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It is the spectral emittance which governs the thermooptical behaviour of surfaces and coatings. It is also the spectral emittance which causes the most serious measuring problems in characterizing coatings. In principle there are two methods: the measurement of the emitted thermal radiation of heated samples and the measurement of the reflectance of opaque samples ( E( X) = 1 - 9(X)). The determination of the reflectance requires the measurement of the radiation reflected into the whole hemisphere, since most of the investigated surfaces are reflecting diffusely. In the spectral range from 1 pm to about 15 μm this can be done with the aid of integrating spheres.

The performance ofmaterials used for solar energy conversion should be maintained for some decades of years. During the development of new products their durability must be investigated in short-term tests, in which the applied loads should be as similar as possible to the natural stresses under working conditions, but the tests should accelerate the degradation considerably. For these purposes the validation of accelerated indoor tests by a comparison with the results of natural ageing is essential. The naturally aged selective solar absorber coatings were only exposed to a part of the required lifetime. Therefore the observed degradations often are very small and a reliable comparison with the degradation achieved in accelerated temperature and moisture tests demands very sensitive analytical methods. Optical spectroscopy (0,3 - 15 μm) has also been used for the investigation of specific absorption bands as well as for the evaluation of differential spectra. Depending on the sample type, the analysis of the spectra has yielded optical thicknesses and refractive indices, or surface roughness parameters. The methods used for lifetime estimation can also be helpful for the comparison of the degradation of indoor and outdoor ageing.

Absorber coatings for solar-thermal energy conversion must have a low reflectance in the wavelength range of the solar irradiation and a high reflectance in the range of thermal radiation. There are several physical principles providing such a spectral behaviour. One of them is a surface microroughness with an rms-roughness smaller than 0.5 µm and a correlation length of the same order of magnitude or smaller. Only few measurement techniques can be used for the characterization of such a microstructure. This paper will discuss the applicability of non-stereoscopic scanning electron microscopy (SEM) with digital image processing for the evaluation of the correlation length of the microstructure. Emphasis will lie on the investigation of pyramidal structures, for which angular dependent SEM can yield additional information about the mean slope of the structure. The results are compared with the outcome of the evaluation of spectrooptical measurements in the wavelength range from 0,36 - 15 µm by means of statistical scattering theories. These measurements are carried out with a Zeiss PMQ3-spectrophotometer-system with a BaSO4-coated integrated sphere (0.36-2.5 µm), and with a Bruker FTIR spectrometer equipped with a diffuse-gold coated integrating sphere (2,0 - 15 µm), respectively.

Selective optical properties can be achieved by a suitable surface roughness. Characteristic parameters describing the roughness of the surface profile are the rms-roughness, the rms-slope, and the autocorrelation length. The relation between these parameters and the spectral reflectance of the considered surface is known approximately from the scalar scattering theory. Due to the dimensions of the investigated surface profiles the roughness measurements require scanning electron microscopy for surface imaging. For the statistical evaluation the image data are processed in a computer system, which performs the Fourier-transformation of the whole image. The square of the Fourier transform yields the spectrum of the spatial frequencies from which the autocorrelation function and the correlation length can be derived by inverse Fourier-transformation. The scanning electron microscope causes a systematic imaging error, which can be corrected by approximating the image signal as a superposition of the surface height and its slope. The results are compared with the results of spectral measurements of the specular as well as the hemispherical reflection in the IR-rage applying the scalar theory mentioned above for copper-oxide and chromium absorber coatings.

Measurements of the near-normal/hemispherical reflectance in the infrared spectral range can be taken by using integrating spheres as photometric integrators. In the infrared region, where the reflectance of the sphere coating significantly differs from one, the specular component of the reflected flux causes problems due to the different weighting of the specularly and diffusely reflected part by the sphere wall. An evaluation method for circumventing these difficulties is presented. It is shown that the influence of the sample indicatrix can be eliminated with a good accurracy if the specular reflectance factor is known. The considerations are based on investigations in the spectral range from 1 μm to 15 μm performed with a Fourierspectrometer and a gold coated integrating sphere. Examples demonstrate the application of this method for the investigation of samples whose specular reflectance factors change with wavelength. That especially occurs if the selective properties of solar absorber coatings is caused by surface roughness. The described method can be applied to estimate the surface roughness parameters and to characterize the optical properties of selective solar absorber layers.

Based on a FTIR spectrometer a measuring device for the determination of the spectral near-normal/hemispherical reflectance and the spectral directional emittance, respectively, of opaque, diffusely reflecting samples has been built up. The device is mainly used for the measuring of the spectral emittance of low-emitting selective solar absorber coatings. The measuring arrangement for hemispherical reflectance measurements with a gold coated integrating sphere is shown. The measuring geometry of the diffuse reflection attachment, the performance of the whole device as well as the calibration and evaluation of the measurements are described. Emphasis lies on the properties of the used integrating sphere and sphere design: on the angular response of the sphere and the influence of the limited reflectance and partly specularity of the sphere wall. It is shown that the integrating capacity of photometer spheres with an IR-active diffuse gold coating is partly reduced due to the relatively high deviation of the sphere wall reflectance from unity as well as due to the deterioration of the Lambertian behaviour of the wall indicated by a specular reflectance factor increasing with growing wave-lengths. Nevertheless the reflection method has some advantages over the radiometric measurement of the thermal radiation emitted by heated samples.

Good durability of solar absorber coatings is one of the most important preconditions for their application. Since the lifetime of an absorber should be at least ten years, a prediction of the ageing behaviour by accelerated ageing tests is very desirable. In general the ageing due to temperature loads of the coatings is caused by chemical or diffusion processes. The temperature dependence of the chemical reaction velocities as well as of the diffusion constants is described by Arrhenius'law. Therefore an increase of the test temperature leads to an accelerated ageing. The degradation of the optical properties of the layer (e. g. solar absorptance and thermal emittance) at high temperatures as a function of time, caused by the enhanced temperatures, can be transformed to lower temperatures using Arrhenius'law. Thus an estimation of the ageing behaviour and the lifetime at operating temperatures is possible. This method is applied to copper-oxide coatings and to Ni-MgF2 cermets. The specific ageing processes have been investigated by Auger-electron-spectroscopy combined with energy-dispersive x-ray analysis, which has shown, that the degradation of the copper-oxide layers is caused by chemical processes while the ageing processes in Ni-MgF2 coatings are dominated by diffusion processes.

Metal-dielectric composite materials (cermets) have been often investigated as solar thermal absorber coatings. The cermet, deposited on an infrared-reflecting metal substrate, is used as interference layer absorbing in the wavelength range of the solar irradiation. The attainable solar absorptance is limited by reflections at the cermet surface. A reduction of these reflection losses can be achieved by additional dielectric interference layers, grading of the filling factor of the metal in the cermet (decreasing from the substrate to the surface of the layer), or a suitable microroughness of the substrate-cermet interface and/or the surface of the coating. The latter way is described for microrough aluminium reflectors, deposited by physical vapour deposition on heated substrates, in combination with Ni/MgF 2 cermet layers. The roughness parameters are evaluated by means of spectral reflectance measurements and scanning electron microscopy in order to investigate their dependence on the production parameters. The obtained optical properties are compared with those of absorber coatings produced according to the other methods mentioned above for achieving an enhanced absorptance of cermet absorber coating. The durability at temperature loads in air and in vacuum, respectively, is another topic of this paper.