Evaluation methods for retroreflectors and quantitative analysis of near-infrared upward reflective solar control window film — Part I: Theory and evaluation methods

Highlights

• Upward and downward reflectivities as reflection characteristics for walls are proposed.

• Upward reflectivity is obtained through evaluation methods using an integrating sphere.

• The reflectivities for retroreflectors at large incident polar angles are obtained.

Abstract

In recent years, research has been actively conducted on the application of cool materials that have high solar reflectivity and high thermal emittance to building envelopes (roofs and walls) and other surfaces in the urban environment. They have been applied to decrease solar absorption and increase the urban albedo as an effective technique to mitigate the Urban Heat Island effect and to reduce global warming. It has been a concern that reflected solar radiation onto the streets (downward reflection) leads to thermal environmental deterioration of pedestrian spaces. In cases where building facades have specular or diffusive reflectivity, solar radiation is reflected downward to pedestrian spaces.

As there is currently a lack of evaluation methods for such retro-reflective materials, this study mainly aims at developing evaluation methods and measurements of reflectivity and transmissivity properties of retro-reflective materials. This required further development of evaluation methods and the apparatus including an integrating sphere, allowing the measurement of directional-hemispherical spectral reflectivity (DHSR) at large incident polar angles that are equivalent to the daytime solar elevation at a south-facing vertical surface, as well as the selective measurement of spectral reflectivity in the upward direction with the horizontal plane as the boundary. Furthermore, evaluation methods of DHSR including the retro-reflected beam that leaks from the incident opening are developed with a new measurement method using an integrating sphere and half mirror.

This article mainly describes theory and methods of the above-mentioned evaluation, and evaluates results of beam leakage compensated DHSR. Verification of the measurement results is discussed in Part II of this article, together with the properties and effects of the upward reflective solar control window film that was evaluated in this study.

Introduction

In recent years, research has been actively conducted on the application of cool materials that have high solar reflectivity and high thermal emittance to building envelopes (roofs and walls) and other surfaces in the urban environment, in order to decrease solar absorption and increase the urban albedo, as an effective technique to mitigate the Urban Heat Island effect (Santamouris, 2014, Santamouris et al., 2008, Synnefa et al., 2008) and to reduce global warming (Akbari et al., 2012a, Akbari et al., 2012b). Many studies on the use of highly reflective materials for building roofs and facades have also confirmed that buildings’ cooling energy can be saved by suppressing the temperature rise due to incident solar energy that is absorbed by or transmitted into the buildings (Levinson and Akbari, 2010, Levinson et al., 2005c, Synnefa et al., 2007, Takebayashi and Yamada, 2015). Furthermore, detailed studies on the optical properties of these cool materials have also been documented (Levinson et al., 2005a, Levinson et al., 2005b).

However, such highly reflective materials have specular or diffusion reflectivity, and when they are employed for non-horizontal surfaces, all or part of the reflected solar radiation goes downward or towards the ground. An increase of the downward reflection leads to thermal environmental deterioration of pedestrian spaces (Erell et al., 2013, Kondo et al., 2009). In recent years, intensive studies on the mitigation of the Urban Heat Island effect by using retro-reflective materials for roofs and facades have been carried out (Rossi et al., 2014, Yuan et al., 2016), and methods for quantifying the reflectivity properties of retro-reflective materials have been proposed (Rossi et al., 2015, Sakai et al., 2012).

In order to quantify the reflected solar radiation, it is indispensable to clarify the reflectivity and transmissivity properties of retro-reflective materials, which is desirably performed through measurement in a stable environment using artificial light sources. Commercially available spectrophotometers can be used to measure spectral reflectivity and transmissivity. Some of these, which are equipped with a gonio-spectrophotometer, can measure with large incident polar angles that are equivalent to the daytime solar elevation at a south-facing vertical surface (Yu et al., 2015). One of the challenges in quantifying the reflectivity properties of retro-reflective materials is how the retro-reflected beam that has the same direction of the incident beam should be measured, and evaluation methods have been reported using a gonio-spectrophotometer and beam splitter (half mirror) (Belcour et al., 2014), as well as coaxial optical fiber (Iyota et al., 2012).

Depending on the characteristics of the retro-reflective material, the incident beam can be reflected or transmitted to a wide angular range. In order to analyze the reflection and transmission energy over a wide angular range, such as reflection towards the ground, an integrating sphere should be used for measuring directional-hemispherical spectral reflectivity (DHSR) and directional-hemispherical spectral transmissivity (DHST). The integrating spheres incorporated in commonly used spectrophotometers, however, are designed to cover incident polar angles of only approximately 10°.

Moreover, measurement of DHSR using an integrating sphere still requires a technique to quantify the retro-reflected beam that leaks or passes back from the incident opening of the integrating sphere. Yuan et al. (2015) have proposed a method that determines the solar reflectance and retro-reflectance of retro-reflective materials by subtracting the solar reflectance measured with a spectrophotometer from the solar reflectance obtained through an outdoor heat balance evaluation.

This study aims to develop evaluation and measurement methods based on the challenges and research mentioned above. In this article, evaluation methods of DHSR and DHST at large incident polar angles described, which have been enabled by development of the apparatus including an integrating sphere and measurement techniques. Furthermore, evaluation methods of DHSR, including of the retro-reflected beam that leaks from the incident opening, are described, which have been developed by new measuring methods using an integrating sphere and half mirror.

Additionally, it is necessary to quantify the impact of the reflected solar radiation in an angular range of specific directions, so that the benefit of reflection towards the sky by retro-reflective materials or the negative impact of reflection towards the ground can be clarified. This study aims to propose separating the hemisphere surrounding the surface element into upward and downward directions with the horizontal plane as the boundary, in order to obtain both spectral reflectivities. It is assumed here that upward reflection goes to the sky and downward reflection goes to the ground. The directional-upward spectral reflectivity (DUSR) and the directional-downward spectral reflectivity (DDSR) can be considered to be useful indices to quantify the benefits obtainable from retro-reflective materials and the negative impact on the thermal environment of pedestrian spaces, respectively.

In this study, near-infrared (NIR) upward reflective solar control window film (URSCF) has been evaluated. This article mainly describes the theory and methods of the above-mentioned evaluation, and evaluates results of the beam leakage compensated DHSR. Verification of measurement results is discussed in Part II of this article (Harima and Nagahama, 2017), together with the properties and effects of the URSCF that was used in this study.