Pietro Polato

Angular-dependent optical properties of low-e and solar control windows—: Simulations versus measurements

Abstract The angular-dependent optical properties of low-e and solar control glazings have been investigated in a European project, ADOPT, within the Standards Measurements and Testing programme. The object of the project has been to identify reliable ways of predicting the angular dependency without having to perform measurements or detailed calculations. Two new predictive algorithms have been developed and validated. For the investigated coatings the accuracy of these predictive algorithms is mostly within 1% of the value obtained by measurements or Fresnel calculations. This is an improvement over previously used algorithms, which have failed to distinguish between different types of coatings. The improved accuracy is of importance in energy simulation of buildings and makes improved product specification possible.

Characterisation of antireflective TiO2//SiO2 coatings by complementary techniques

Abstract Reactive magnetron sputtering deposition processes based on rotating or double cathodes allow the deposition of multiple TiO2//SiO2 layers on glass for obtaining antireflective coatings for large area applications with a good homogeneity and high resistance against environmental attacks. By increasing the number of TiO2//SiO2 layers, the luminous reflectance of the coated glass decreases but the calculation of angular daylight and energy parameters requires the use of an increasing number of complementary analytical techniques due to the complexity of the coating. In this paper we show how Rutherford back-scattering, neutron reflectivity and spectrophotometry can be successfully utilised to perform a quite complete characterisation of commercial antireflective coatings based on three TiO2//SiO2 layers.

Measurement and prediction of angle-dependent optical properties of coated glass products: results of an inter-laboratory comparison of spectral transmittance and reflectance

The European Union ADOPT project has been concerned with the accurate determination of the optical properties of coated glass products, describing reliable procedures for angle-dependent measurements and the development of predictive algorithms to enable accurate angle-dependent performance characterisation based upon near-normal values only. An essential element of the ADOPT project was the inter-laboratory comparison of transmittance and reflectance measurements for commercially available glazings. Thirteen European and three USA laboratories participated. The selected samples represent major categories of coated glass products used in buildings. Each participant received a set of samples and measured the angle-dependent spectral transmittance and reflectance under appropriate polarisation conditions for incident wavelengths within the solar spectral distribution. Calibrated mirrors were also provided for reference purposes. This paper presents the results of the inter-comparison which show that near-normal solar, visible and spectral transmittance and reflectance can be measured with an uncertainty of better than ±0.02. The inter-comparison illustrates the difficulties still experienced in making accurate oblique incidence measurements. The predictive algorithms, which require near-normal input data only, are capable of predicting the angle-dependent transmittance, reflectance and total solar energy transmittance for double and triple glazed windows to an accuracy which enables reliable calculation of the annual solar heat gain through windows.

Characterization and calibration of a variable-angle absolute reflectometer

The instrument described is a newly designed reflectometer. Measurements are made with a polarized light beam as a function of wavelength and angle of incidence. Calibration tests have outlined the possibility of using the instrument for reflectance measurements on mirrors with good reproducibility (0.2%) and accuracy (better than 1%). This has been obtained by the alignment system of the sample and the absolute method used for the determination of its reflectance.

Hybrid equivalent model algorithm for the prediction of glazing angular properties

Daylighting and energy consumption evaluation for actual rooms requires knowledge of the luminous and energetic parameters of the fenestration, both at normal and off normal incidence. In the case of coated glazings, the off normal parameters can be achieved only with very complex measurements. The Hybrid Equivalent Model Algorithm allows the prediction of the fenestration characteristics solely on the basis of the transmittance and reflectance spectra, measured at normal incidence, without need of any information about the coating composition. In this paper, the algorithm effectiveness is tested on single, double and triple-pane glazings by applying it to a set of synthetic coated glasses, inspired by real specimens; the variety of the considered coatings makes this set quite representative of the industrial production. The results are very encouraging: for 19 samples, on the 22 considered here, the mean deviation of the g-factor from the true values is less than 1%; for the remaining 3 it is not larger than 3.7%, and probably it could be reduced by further refining of the hybrid model because these 3 samples belong to the same product class.

Wideband optical coatings for protecting artwork from ultraviolet and infrared radiation damage

The damaging effects of illumination on artwork are well known. Art conservation requires protection from both vandalism and radiation damage. Glass is an appropriate material for these requirements but it partially transmits UV and IR radiation. An optical coating on glass that eliminates UV and IR radiation coming from natural or artificial illumination, is proposed. This coated glass, positioned in front of the artwork, is also able to reduce reflection without altering the appearance or colour.

Characterization of TiNxOy thin films on architectural glass by X-ray reflection and spectrophotometry

Abstract Conveniently selected thin films, such as magnetron-sputtered TiNxOy films, are deposited on flat glass for architectural purposes to reduce daylighting and solar intake in indoor environments. The luminous and solar behaviour of fenestration in real situations can be predicted if the coating structure (number of layers and thicknesses) and the material optical parameters (refractive index and absorption coefficient) are known. Unfortunately this information is often not available. Therefore a number of measurements are necessary to work out these parameters. In this paper, spectrophotometry has been applied for the optical characterization of commercial glass sheets coated with TiNxOy films. This technique was supported by X-ray specular reflectance measurements which supplied useful information about surfaces, thicknesses and layered structure of the thin film coatings.

Variable angle photometric characterization of a laminated glass embedding a lamellae system

Glazing units containing sunshading devices (vertical blades, roller shutters, Venetian blinds, etc.) are available on the market nowadays. These products are roughly divided into two families: double-glazed units with automatically controlled devices in the interspace and monolithic glasses embedding fixed devices. The luminous and solar behaviour of the incorporated sunshading devices is intentionally strongly dependent on the position of the sun in the sky, and therefore the optical characterization of this kind of advanced glazing requires transmittance and reflectance measurements as functions of at least one angular coordinate. In this paper a lamellae system embedded in a plastic matrix and sandwiched between two glass sheets is considered. Transmittance and reflectance of the sample computed from the geometrical parameters of the lamellae system are compared to the corresponding values measured with global and bidirectional variable angle measurement systems. As an example of application of the obtained data in real conditions, the laminated glass containing the lamellae system closes the window of a room and daylighting inside the room is evaluated.

Reflectance measurements on second-surface solar mirrors using commercial spectrophotometer accessories

Abstract Commercial spectrophotometers can be used with suitable accessories (specular reflectance accessories, integrating spheres) to perform accurate measurements on second-surface reflecting mirrors normally used for solar collectors. In this article the results obtained with different spectrophotometers and accessories are reported and discussed. Good convergence of the measurements was obtained with particular expediency in the visible range. In the case of specular reflectance accessories, a correction of the results is necessary account for the lateral shears of the second surface reflected beams. When using an integrating sphere as an accessory, its reflectance port must be sufficiently large in order to capture the most significant reflected beams. In the near infrared range, acceptable values can be obtained only for good alignments of the specular reflectance accessories and for internal coatings of the integrating spheres with a high diffuse reflectance in the entire solar range.

Multilayer coatings on glass for painting protection and optimized color rendering

Optical coatings offer a solution to the problem of damage to paintings, caused by ultraviolet and infrared radiation, by cutting radiation wavelengths outside the visible range. Simultaneously, these coatings can enhance an observers viewing of the paintings by reducing the reflections from ordinary glass panes. All these functions should be performed by the same coating. The design of such a coating, as well as the evaluation of existing products, requires the definition of an appropriate merit function in which coating absorption, high transparency, and color rendering are combined.