A review is given of the main application fields of optical thin films and various mathematical methods of their design are briefly characterised, distinguishing between analysis, synthesis, recognition and the graphical methods. Some typical design procedures, such as the use of the Argand diagram, the equivalent layer or of buffering, are demonstrated. Finally, the present and future use of computers is assessed.

Refractive indices and index profiles of dielectric coatings have been measured during their deposition. The measurements can be applied in the case of single layers as well as complex layer systems with arbitrary layer thicknesses. The refractive index of homogeneous layers is investigated using transmission measurements of monochromatic light, whereas inhomogeneous layer profiles are measured at different wavelengths.With an accurately determined index, the production of special dielectric multilayer stacks as steep edge filters or antireflection coatings with low residual reflection can be realized. Thus it is possible to reach the theoretical steepness by monitoring the actual refractive index and correcting the layer thickness accordingly. Considering inhomogeneities the performance of antireflection coatings can be improved.

Software packages have been developed for use with the Perkin-Elmer 3600 computer control system for a Perkin-Elmer 983 infrared spectrophotometer. A system capable of automatic quality control for production processes, with minimum operator intervention has been established. A secondary achievement is the development of a thin film design package, including automatic optimisation, using the Perkin-Elmer 3600 as a stand-alone computer.

First, an introductory review of deposition dependent microstructural peculiarities of real optical thin films is presented. Then, some physical and chemical properties of particular interest for the user of coated optical elements are surveyed. Of the optical properties, the refractive index is discussed in detail, and reflectivity, light scattering and surface plasmon excitation are also included. Of the mechanical properties, adhesion, hardness, abrasion as well as some aspects of intrinsic stress are illustrated with practical examples. The importance of the thermal stability of optical coatings is emphasized, and some examples of severe alterations of coating properties after thermal treatment are given. The discussion of diffusion processes within some coatings and in the surface region of substrates leads to a survey of selected chemical reactions between the substrate and the coating, within the coating itself and between the coating and the environment. Finally, the degradation of coatings as a result of radiation impact is discussed shortly.

ℓThe state of polarization of light reflected from a strained metal surface is analyzed. It is demonstrated that the reflected light contains contributions which are linear and quadratic functions of strain. As shown, by our measurements on polycrystalline Aℓ, these can be separated by ellipsometry. The reflected light is measured as a function of the orientation of the principal axes of strain and qualitative agreement with theoretical predictions is obtained.

Measurements were carried out concerning the intrinsic tensile stress of chromium films evaporated onto bare and precoated glass substrates. Additionally the structure and microstructure of the films were investigated. Using these results together with the elastic data of chromium films a quantitative interpretation based on a grain boundary model could be performed.

By direct measurement of the evolution of the optical properties of a dielectric multilayer during its deposition we have access in real time to data on the thickness, the index and the structure of each individual layer. We describe such techniques which have been put into operation in Marseille to tackle the specific problem of monitoring the deposition of optical coatings on surfaces. We analyse the causes of the performance limitations of these monitoring methods.

An automated control system for evaporation and sputtering plants is presented. It consists of an optical monitoring system (OMS 2000) and a control unit (LEYCOM) based on a 16 bit microprocessor.This system has been successfully in operation since three years. The OMS 2000 is a single wavelength monitoring system designed for measuring the reflectance or transmittance of a test glass during deposition of the coating. It has the additional capability of terminating the evaporation when a preselected intensity value is reached or when the measured intensity passes through an extremum. Different optical monitoring methods, which can be executed with the OMS 2000, are discussed with respect to their error compensation effect. When using a special 6 fold test glass changer a special monitoring method is possible which makes monitoring of non-quarterwave layer systems rather easy and which exhibits a useful error compensation effect for edge filters.

A Kalman filter algorithm has been applied to interpret the optical reflectance excursions during vacuum deposition of infrared coatings and multilayer thin-film filters. The application has been described in detail elsewhere and this paper now reports on-line experience for estimating deposition rate and thickness. The estimation proved sufficiently reliable to firstly 'navigate' regular manufacture (as controlled by a skilled operator) and to subsequently reproduce the skill without interpretation or intervention whilst maintaining exemplary product quality. Optical control by means of this Kalman filter application is therefore considered suitable as a basis for the automated manufacture of infrared coatings and multilayer thin-film filters.

There are many processes known for fabricating thin films/1, 2.Among them the group of physical vapor deposition processes comprising evaporation, sputtering and ion plating has received special attention.Especially evaporation but also the other PVD techniques are widely used to deposit various single and multilayer coatings for optical and electrical thin film applications/3,4/.A large number of parameters is important in obtaining the required film properties in a reproducible manner when depositing thin films by such processes.Amongst the many are the film thickness, the condensation rate,the substrate temperature,as well as the qualitative and the quantitative composition of the residual gas of primary importance.First of all the film thickness is a dimension which enters in practically all equations used to characterize a thin film. However,when discussing film thickness,definitions are required since there one has to distinguish between various types of thicknesses e.g.geometrical thickness,mass thickness and optical thickness.The geometrical thickness,often also called physical thickness,is defined as the step height between the substrate surface and the film surface.This step height multiplied by the refractive index of the film is termed the optical thickness and is expressed generally in integer multiples of fractional parts of a desired wavelength.The mass thickness finally is defined as the film mass per unit area obtained by weighing.Knowing the density and the optical data of a thin film its mass thickness can be converted into the corresponding geometrical as well as optical thickness.However,with ultrathin films ranging between a few and several atomic or molecular "layers"the concept of a film thickness may become senseless since often no closed film exists of such minor deposits.Although film thickness is a length,the measurement of it can,obviously,not be accomplished with conventional methods for length determinations but requires special methods.The great efforts made to overcome this problem led to a remarkable number of different,often highly sophisticated film thickness measuring methods reviewed in various articles such ase.g./5,6/.With some of the methods,it is possible to carry out measurement under vacuum during and after the film formation other determinations have to be undertaken outside the deposition chamber only after the film has been produced.Many of the methods cannot be employed for all film substances,and there are varying limits as regards the range of thickness and measuring accuracy.Furthermore, with these methods the film to be measured is often specially prepared or dissolved during measurement and therefore becomes useless for additional investigations or applications.If only those methods which can be employed during the film deposition are considered,then the very large number of methods is considerably reduced.Insitu measurements,however,are highly desired since many basic investigations and practically all industrial applications require a precise knowledge of thefilm thickness at any instant to enable termination of the deposition process at the predetermined right moment.Apartfrom few exceptions in practical film deposition only optical measuring units andmass determination monitors are used.

A system which includes a wide-band scanning monochromator has been developed for the monitoring of the deposition of optical thin film coatings. This instrument measures the transmittance of the growing films over the spectral region 400 to 1100 nm. The monochromator uses a flat-field holographic concave grating in conjunction with an array of electronically-scanned photodiodes to measure the spectral characteristics of the films over the entire region once every 0.2 second. The data from the scans are used in the measurement and control of film thickness during deposition, but are also recorded on floppy disks so that they can be analysed afterwards, and they are now being used in the study of the behaviour of optical thin films during deposition. From these results it should be possible to calculate the profile of refractive index throughout each film yielding useful information on film structure. Methods which are used for this calculation involve as a first step the identification and measurement of extrema in the transmittance records for each wavelength permitting the calculation of the effective indices between these extrema and the eventual derivation of the refractive index profile of the films as a function of wavelength. Results which have so far been obtained for zinc sulphide are consistent with measurements of the refractive index profile made by other workers at single wavelengths, and therefore give additional support to the currently accepted models of zinc sulphide microstructure.

During the production of dielectric thin film stacks for optical use, small thickness errors are unavoidable. These can be detrimental for the reflectance curve R as a function of the wavelength λ. If the thickness error for a certain layer is known, however, its influence on the reflectance can be reduced by correcting the thicknesses of the following layers. Starting from the matrix of derivatives ∂R_j/∂^tk, where Rj is the reflectance of the j-th extremum and tk the thickness of the k-th layer, a method is developed which calculates these corrections during the production process of the stack. Examples will be given, using a quartz crystal monitoring system by which an error is easy detectable. Using this method, the deviations in the reflectance curve can be reduced by a factor of about five. This resulting reduction is strongly dependent on the error in the last layer of the stack for which no compensation is possible.

Classical broadband antireflection coatings utilize a single halfwave layer to broaden the spectral bandwidth over which antireflection takes place. A logical extension of this design technique is to use multiple halfwave layers to further broaden and/or flatten the spectral response. The generic category of multiple halfwave antireflection coatings are examined. Examples are presented which include both dispersive and non-dispersive calculations. The spectral response of these designs will be compared to a classical quarter-half-quarter antireflection coating. Design characteristics at non-normal incidence are examined as well as sensitivity to errors in thickness, index, and the effects of film inhomogeneity.

A, method of designing broad, double-band antireflection (AR) coatings on various glasses is presented. The method, based on Fabry-Perot interference filters theory, utilizes the effective interfaces concept. A simple, four-layer system is shown to fulfil reflectance-phase shift conditions, which cause transmittance maxima at λο and near λο. The model was employed to design an AR coating on BK-7 in the visible and at 1064nm. The Herpin equivalent indices were used to transform the basic structure into a six-layer stack com-posed of two materials only. [P such systems, consisting of MgF₂/ZnS and SiO₂/TiO₂ pairs are discussed. The method was extended to design a double-band AR coating on quartz in the UN/ (335-365n0 and at 1064nm. Four-layer systems, composed of Si02/Zr02 or Si0₂/Y20₃/Zr0₂ are described. Experimental results are given to illustrate the potential use of this design method to produce low-loss AR coatings by standard vacuum depo-sition techniques.

A generalised scheme for designing non-quarter wave higher order multilayer ARC systems is developed. In this scheme, one or more than one layer of the initial single/double/ triple layer system, are considered as Herpin equivalent layer and exact optical thicknesses of the constituent layers are calculated. It is seen that the method can yield broad band ARCs and thickness corrections can be applied under in situ conditions. A four layer ARC system, designed and optimised by this method has reflection losses less than 0.5% over a spectral range of 6.55 - 11.25 micrometres.

The molecules which comprise aLangmuir-Blodgett film are often long chain fatty acids. The refractive index of these molecules may be selected by choosing the CH₃ chain length, and modified by incorporating a metal ion into the OH termination of the structure. A monomolecular layer of such molecules may be created by spreading the fatty acid on a water surface. The monolayer may then be removed from the water surface by immersing a glass, or similar substrate, through the monolayer; the molecules will adhere to the glass. It is necessary to deposit tens of monolayers onto the substrate, by sequential dipping, in order to fabricate a planar optical waveguide. Some organic fatty acids may be modified by uv Tight, x-ray exposure, orbyanelectron beam, to form stripe waveguides and other integrated optical components. The attraction of such guiding structures is that by fabricating them from molecular layers, the film refractive index and the film thickness can be precisely determined. The energy propagation velocity within the waveguide may thus be accurately predicted.

Planar monomode and multimode Si0₂-Ti0² waveguides were prepared with a dip coating method from Liquicoat solutions supplied by E. MERCK. By varying the Si0₂:Ti0₂ mixture ratio the value of the refractive index nF of the waveguiding films on Pyrex glass substrates can be chosen to lie between nF-1.6 and n_F =1.36 First results on the preparation of LiNb0₃ waveguides are also presented. Thicknesses, refractive indices and losses of the waveguides were determined at the blue-green Ar laser wavelengths and at the He-Ne laser wavelengths λ=632.8 nm and λ=1.153 μm. With an embossing technique we fabricated surface relief gratings on Si0₂-Ti0₂ wave-guides. We used them successfully as input grating couplers. We propose to use this emboss-ing technique to fabricate channel waveguides and other integrated optical components in inorganic hard waveguiding materials such as Si0₂-Ti0₂.

A method for analyzing slanted anisotropic gratings is presented. The method allows treatment of scattering and guiding of waves by a general dielectric grating with fringes of any orientation in three dimensions. Generalized coupled-wave equations are derived in a unified matrix form expressing the coupling of the space harmonics in the grating region. By introducing the concepts of the transmission and boundary matrices, the solutions are obtained rigorously by systematic matrix calculations that are easily implemented on a computer. The diffraction efficiencies of anisotropic gratinas and the properties of dielectric waveguides with slanted gratings are shown as numerical examples.

Computer codes which simulate the photolithographic process are open loop problem solvers: given the inputs and parameters , a result is predicted . We suggest herewith that control theory may help in closing the feedback loop and allow process design and optimisation. As an example we deal with polychromatic exposure, which aims at reducing the standing wave effect. We consider resist activator dynamics and scalar field equation. A minimisation problem is defined , to which we apply complex Lagrangian theory. Optimal inputs can be searched for by an algorithm of which we give the flow chart.

The novel sol-gel technique has been implemented to deposit electroconductive tin oxide films to be used as transparent electrodes, mainly for display applications. Thin films of antimony-doped tin oxide were deposited on several types of glass substrates (soda-lime-silica, borosilicate and fused silica) using a dip-coating procedure. Alcoholic solutions of tin and antimony organometallic compounds were prepared under controlled conditions. The dipcoating procedure is described° in detail as well as subsequent thermal treatments under controlled atmosphere and temperatures up to 630 C. The optical and electrical characteristics of the films were studied as a function of the process parameters, firing conditions and number of coatings. After the subsequent thermal treatments, 2film resistances of about 200 ohms square could be measured corresponding to resistivities of about 10 ohm cm. The films optical transmission was above 80 percent.

A technique has been developed to determine one-dimensional linear refractive index profiles for planar thin films from angular reflectivity measurements at one wavelength. The technique uses a nonlinear least-squares fitting algorithm, together with a multilayer characteristic matrix model. The fitting error is examined as a function of adjustable parameters using a three-dimensional graphing routine. From synthetic data it is demonstrated that, within the domain of chosen adjustable parameters and realistic experimental uncertainties, a best linear profile fit can be obtained. The technique is demonstrated using reflectance data on laboratory samples of In₂0₃:Sn.

A problem of interest in thin film optics is to nondestructively measure one-dimensional refractive index profiles. We have been investigating the use of measurements of reflectivity magnitude as a function of incidence angle to determine such refractive index profiles. One approach is to use the Gel'fand Levitan algorithm as outlined by Moses and deRidderl. The algorithm requires knowledge of the Fourier transform of the complex reflectivity function. In this paper we present the results of computer testing an iterative technique for generating the complex reflectivity function from its magnitude and from appropriate constraints. The empirical results indicate that the iterative technique is capable of obtaining complex reflectivity estimates that could be within laboratory ins trument"generated uncertainty.

The formation of leached layers on optical glass BaK 4 by immersion in an acidic solution has been studied both by spectrophotometric and ellipsometric reflectance measurements after various immersion periods. From preliminary spectral reflectance measurements as well as from additional Auger depth profiling results, it can be assumed that the composition and refractive index of the leached layer are homogeneous. However, from physicochemical consi-derations of the diffusion process, a very thin transition layer between the homogeneously leached layer and the bulk glass should exist, and this was not found by these methods. Hence, ellipsometric measurements of various leached layer samples were taken. In a first approximation, the homogeneous layer model holds also for the interpretation of the ellipsometric data. However, the addition of a thin transition layer to the homogeneous leached layer enables better interpretation of the ellipsometric results.

Efficient integrated frequency doubling devices transparent in the visible and near I.R. are demanded by a number of applications. The optimization of both wave interaction configurations and material intrinsic nonlinear susceptibility are successively discussed within this scope. Basic features such as power confinement, interaction length dependence, phase matching techniques, underlying the second harmonic generation conversion rate in bulk and waveguided structures are compared. Undoped Ga As film epitaxied over n+ doped Ga As substrate and TIPE Lithium Lobate waveguides exemplify the possibilities of non linear thin films. The higher non linear susceptibility of certain organic molecular single crys-tals should help raise the efficiency of doubling devices. We report the definition and bulk performances of two non linear organic crystals, namely POM (3-methyl-4 nitropyridine-1-oxyde) and MAP (methyl-(2,4-dinitropheny1)-aminopropanoate) with a figure of merit up to one order of magnitude above that of Li Nb 0₃. The combination of organic materials and waveguided configuration should lead to a new generation of non linear devices.

A two-stage replication technique has been developed that will enable microroughness measurements to be made on large or unusually shaped optics that cannot be measured using existing techniques. A small, first-stage replica of room-temperature-vulcanizing (RTV) silicone rubber is made of one or more parts of the surface. This material exactly replicates surface details and separates easily from most glass and metal optics. A second-stage epoxy replica is then made from the first-stage replica. It also preserves surface details and can be easily separated from the first-stage replica. The epoxy replica can be aluminized for Nomarski microscope examination, interferometric or scattering measurements, and can also be profiled with a diamond stylus probe. Examples are given of various types of surfaces and their replicas. Surface profiles are also shown of two masters and their replicas. This two-stage replication technique has been found to give excellent replication of surface detail, along with adequate preservation of surface flatness and a minimum lateral dimensional change, 0.3%. It is thus an excellent method for sampling small surface areas of large optics.

Although it is possible to accurately predict angle-resolved scattering from surfaces covered with multilayer dielectric films and some of the scattering from metal-coated surfaces, there are still areas of disagreement between theory and experiment. In particular, the ratio of p- to s-polarized scattered light at a given scattering angle should be a function only of the optical thicknesses and optical constants of the surface layers (for dielectric films) or optical constants (for opaque metal films) , and the angles of incidence and scattering; it should be independent of surface topography. It was found experimentally that the p/s polarization ratio for 60-degree incidence, 60-degree retroscattering at λ6328 Å varied widely from the theoretical values for silver- and aluminum-coated glass samples and for polished and diamond-turned copper samples. In addition, theory predicts no cross-polarized scattered light (p-incident/s-scattered or s-incident/p-scattered), but in some cases large amounts were measured experimentally. It was found that the polarization ratio could be increased to approach the theoretical value by heating the films in vacuum, with consequent recrystallization and roughening of the films. It was hypothesized that some of the measured effects could be caused by fluctuations in the optical constants in the plane of the film; a first-order vector perturbation theory was derived to cover this situation. Calculations made using the theoretical equations look encouraging, but the theory does not predict any cross-polarized scattered light. More theoretical work needs to be done and, in addition, some structural characterization of the films.

The motivation for research on laser induced damage in optical thin films is quite obvious to any individual working with high power lasers, since the principal limiting constraint in the design and operation of high power laser systems is performance degradation due to the interaction of intense coherent light with optical elements in the system. It is of concern in the normal operation of high power laser systems and in the design and performance of efficient lasers. Increased power loading (watts per square centimeter of laser irradiation) would enable the design, fabrication, and utilization of more compact laser systems with greater yield and lighter weight, which, for most applications, offer tremendous advantages. Mechanisms involved in laser induced damage in optical thin films will be discussed.

Outlines are given of a theoretical model for predicting the spectral and spatial distributions of radiation scattered at rough interfaces of optical coatings, as well as the apparatus for measuring these distributions in terms of differential scattering coefficients (DSC). Measurements of DSC for a reflector, narrow-band spectral filters deposited at two extremes of substrate temperature and an uncoated substrate are compared with the corresponding theoretical predictions. The values of DSC integrated over all space are also compared with the predictions of the model and with values of the total integrated scatter (TIS) obtained from photometric observations of the direct transmittance and reflectance of the filter coatings.

The surface roughness of ultrasmooth laser mirror coatings and substrates can be assessed in various ways: mechanically, optically and electron-optically. In this paper, a selection of useful methods like stylus measurements, optical heterodyne profilometry, light scattering measurements and electron mirror interference microscopy is presented. Results obtained with these methods at various laboratories from our inhouse superpolished optical flats are given. Some problems of comparing the results from the different methods are addressed and aspects of the practical utility of the methods discussed.

Laser induced damage threshold measurements at 248nm have been made on thin film deposited aluminium and dielectric overcoated aluminium mirrors. The damage threshold of the aluminium coatings was 0.2Jcm^-2. Three dielectric overcoats have been considered, SiO, MgF₂ and ThF4. The damage threshold of the SiO and MgF? overcoated aluminium mirrors was about 0.2 - 0.25Jcm^-2. The ThF4 overcoated aluminium mirrors with reflectivities at 248nm comparable to the MgF₂ and SiO overcoated aluminium mirrors had damage thresholds of 0.4Jcm^-2. The influence of U.V. irradiation on the U.V. reflectivity and stoichiometry of the SiO overcoated aluminium mirrors was also investigated. Improvements in reflectivity of 20% at wavelengths below 250nm were observed.

Magnetron Reactive sputtering presents interesting possibilities for the deposition of optical coatings. Many oxydes and nitrides have been obtained in this way from metallic targets, in while the synthesis of other compounds (fluorides, sulfides) is still problematic. Starting with relatively rough optical applications, as: colored window-glass coatings and indiumtin oxide layers for IR-reflectors, the high-rate magnetron sputtering becomes available for precision optical (interferential) coatings, owing to the progress made in the mastering of the reactive sputtering machinism.

Integrated optic devices are normally confined to within a few microns of the surface. This is the depth scale which is readily accessible to ion implantation techniques. Optical waveguides and associated components may thus be written directly into a substrate or thin film by a combination of masking and ion implantation. The earlier development of implantation for semiconductors has been extremely successful and liberated circuit design from the constraints of diffusion controlled circuits and allowed high tolerance, high density inte-grated circuits to be formed. For optical systems similar advantages allow refractive index depth profiling, strong lateral confinement of low loss waveguides and spatial variation of electro-optic and SAW properties. A full exploitation of the implantation techniques to thin film systems has still to be made. This paper will give examples of current progress in ion implanted waveguides and electro-optic devices with materials ranging from silica and lithium niobate to gallium arsenide. These will demonstrate the mechanisms by which the refractive index or optical properties can be controlled. Related ion implantation effects will be briefly mentioned.

Visibly transparent highly conducting indium oxide has been prepared on flexible plastic sheet in a continuous roll-to-roll reactive magnetron sputtering apparatus.Results are presented which show the sensitive dependence of the optical and electrical properties of the coatings on the deposition conditions. Attention is drawn to the control of gas admittance and maanetron current and the necessary procedures are described.

Sol-Gel technology can be used to deposit a wide variety of oxide coatings on substrates such as glass, ceramics or metals. This presentation outlines the fundamentals of the process and the possibilities offered by this technique. Several oxide layers have been prepared in our laboratories including single oxides like SiO2, TiO₂, ZrO₂, VO_2+x, V₂0₅, as well as multicomponent systems such as SiO₂-Ge0₂, SiO₂-TiO2, SiO₂-Y20₃ and SiO2-TiO₂-Al₂0₃. The coatings are deposited by dipping the substrate into a solution containing the appropriate organometallic compound with control of the surrounding atmosphere. Subsequent thermal treatments are required to eliminate the volatile solvents, which are physically and chemically absorbed into the oxide layer and to densify the layers. The first thermal treatments are carried out at temperatures up to 150°C. Further treatments up to about 600°C (depending on oxide composition) are necessary to eliminate organic traces and residual moisture as well as to harden further the oxide coating.

The cost of materials is often a major part of the total cost of thin film products such as solar cells. When evaporation is used to manufacture a thin film product, the material cost depends upon the efficiency of material utilization which, in turn, depends upon the vapor source distribution and the portion of the vapor stream that is condensed on the substrate. The vapor incident on a flat substrate suspended and conveyed over single and multiple evaporation sources is analyzed. The sources are assumed to have symmetric vapor distributions described by fcos⊕ + gcos³⊕(P. The results are then applied to the design of masks which attenuate a portion of the vapor stream to provide a coating with uniform thickness. Graphs of the material usage efficiencies are provided for common configurations.

Hydrogenated amorphous carbon (a-C:H) thin films were prepared from hydrocarbons such as CH4, C2H4 and C₆H₆ in a capacitively coupled RF-discharge. a-C:H films which are electrically insulating (10 12 2Ωcm), transparent in the IR and very hard (on the order of 1400 kp/mm2 Knoop hardness) were deposited on glass, quartz, Si, Ge, SiC, GaAs and Gd3Ga₅O1₂. This method allows large area (several inch diameter) homogeneous coatings and using C₆H₆ also gives high deposition rates on the order of 1000 Å/min. The deposition process is described and the influence of various deposition parameters is discussed. It is shown that by means of the two parameters negative self bias and gas pressure coating properties such as refractive index can be easily tuned and controlled. The application of a-C:H as a single layer antireflection coating on Ge for 10.6 [im is demonstrated; the reflection at 10.6 pri is less than 0.2%.

A brief survey is given of some recent trends and developments in the field of optical coatings for energy efficiency and solar applications. The discussion is focused on spectrally selective coatings and embraces transparent heat-mirrors, surfaces for selective absorption of solar energy, coatings for passive cooling by selective infraredemission, and optical switching coatings. A number of examples of coatings for different purposes are treated; most of these are taken from recent work performed at Chalmers University of Technology.

The enhancement of the photothermal conversion efficiency of solar energy in solar collectors can be achieved using spectrally selective absorbing surfaces limiting the infrared radiative losses. The interest and the limits of the spectral selectivity, in terms of con-centration ratio and temperature, are discussed and the global quantities a (solar absorptance) and ε (infrared emittance) characteristic of the selectivity are pointed out, considering the energy balance. The useful parameters λ (wavelength) and θ , cp (incidence) related to real problems and real surfaces are then considered. The connections between the measured characteristic quantities a' (λ,θ,Φ, c'(λ,θ,Φ and the optical properties of so-lids are established for the case of homogeneous, inhomogeneous and rough films. Finally the measurement techniques of these quantities are presented. An original system, including integrating spheres, developed at Laboratoire d'Optique des Solides, is described in detail.

The provision of coatings to utilise solar radiation and to Prevent heat radiation loss creates many Problems in their design and creation. They must be simple and be able to be manufactured in very large areas to meet the cost and application requirements. The use of optical Properties given to materials by their free carrier concentration is considered and it is shown that the outimum performance is obtained with thin films between those of metals and semi-conductors. The problems encountered with the use of non-optimum films are discussed. The process of Planar magnetron sputtering is discussed in detail to indicate how it forms a solution for the deposition of complex compound films which are suitable for this application, with close stoichiometry control, onto large area substrates of glass or plastic sheet.

Films with spatially graded refractive indices can be formed by a variety of techniques to produce high efficiency coatings. Cermet films with graded compositions are useful as selective surfaces and their absorptivity can be enhanced by coating with a microscopically textured surface to provide solar absorptivities approaching 0.99. High temperature stable Mo-Al₂0 and Pt-Al₂0₃ have been formed with this enhancement. Microscopically textured but chemically homogeneous materials can also be used for simple and effective selective surfaces. Semiconductors have intrinsic selectivity in addition to that induced by the structure. The use of reactive ion etching and selective chemical etching to produce the desired microstructure of submicron sized cones or columns in semiconductor and other materials is described.

The study of optical characteristics of "Black" molybdenum films shows that the thickness of thin films and the texture of the substrate surface have a great influence on the selectivity of the absorbing surfaces. The conjugation of the effects of the two parameters permits to enhance the selectivity of the stacks. The results are interpreted in term of an application of the approximated electromagnetic model of Beckman.

Chromium nitride (CrN_x) layers have been prepared by d.c. reactive sputtering of chromium in an argon-nitrogen atmosphere. Structural, electrical and optical properties of samples, deposited at various nitrogen pressures, have been determined. Spectrally selective CrN_x layers on a polished copper substrate show a moderate solar absorptance (α_s≈ = 0.80) and a low thermal emittance(ε_n 0.04). The absorptance has been improved by depositing CrN_x films on a rough copper base-layer. The thermal stability in vacuum and the corrosion resistance of CrN_x, samples have been tested.

The solar selective properties of thin titanium nitride films have been studied. High quality TiN-films were made on fused silica and glass substrates by reactive sputtering of titanium in a nitrogen atmosphere. The resulting film thicknesses ranged from 5 - 120 nm. The reflectance and transmittance measurements confirm the high solar and visible transmission previously calculated from optical constants. The infrared reflection is lower than calculated, but sufficiently high to make TiN a new competitor for selective transmission applications. Three optical parameters were determined and used to obtain the optical constants as well as the film thickness. The refractive index increases with decreasing thickness. The extinction coefficient is almost constant above the thickness of 12 nm, but strongly reduced for films thinner than this.