-12069
