Single-beam sphere measurement

When room permits, single-beam spheres can be designed with an additional dummy port. The background correction is performed with the sample to be measured in the dummy port and the physical reference in the sample port. A scanis then run with the positions of the two switched. In this comparison method the average reflectance (hence the throughput) of... 

For single-beam spheres without dummy ports in applications in which substitution error is a concern, the spheres can be calibrated with a set of standards such as one of Labsphere s Reflectance Standards Sets, which has been measured on a sphere without substitution error. With these standards, a table of measured versus actual readings can be generated and used to correct for substitution error (Fig. 9). 

Reference and sample measurements are performed consecutively, and the resultant (sample) spectrum is obtained as the ratio of the two photon fluxes onto the detector. In a single-beam spectrometer, there are no other options in a double-beam spectrometer, the photon fluxes of the sample and reference beam path are compared. When an integrating sphere is used with two ports and a white standard in the reference position, the photon fluxes are comparable to each other, and no problems occur. Note that the ports are part of the sphere and that any material change in the reference or sample position will change the average sphere reflectance pave. The reference measurement should be conducted with exactly the same components (windows) as the sample measurement otherwise, "substitution errors" may occur. 

A second problem that can be overcome is single-beam substitution error, as discussed previously. By use of a so-called dummy port, the overall throughput of the sphere can be equalized between reference and sample scans by placing the sample in the dummy port while measuring the reference, then changing the position of sample and reference for the actual sample measurement scan. This dummy port is also illustrated in Fig. 17. 

Spectral hemispherical transmittance of samples is measured using a Perkin-Elmer Lambda-9 dual-beam UV-VIS-NIR spectrophotometer with an integrating-sphere attachment. Use of such a device allows the absolute transmittance to be measured as po- ASTM E903. Such spectral measurements can then be eonvolved with an appropriate standard terrestrial spectrum (as specified by ASTM G173) to compute a solar-weighted hemispherical transmittance, T2 (>.=200 nm to 2600 nm), as a meaningful single measure of optical performance. 

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