Standards for Solar Optical Properties of Specular Materials



There are at least four standards based on the use of spectrophotometers to measure the spectral solar optical properties of glazing. The test procedures in these standards are often accompanied by calculation methods for combining two or more glazing layers into glazing systems. All of these standards are primarily concerned with materials having specular properties and with optical properties at normal incidence although reference is sometimes made to scattering materials and properties at oblique incidence. Scattering materials are discussed in more detail under standards for shading materials and a special section on angular dependence summarizes the few provisions that are present in these specular standards.

ISO 9050

2nd Edition 2003-08-15
Glass in building — Determination of light transmittance, solar direct transmittance, total solar energy transmittance, ultraviolet transmittance and related glazing factors
ISO TC 160/ SC2 / WG2

Choice of solar spectral irradiance function.   It is easy to agree that all standards should adopt the most technically correct solar spectrum. It is difficult or impossible, however, to say that a single spectrum is better than another spectrum for all possible conditions. It is most important to make a choice that is in harmony with other widely used standards. ISO 9050 currently uses the global spectrum of ISO CIE 9845 (from ASTM E892), while NFRC uses the direct spectrum of ISO/CIE 9845 (ASTM E891). This is significant because it can result in differences of a few percentage points in the solar average properties of strongly spectrally selective glazing products. 

Experimental guidelines in ISO 9050 are very sketchy. It is often assumed that purchase of an expensive spectrophotometer is a guarantee of good data. Even when an experienced technician carefully follows the instrument manufacturers instructions with further guidance by standards such as ISO 9050, the results are often greatly in error. This has been demonstrated many times during the course of interlaboratory comparisons (ILCs). Experimental guidelines should be expanded and put in a step-by-step form, so that any user of the standard could be expected to perform well in an ILC. The equations for calculating the properties of multilayer systems in ISO 9050 are correct, but they are messy. Little use is made of mathematical substitution and the equations are written out in fully expanded form. This makes them difficult to read, verify or program. The spectral integrals are written out within the multilayer equations, which looks impressive but is simply confusing. These equations should be written in much more compact and comprehensible form. Better yet, a simple reference should be made to one of many archival publications dealing with this subject.
Angle dependent optical properties. Currently ISO 9050 does not include procedures for measuring or extrapolating angle dependence of coated glass. Perhaps a review of the ADOPT work and other key papers on the subject would lead to something going into the standard. It is actually very important. Proper angle dependence can have a much larger effect than say choice of solar spectrum.
Nonspecular glazing: Currently ISO 9050 does not include the possibility for measuring the properties of diffuse glazing or calculating the properties of combinations. This would be a big change and the experimental techniques have not been fully researched. Nevertheless this is an important class of products and the extension should be seriously considered.

EN 410

1998
Glass in building - Determination of luminous and solar characteristics of glazing
CEN TC 129

EN 410 is virtually identical to ISO 9050 (see above). Unlike ISO, CEN TC 129 has been active and has a new draft of EN 410 at an advanced stage of development. The decision has been made to retain the CIE 98 solar spectrum which is global but not quite the same as either the ISO or the new NFRC choices.

ASTM E903

1996 (lapsed)
Standard Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres
ASTM E44

Unlike ISO 9050 and EN 410, ASTM E903 is mainly concerned with experiment, giving detailed instructions for the use of integrating spheres. Also, E903 applies to the general case of sheet materials, for any purpose related to solar energy use, rather than glazing in particular. This could be considered an undesirable degree of generality because different applications and material types may require different wavelength and angular ranges, different average types and different allowances for error. Nevertheless, ASTM E903 is a useful standard, written by technical experts.

E903 is written more as a reference document than a prescriptive standard and it is sometimes ambiguous, as in the treatment of nonspecular materials. As proof of this assertion, consider that many operators following this test method have been unable to measure with acceptable accuracy in ILC tests without further guidance. With careful revision and simplification of the experimental guidelines E903 could serve as an excellent reference on experimental methods for glazing-specific standards such as NFRC 300.
Note: ASTM E903 has officially lapsed but committee E44 is working on reinstatement.

NFRC 300

2009
Test Method for Determining the Solar Optical Properties of Glazing Materials and Systems
NFRC Optical Properties Subcommittee

This method has importance in the U.S. because it is used by the NFRC which rates and labels window systems sold in the U.S. It has often been resolved in principle that an ISO standard or other consensus standard developed outside of NFRC would be prefereble to developing internal procedures in the optical properties area. With the upcoming revision of ISO 9050, the time may be ripe for this change. The existing standard is not too bad but suffers from some of the same deficiencies as ISO 9050 and others:

  1. NFRC recently voted to "prefer" the new global spectrum ASTM G197, reasoning that G197 is the result of the latest research and superior instrumentation to ISO/CIE 9845 (ASTM E892). Implementation of this change in NFRC has been deferred pending investigation of the direction that ISO 9050 is likely to take. The result of a change in ISO 9050 from ISO/CIE 9845 global to ASTM G197 would be relatively small since both of them are global spectra.
  2. Missing extrapolation method for properties at oblique incidence. This was not important until recently because the NFRC rating system is based on normal incidence. The new CMA program requires angle-dependent properties for output to energy performance programs.
  3. Missing experimental procedures for diffusing materials such as fritted glass. Computational procedures for combining diffuse glazing are already built into Window6.
  4. Experimental guidelines are not adequate to ensure measurement to the stringent level of accuracy required to calculate the properties of glazing materials from the properties of components. See detailed discussion of this issue.

NFRC 302

The discussion of NFRC 300 would not be complete without discussion of NFRC 302. Not a technical standard or test method, NFRC is an unusual document which serves as a set of directions or rules for gathering optical data for the IGDB. As such it references NFRC 300 and other technical documents.

EN14500

Finally there is a new standard EN 14500 developed specifically for shading materials, which also has strong potential relevance for specular glazing. In fact, it could be considered as a unified standard for spectrophotometer tests on all kinds of materials. It is discussed in detail under standards for shading materials.





Subpages (1): Angle Dependence
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