The results of full-scale experiments on models of a smart window with a double-grating optical filter are presented. Such a filter has an angular-selective direct light transmission due to two thin-film gratings with transmissive and non-transmissive (absorptive, reflective, scattering, or chromogenic) parallel strips. The gratings are angled to match the solar path relative to the window to optimize the daylight and solar heat gain without the use of shading/redirection devices. Due to the strip widths, relative positions, and slope angles of both gratings, calculated to minimize transmission at a preset time, such a smart window self-adapts to daily and seasonal changes in the position of the Sun. Diffused radiation passes through the transmissive strips of both gratings, increasing the daylight and thermal comfort indoors. Experimental data for the four hottest months confirmed the results of numerical simulation based on a previously developed method for calculating the temporal characteristics of the directional light transmission of filters. The minimum transmittance was achieved at a preset time on June 15, for which all geometric parameters of the filters were calculated; on July 15, August 15, and September 15, the light transmission consistently increased, indicating good seasonal adaptation. Conclusions regarding the selection and optimization of the strip widths of the gratings, as well as the processing of experimental data, were obtained.
This paper presents methods for calculating the direct light transmittance of a double-grating optical filter. Based on these methods, numerical simulations were carried out, the results of which were confirmed by experiments under direct collimated white light without the influence of diffuse light using two grating smart window models with different sets of strip widths, as well as a model of a conventional window with clean glass. The results showed that by pre-calculating the angle of inclination, the widths of the strips of both gratings and their relative position, taking into account the coordinates of the building and the azimuth of the window, the filter provides angular selectivity of transmission without the use of shading/redirection devices. Window transmission with such a filter is minimized during specified times of the selected day when the need for sun protection is greatest. Due to the angular selective light transmission and the ability to adjust the inclination of the gratings to the trajectory of the Sun, such filters are promising for use in smart windows for optimal filtration of direct solar radiation indoors.
Features of the application of a novel optical filter with angular selectivity of the light transmission to architectural glazing are considered. The filter consists of a sheet transparent substrate with thin-film grating layers on both surfaces. The gratings formed by directionally transmissive strips, alternating with absorptive, reflective, or scattering strips. Their relative position on the input and output surfaces provides angular selectivity of the directional light transmission – as the incidence angle changes, the proportion of radiation that passes through both gratings of the filter also changes. Chromogenic materials currently used in the laminated smart windows, providing control over the intensity and spectrum of the transmitted solar radiation, cannot achieve the selective regulation on the ranges of incidence angles. Such a regulation requires the use of additional daylight-redirecting devices, especially blinds, to dynamically adapt to the position of the sun. The grating optical filter provides angular selectivity of the light transmission of a window without such devices. The features of using this filter in the single and double glazed windows are described. A graphic analytical calculation method is proposed for estimating the effect of geometrical and optical parameters of the filter on the angular characteristics of the light transmission. An algorithm to optimize filtering solar radiation taking into account the geographical coordinates of terrain, time of day and year and the orientation of the window to the cardinal is set. An algorithm to calculating geometrical parameters of the filter with pre-specified characteristics of the light transmission is obtained.
Classical multilayer filters are optically homogeneous in each layer of the surface coatings. Diffraction gratings and photonic crystals used in the filters have micro- and nanoscale inhomogeneities. An optical filter consisting of sheet transparent substrate with thin-film grating layers on both surfaces is developed. Macroscopic gratings of submillimeter widths are formed by directionally transmissive strips, alternating with absorptive, reflective, or scattering strips. Their relative position on input and output surfaces provides angular selectivity of light transmission – part of the radiation that has passed through input gratings is blocked additionally by output gratings depending on incidence angle. Graphicanalytical calculation method determines the influence of optical and geometric parameters of alternating strips on angular characteristics of transmission. Characteristics are broken lines with areas of decreasing or increasing transmission, which practically linear for the angles up to approximately 60°, with further increase in the angles, the lines are curved more because of sinusoidal dependence under Snell's law. Results of graphic-analytical calculations are confirmed experimentally. Preset the desired dependence of transmission is adjusted considering reflection (Fresnel formulas) and absorption (Bouguer-Lambert law). The adjusted dependence is approximated to a broken line taking into account obtained regularities. Finally, all parameters of alternating strips of gratings are chosen. Thus at pre-known trajectory of light source relative to the filter pre-adapted angular selective regulation of its light transmission is provided. Architectural glazing to control the transmitted solar radiation without special redirecting devices and provide invisibility through window at predetermined angles is the most promising area of application.
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