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.

ISO 9050

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

After a long period of inactivity, a new convenor has been appointed to the WG and an overdue revision of ISO 9050 was discussed at the ISO TC 160 plenary meeting in September 2009. The following issues regarding the revision could be said to hold for other standards as well:

1. Harmonization among countries: ISO standards often follow a "parallel development" with a CEN counterpart. In this case the corresponding CEN standard is EN 410 (see below). Since the EU member countries are bound to follow the CEN standard, this leads to reluctance to allow deviations from the CEN standard when the ISO standard development process begins. This situation is again presenting itself and needs to be discussed in ISO WG as soon as possible.
   
   
2. Harmonization among ISO committees: At one time ISO TC 163 and ISO TC 160 were supposed to be working together. In particular, standard ISO 15099 on window thermal performance was supposed to reference the optical properties standards developed by ISO TC 160. At least one organization, NFRC, has adopted ISO 15099 as the official framework and guide for all software development.
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3. Choice of solar spectral irradiance function. How can it be true that the choice of source function is both very important while being almost arbitrary?  It is easy to agree that all standards should strive for the most realistic solar spectrum and eliminate those that are known to contain experimental error. It is difficult or impossible, however, to say that a particular spectrum is "better" than another spectrum based on characteristics such as air mass, atmospheric conditions or direct/diffuse split. This is an inescapable problem when using a comparative standard based on a single set of environmental conditions. From this point of view, it is most important to make a choice that is in harmony with other widely used standards.
   
   ISO 9050 (and EN 410) currently uses the global spectrum of ISO CIE 9845 (from ASTM E892), while the U.S. 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. 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.
   
   
4. 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. Furthermore this procedure, while adequate for traditional purposes, is not good enough to ensure accuracy for calculation from component properties. Furthermore, it could be argued that the experimental guidelines should be a separate test procedure from the multilayer and spectral calcs to isolate this troublesome and long-running harmonization issue
   
   
5. The equations for calculating the properties of multilayer systems in ISO 9050 are correct, but they are messy. No use is made of the basic mathematical principle of substitution and the equations are written out in fully expanded form. This makes them difficult to read, verify or program. Also, 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.
   
   
6. 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.
   
   
7. 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

revision 1998
title Glass in building - Determination of luminous and solar characteristics of glazing
committee 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. It is unclear what changes can still be made.

ASTM E903

revision 1996 (lapsed)
title Standard Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres
committee 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

revision 2009
title Test Method for Determining the Solar Optical Properties of Glazing Materials and Systems
committee NFRC Optical Properties Subcommittee
chair Sneh Kumar

Although not developed by one of the usual national or international standards body, 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 omissions as ISO 9050 and others:

1. 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.
2. Missing experimental procedures for diffusing materials such as fritted glass. Computational procedures for combining diffuse glazing are already built into Window6.
3. 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.