The alphameter was constructed for measurements of the solar absorptance.
The alphameter is based on an integrating sphere equipped with spectrally narrow-banded light sources. In order to get spectral information, 7 LED´s in the blue to infrared wavelength region (0.38 – 1.55µm) illuminate the sphere subsequently. The reflected LED signals are measured by using a Si and a Ge detector.
The reference standards or samples to be measured have to be placed at the hole on the sphere surface. Opposite to the sample port are two collimated detectors. When one of the light sources illuminates the sphere, the light reflected by the sample is measured by the detector. From this reflected signal, reflectance or absorptance values are evaluated by employing the reference spectrum for Air-Mass 1.5 radiation.
The alphameter is a very stable but low resolution type of spectrometer. It is very well suited to measure spectra with gentle slopes such as spectra of typical solar selective absorber coatings, strongly pronounced interference fringes cannot be resolved and measured accurately, however. Curved or tubular samples require an adaptor.