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Fabry–Perot interferometers have stimulated numerous scientific and technical applications ranging from high-resolution spectroscopy over metrology, optical filters, to interfaces of light and matter at the quantum limit and more. End facet machining of optical fibers has enabled the miniaturization of optical Fabry–Perot cavities. Integration with fiber wave guide technology allows for small yet open devices with favorable scaling properties including mechanical stability and compact mode geometry. These fiber Fabry–Perot cavities (FFPCs) are stimulating extended applications in many fields including cavity quantum electrodynamics, optomechanics, sensing, nonlinear optics and more. Here we summarize the state of the art of devices based on FFPCs, provide an overview of applications and conclude with expected further research activities.

The preparation of a novel class of e-caprolactone (CL) monomers, modified at the β-position of the ester function, is described. The efficient thia-Michael addition to cyclohex-2-en-1-one and subsequent Baeyer–Villiger oxidation provided the regioselectively modified CL monomers. To enable a sustainable Baeyer–Villiger oxidation, several reaction procedures were investigated. In order to test a controlled ring-opening polymerization of the prepared monomers, the kinetics were studied and the monomer to initiator ratios were varied in order to prepare poly(e-caprolactone)s with different molecular weights and different side groups.

For economical finite element calculations of aero-engine disks it is advantageous to use an axisymmetric FE-model. However, disks often contain noncentral holes, for example bolt holes which disturb the axial symmetry. In the present article methods are described on how to find axisymmetric substitute structures for these local disturbances. The substitute structures have been constructed in such a way that the stresses they predict reproduce those in the original structure far away from the hole region. The stresses in the original structure were calculated by applying the formulas obtained by Muskhelishvili for a ring and for a hole in infinite space in an alternating way. An important asset of the substitute structures is that they allow for the prediction of the stress peaks occurring at the hole surfaces. This has been achieved by the determination of appropriate stress concentration factors which must be applied to the respective model stresses. For the practically relevant cases, the developed substitute structures are independent of the specific load conditions and only depend on a single geometric parameter characterizing the relative distance between the holes.

An ellipsometer attachment for a commercially available Fourier transform spectrometer (Bruker IFS 66) was developed. Almost no modification of the basic spectrometer was necessary. The optical set-up, providing a parallel light beam with variable diameter, is placed in the sample compartment. The beam is deflected to an external goniometer attachment which allows measurements to be taken with incident angles of up to 85° depending on the sample size. The possible combination of our optical components (light sources, beam-splitters, polarizers, and detectors) allow measurements in the wavenumber range from 30 000 to 400 cm −1 . The polarizers are aligned automatically. Determination of the refractive index of thin films is based on measurements at multiple angles of incidence and evaluation by the least squares fit method.

This paper is concerned with an estimate of the size of the resulting engine disk fragments after failure due to radial cracking. The disk is modelled by a rotating ring in which the stress history after cracking is calculated. The results show that immediately after failure bending moments and normal forces propagate away from the crack region while increasing. However, arriving at about 100–130° away from the crack surfaces the propagation speed of the maximum bending moment and normal forces noticeably decreases, though their value is still strongly increasing. An exceeding of the ultimate tensile strength in this situation leads to a second failure at about 120° away from the original crack. The resultant fragments, each of them approximately one third of the original disk, have the biggest translational energy of all possible failure modes.

Liapunov's direct method in an extended version suitable, for spatially continuous systems is used to derive sufficient conditions for the stability of marine risers. The method has its origin in considering the system energy and represents a dynamic extension of Lagrange's static stability approach which relates the existence of a positive definite potential energy to stability. Since the direct method utilizes only the differential equation and the boundary conditions to construct energy-like functions from which stability statements are deduced, it is not necessary to solve the system equations. The applicability of the method to nonlinear systems makes it possible to include the nonlinear hydrodynamic drag force due to the sea water in the stability analysis. As an example, the dynamic stability of the vertical riser equilibrium position is analysed here for pinned-pinned boundary conditions, resulting in a confirmation of the “stability limit” already known through purely static stability analysis. Liapunovs direkte Methode in einer fur raumlich kontinuierliche Systeme erweiterten Fassung wird angewandt, um hinreichende Bedingungen fur die Stabilitat von Meeres-Risern herzuleiten. Die Methode hat ihren Ursprung in energetischen Betrachtungen und stellt eine dynamische Erweiterung des Lagrangeschen statischen Stabilitatskriteriums dar, welches die Existenz einer positiv definiten potentiellen Energie mit Stabilitat in Beziehung bringt. Da die direkte Methode nur die Differentialgleichung und die Randbedingungen zur Konstruktion energieahnlicher Funktionen, aus denen Stabilitatsaussagen abgeleitet werden, verwendet, brauchen keine Losungen der Bewegungsgleichung bestimmt zu werden. Die Anwendbarkeit der Methode auf nichtlineare Systeme gestattet es, die nichtlineare hydrodynamische Dampfung durch das Seewasser in die Stabilitatsanalyse einzubeziehen. Exemplarisch wird hier die dynamische Stabilitat der vertikalen Riser-Gleichgewichtslage fur beiderseits gelenkige, unverschiebliche Lagerung untersucht; das Ergebnis bestatigt die aus rein statischen Berechnungen her schon bekannte „Stabilitatsgrenze”.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 3386-3389, Stabilized performance parameters of PV-modules are necessary for energy yield prediction as well as for the investigation of module degradation effects. The electrical parameters of thin-film modules show stabilization behaviors which are typical for the applied technology. However, this behavior is not satisfyingly understood yet. Different types of thin-film modules have been exposed to artificial irradiation and controlled temperatures in a climatic cabinet with a class B solar simulator for up to 330h. The modules have been connected to electronic loads to perform IV-curve measurements every 15 minutes and MPP-tracking between the measurements. The stabilization of the different parameters (Uoc, Isc, FF, Pmpp) has been analyzed using this data. Temperature correction was done with temperature coefficients which have been measured after a certain irradiation dose had been applied. Flasher-measurements have been used for confirmation of the DC-measurements after the relaxation of the modules after the continuous irradiation exposure was finished.

Bei der Planung eines Versuchs ist a priori Berucksichtigung statistischer Aspekte unerlaslich. Nach der Formulierung der Arbeitshypothesen mussen die Auswertungsstrategie und die zu verwendenden statistischen Testmethoden festgelegt werden. In die anschliesende Fallzahlberechnung fliesen das Signifikanzniveau α, die Macht des statistischen Tests und die Effektgrose, das heist der kleinste nachzuweisende Unterschied, ein. Nur wenn diese Angaben auch bei der Publikation von Versuchsergebnissen zitiert werden, last sich die Qualitat und die Aussagekraft eines statistischen Tests beurteilen. Fehlen diese Angaben, kann nicht verifiziert werden, ob die Ergebnisse in einem methodisch einwandfreien Kontext erzielt wurden oder aber—wegen der meist zu geringen Fallzahl—zu Fehlschlussen verleiten. Am Beispiel des t-Tests, der Varianzanalyse und des Vierfeldertests werden Methoden zur Fallzahlberechnung und zur Bestimmung der Macht eines statistischen Tests beschrieben und an Beispielen erlautert. Ein Literaturuberblick gibt einen Eindruck von den verwendeten Fallzahlen bei Versuchen in der forstlichen Forschung. Bei den untersuchten Literaturstellen werden leider in der Regel zu wenige Angaben zu den statistischen Tests gemacht, so das eine Beurteilung der Aussagekraft der Ergebnisse aus statistischer Sicht fast nie moglich ist. Um die Vergleichbarkeit wissenschaftlicher Studien zu gewahrleisten, sollte sowohl bei der Versuchsplanung als auch bei der Veroffentlichung der Ergebnisse im Methodenteil auf die verwendeten statistischen Methoden intensiver eingegangen werden. Insbesondere sind Angaben zur verwendeten Testprozedur, zur Macht des Tests, zum Versuchsumfang, dem nachweisbaren Unterschied und dem Signifikanzniveau α, das heist Wiedergabe der exakten p-Werte und nicht das Uberschreiten eines bestimmten Niveaus notwendig.

Publisher Summary This chapter summarizes the optimization of ventilation and ventilation rate on the amount of moisture inside the box of flat-plate collectors. Most flat-plate collectors are not air tight, which is in marked contrast to evacuated tubular collectors and hermetically sealed flat-plate collectors, and humidity can enter or leave by ventilation. It is shown by measurements of the ventilation rate and of the transient behavior of the humidity in collectors after rapid changes of the ambient humidity, that the microclimate in collectors depends strongly on the collector type for three principle reasons—that is, the type of and amount of air ventilation, insulation materials and framing materials, and changes by aging over time.

Publisher Summary The purpose of this chapter is to outline various ways for quantifying changes in performance. The choice of an appropriate and sensitive measure of degradation is critical to provide a rapid indication of performance loss. By relating such performance indicators to relevant environmental stress parameters, it is possible to derive correlations between outdoor and accelerated exposure test results. Several photometric performance parameters that characterize optical degradation are also discussed.

Publisher Summary This chapter summarizes the methods for accumulating data for climatic stresses, for evaluating the data, and reporting the effects of different climates on several flat-plate collectors. The environmental impact on materials exposed directly to weathering or indirectly to weather dependent stress conditions, as in solar systems, varies over time. Therefore, it is necessary to accumulate stress data over a significant time period. Appropriate periods are usually at least one year or better still, several years to obtain the data needed for assessing the durability of a new material in comparison with others. If the data are intended to be a basis for general accelerated life tests, several different climates should be monitored at different locations and for different periods to identify the most challenging stresses. The impact of evaluation parameters like sampling time or integration intervals is considered. The chapter shows the effect of different climates, different materials, and the different collector types for heating domestic water based on the amount of stresses. Statistical analysis tools and the interpretation of the results are also discussed.

Publisher Summary This chapter provides an overview of the most important environmental stress factors that may limit the useful life of solar thermal systems. Service life estimation procedures are based on the knowledge of the stress factors acting on the product. The stress factors on materials in solar thermal systems are discussed qualitatively and their quantitative assessment is discussed in the following chapters. The most important climatic stress factors are: ambient temperature, relative humidity in the air, the total solar irradiation especially UV-A and UV-B, temperature of the sky, rain, wind, environmental pollutants, and micro-biological contamination.

Publisher Summary This chapter summarizes the concepts of system performance, performance criteria, and loss in performance, and infers that system performance usually depends on a few important properties of the materials used. Service life prediction needs a clear definition of the end of service life. The total failure of a product is easy to detect, but a slow and continuous degradation in the properties of materials, which results in a continuous decrease of the product performance, needs special criteria for the definition of service life. The evaluation of performance criteria for some materials in solar thermal systems is discussed for glazings, reflectors, and absorbers by using parametric studies on the impact of material properties on the energy performance of a system. For a specific example, the results from modeling the system performance from changes in the solar absorptance and thermal emittance are presented for flat-plate solar thermal collectors operating at ca. 45°C and 70°C.

Growth of European forests has changed considerably in recent decades. Although the methods applied in the 22 studies varied according to the data available, most studies showed the same general trend: site productivity has increased on many sites (Table 1). An increasing growth trend has been observed in the southern regions of Northern Europe, in most regions of Central Europe and in some parts of Southern Europe (Fig.1). No clear trend was found in the most northern part of Europe, in rare cases in Central Europe and in some observations in Southern Europe. A decreasing trend was found in exceptional cases where extreme growth conditions such as intense exposure to pollutants or exceptional climatic conditions occurred.

A 31-year-old woman was admitted with recurrent pulmonary emboli. The patient subsequently developed right sided hemiplegia. A mobile left atrial mass attached to the interatrial septum was detected and paradoxical embolization was assumed to be the underlying cause of the cerebral event. Leg phlebography and B-mode ultrasonographic imaging showed that the most probable source of embolization was a right popliteal venous aneurysm containing thrombi adherent to the vein wall. The immediate treatment was restricted to full dose heparin therapy and supportive measures. Thirteen days after admission, the aneurysm was excised. Recovery was rapid and the patient was discharged for further rehabilitation with an only minimal neurological deficit 3 weeks after admission.

This is a presentation of 34 patients with traumatic or intraoperative vascular injuries treated at the Rural Clinic am Plattenwald during the timeperiod from 1978 until 1988. Limb amputation was performed in three cases. Two patients died. Our goal is to show the importance in early recognition and treatment of such injuries which requires an efficient interdisciplinary cooperation between orthopedic and vascular surgeons, as well as the radiologist. The employment of angiography should be generous.

Selective optical properties can be achieved by a suitable surface roughness. Characteristic parameters describing the roughness of the surface are the rms-roughness, the rms-slope, and the correlation length. 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 calculation of the autocorrelation function and the correlation length of the surface structure. Another approach uses a relation between the surface roughness parameters and the spectral reflectance, as given by a statistical scattering theory. The approximation of the measured spectral reflectance in the infrared range by this model as well as the SEM image analysis is used for characterizing the surface roughness of copper-oxide and chromium solar absorber coatings.

Ni/MgF2 composite films deposited onto an IR reflector are well suited for spectrally selective solar absorbers. The optical spectral properties of these layers are calculated using effective medium theories to evaluate the effective refractive index as a function of the volume fraction of the metal. The depth gradient of the metal concentration is treated using a multilayer model. The coatings were produced by co-evaporation of nickel and MgF2. The two electron beam evaporators were independently controlled using two separate quartz crystal monitors and a microprocessor unit. This device enables the filling factor to be varied reproducibly. The spectral absorptance of the cermet films was measured in the range 0.36–40 μm. These data were used to calculate the solar absorptance, the thermal emittance and the solar thermal efficiency. The thermal stability was investigated for temperatures up to 723 K.

The possibilities and problems of durability tests of solar absorber coatings are discussed with respect to service lifetime estimation methods. Constant temperature tests in air as well as humidity and condensation tests have been carried out at different load levels on six commercially available solar absorber coatings (black chrome, black nickel, coloured stainless steel and nickel pigmented anodized aluminium). The results of the temperature tests have been used for a lifetime estimation based on Arrhenius relations. The estimated degradation behaviour and the changes of the spectral reflectance of the coatings after the accelerated indoor tests are compared with the results of outdoor tests under stagnation conditions. In this way, the influence and the importance of the particular degradation factors (temperature, humidity, pollutants, etc.), the specific sensitivity or stability of the tested coatings with respect to these degradation factors, and the applicability of the lifetime estimation method have been investigated.

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 of the coatings due to temperature loads is caused by chemical or diffusion processes. The temperature dependence of the chemical reaction velocities as well as the diffusion constants is described by Arrhenius' law. Therefore an increase of the test temperature leads to an accelerated ageing. The temporal variation of the optical properties of the layer (e.g. solar absorptance and thermal emittance) at high temperatures can be transformed to lower temperatures using Arrhenius' law, if the degradation process does not change in the considered temperature range and the investigated time intervals. Then an estimation of the ageing behaviour and of the lifetime at operating temperature is possible. This method has been 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.

The non-linear characteristic of the McDonald emissiometer INTEC Model 2145 is investigated. A reflection model yields a characteristic in very good agreement with the experimental data. The areal distribution of the sensitivity of the measuring head shows no azimuthal dependence. The radial weighting function can be approximated by a Gauss function.

Electroplated nickel oxide layers are often proposed as selective solar absorber coatings. The solar absorptance of these layers can be improved by superposing an appropriate surface structure. The growth conditions of the nickel crystallites can be influenced by the current density. The high current densities required for the deposition of suitable structured surfaces can be realized using short current pulses instead of direct current. The effect of the deposition parameters on the surface structure and the optical properties of the coatings were studied by scanning electron microscopy and by radiometric and spectrophotometric measurements.

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.

Determination of the reflectance for directional irradiation of diffusely reflecting samples requires measurement of the radiation reflected into the whole hemisphere. This can be done with integrating spheres. An arrangement based on a rapid-scan Fourier transform spectrometer equipped with a gold-coated integrating sphere allows measurement of the spectral reflectance in the 1.5μm–∼15μm wavelength range. Problems of calibration and accuracy are discussed. Results are compared with those of conventional techniques: measurement of the directional emittance of heated samples and measurement of the specular reflectance.

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.

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.

Solar absorber coatings are exposed to stresses, which are - depending on the collector and the local climate - temperature, humidity and moisture, solar irradiation and pollutants. The investigation of the durability of the coatings against these stresses requires accelerated short-term tests. Reproducible indoor tests, in which the loads are applied separately, allow a direct and quick comparison of the respective stability of different coatings and the identification of specific sensitivities to single loads. Constant temperature tests in air as well as humidity and condensation tests were carried out with six commercially available absorber coatings (black chrome, black nickel, coloured stainless steel and nickel-pigmented anodized aluminium). They showed great differences in the stability of the optical properties of the coatings. The degradation data obtained from these tests are contributions to a data base for service lifetime prediction models to be developed in the framework of the Task X of the IEA-programme “Solar Heating and Cooling”.

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.

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.

The increased risk of cholelithiasis during intake of oral contraceptives may be due to estrogen-induced saturation of the bile with cholesterol. In a randomized, prospective, crossed-over double-blind study 20 healthy women after roentgenological exclusion of gall-stones received either 1.0 mg of norethindrone acetate and 50 microgram ethinyl estradiol daily - as usual in oral contraception - for 21 days with 7 days of placebo treatment in each cycle or one fifth of this hormone dose in form of a continuous daily medication. After a 4 month's treatment the medication form was crossed-over. At the beginning of the study, before the cross-over and after the study bile was collected by duodenal intubation after a 12-h fast and the lithogenic index as a measure for cholesterol saturation of the bile was determined. No correlation between the dose and the lithogenic index was demonstrated, neither in 15 women, who had used oral contraceptives before the study nor in five women without any previous hormonal contraceptives.The increased risk of cholelithiasis during oral contraceptive (OC) intake may be due to the estrogen-induced saturation of the bile with cholesterol. In a randomized, prospective, crossover double-blind study, 20 healthy women after roentgenological exclusion of gallstones received either 1.0 mg of norethindrone acetate and 50 mcg of ethinyl estradiol daily for 21 days with 7 days of placebo treatment in each cycle or 1/5 of this hormone dose in the form of continuous daily medication. After 4 months of treatment, the medication form was crossed over. At the beginning of the study, before the crossover and after the study, bile was collected by duodenal intubation after a 12 hour fast and the lithogenic index as a measure for cholesterol saturation of the bile was determined. No correlation between the dose and the lithogenic index was demonstrated, neither in 15 women who had used OCs prior to the study nor in 5 women without any previous OC taking history. (author's)

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.

Forest growth and gap models have classically been divided into three main categories depending on the scale they refer to: stand models, distribution models or individual based models. Most of the time, a modeler chooses one of this level and designs his/her model from assumptions derived from the question the model is expected to answer to: yield prognosis, silviculture simulation, dynamic resource inventories. The question addressed in this presentation is the study of the coherence between those three scales. When a detailed model such as an individual based one has been designed, it seems easy to degrade it into a coarser one, by averaging over individual for example. At the same time, it is possible to design a model on the evolution of the average itself, regardless of the individual value distributions. If both methods lead to the same evolution of, say, the average value, then it is said that the individual based model can be aggregated. Our paper will address this topic on two directions: first as a mathematical based approach to study whether this property is a usual or exceptional in dynamical system, where it will be proved related to some eigenvalues in linear systems. Second as a case study between individual based and distribution models on data in Paracou experimental plots in French Guiana, where trees have been measured individually every three years since 1984, using a Forska type individual based model on one hand and an existing Usher-like matrix model on the other. As a conclusion, the feasability and difficulties of aggregation will be discussed.