Angle Dependence

Introduction

Standards on optical properties of glazing materials often focus on properties at normal incidence for various reasons: it is much easier to measure transmittance and reflectance at normal or near-normal incidence and polarization does not come into play. Commercial instruments sometimes have accessories for oblique incidence but they can be expensive and they are not often well tested. Obviously, sunlight falls on window surfaces over a range of angles far from normal incidence. For a simple comparative rating system, such as that used by the NFRC,  normal incidence is considered to provide an adequate ranking of products. It is certainly true that most specular materials have a functional cosine dependence and normalized angular profiles have a similar shape. Nevertheless, there can be a significant difference at say 60 degrees which could misrepresent the solar protection of certain coatings.

Status of Current Standards

ISO 9050 says only that "the sample shall be irradiated by a beam whose axis is at an angle not exceeding 10 degrees from the normal to the surface." Clearly the standard is limited to normal incidence with the small amount of latitude referring to the supposition that the optical properties are acceptably constant to 10 degrees. There would be no need to even mention that point except that reflection can never be taken at exactly normal incidence. EN 410 uses similar brief language as ISO 9050 on the subject of angle dependence. The new version of EN 410 apparently makes no changes to this policy although it is not quite final.

ASTM E903 refers to "near-normal hemispherical" transmittance (or reflectance)." The use of an Edwards sphere (center mount) is allowed and mention is made of the fact that a center-mount could be used for angle-dependent reflectance measurement. Apparently this statement is for information only and the standard is restricted to normal incidence. There is a new version of E903 under development, but like EN 410 an extension to angle dependence has not been considered.

If we consider  standards on properties of shading materials such as  EN 14500 there is some mention of how to incorporate polarized light as measurements are carried out at oblique angles. All diagrams covering the integrating spheres, however, are for normal angles and no recommendations are made for which angles to study or how to carry the measurements out with a spectrophotometer.

Future Work

So, very little has changed even in revisions of two major international standards. There is one more opportunity coming up however with a planned revision of ISO 9050 in 2011. ISO 9050 is arguably the most important standard from the point of view of international harmonization. If a procedure can be outlined then there is a good chance to incorporate angle dependence into ISO 9050. The International Commission on Glass (ICG) TC10 has decided to study a set of coated glazings (solar control, low-e, and antireflection) as well as a second surface mirror and as reference case an uncoated clear glass. The goal is to find a reasonable procedure for supplying information about how glazings perform at oblique angles of incidence. The samples were selected and supplied by the members of TC 10. The samples where selected to cover a range of coating that are representative for a large share of the produced coatings. It was further decided that only two angles of incidence would be selected for the ILC, zero and sixty degrees. The motivation was that intermediate angles would contain the same errors as sixty but less pronounced, it is also of interest to minimize the amount of measurements needed. Any procedure involving oblique angles of incidence might include a fair amount of extra manual labor for the measurements. Instead detail was to be focused on the wavelength dependence. 



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