Standards for Optical Properties of Shading Materials and Systems

Standards for optical properties of specular glazing materials are well established. Standards for nonspecular (partially diffuse or light redirecting) materials such as diffuse glazing or shading systems (called "attachments" by NFRC committees) are beginning to appear, but they are still in a chaotic state. ISO 9050, EN 410, ASTM E903, NFRC 300, are standards for measuring and calculating the properties of specular materials using spectrophotometers. Some of them make vague statements regarding the ability to measure nonspecular materials such as frtted glass, painted slats, etc, but this claim is debatable.

CEN Standards

CEN has been busy in this area and there are quite a few standards of relevance. Apparently there are two TCs involved with overlapping areas of interest: TC 33 (doors, windows etc) and TC 89 (thermal performance of ...building components, e.g., windows). There is also TC 129 (glass in building) which has some overlap in scope (see reference to EN 410 below). Let us first consider the four CEN standards of most direct relevance to optical properties and their complex relationships:

EN 14500

EN 14500: 2008
Blinds and shutter - Thermal and visual comfort - Test and calculation methods
CEN TC33/WG3 Doors, windows, shutters, building hardware
This is a detailed measurement standard whose closest relation is ASTM E903. EN 14500 goes far beyond ASTM E903 in details on experimental technique with spectrophotometers and integrating spheres. Also, it specifically claims to deal with the diffuse materials used in construction of shading systems. In contrast, ASTM E 903 makes a weaker claim to measurement of diffuse materials as a secondary purpose to the goal of testing specular materials. IEN 14500 covers both surface and center mount methods as does E903.

What does this standard have to do with "comfort?" This word in the title seems to be an artifact of the purpose of the TC 33. EN 14500 and its companion document EN 14501 (see below) have the combined purpose of producing a variety of comfort parameters for shading rather than energy parameters. Because of this tie-in, both standards have the word comfort in the title. Nevertheless, because of the more general usefulness of EN 14500, it is also referenced in standards produce by CEN TC 89 (see below) that are more closely related to energy performance.

Both EN 14500 and ASTM E903 are excellent documents in their way. Obviously written by technical experts in the field, they are full of useful information. They fail however to deliver on the often-expressed desire for a clear step-by-step procedure that can produce consistently accurate results in the hands of a competent technician. For example, it would be  highly desirable in performance of our Interlaboratory Comparisons (ILCs) to be able to provide such a direct set of instructions for all to follow.

Even more troublesome is that these "standards" claim to apply to diffuse materials as well as to specular materials. This is a premature claim. For example, we have found that accurate measurement of slat properties, which is possible in principle using a center mount for opaque materials , is quite difficult to obtain. The center mount that we used is a commercial product designed specifically for our integrating sphere. The price of these accessories would certainly imply a degree of confidence which turns out not to be warranted. Other problems are common with surface mount techniques. Procedures are needed quickly, but some restraint should exercised and studies should be referenced to prove that a given technique has been used with success.

EN 14501

EN 14501:2005
Blinds and shutters - Thermal and visual comfort - Performance characteristics and classification
CEN TC33/WG3 Doors, windows, shutters, building hardware
As mentioned above, this standard has the primary purpose of defining various comfort-related parameters constructed from more basic optical properties measured in accordance with EN 14500. These parameters include opacity, glare, privacy, visual contact, daylight, and color rendition. The energy-related g-factor which is also considered a comfort-related factor is included, but other documents are cited for the actual calculation, EN 410 for specular materials and EN 13363 (see below) for shades.

EN 13363-1

EN 13363-1:2003 + A1:2007
Solar protection devices combined with glazing – Calculation of solar and light transmittance– Part 1: Simplified method

CEN TC 89 Thermal Performance of Buildings and Building Components
Documents produced by CEN TC 89 now have titles that reflect the focus on energy. Still a bit indirect, mention of g-factor is omitted in the title although that is the ultimate purpose here. This is a very simple and practical method, with tables of base case optical properties for glazing and blind layers when the specific values are not known. The general term "solar protection devices" in the title seems to mean "blinds" in particular. Light and solar transmittance of combinations of glazing and shading are calculated with the specular glazing assumption which is surely not always accurate in these cases.

Would such a standard be suitable for our purposes? I think we are beyond the stage where falling back on this for lack of a more detailed and accurate method would be necessary.

EN 13363-2

EN 13363-2: 2005 / AC:2006
CEN TC 89 Thermal Performance of Buildings and Building Components
Solar protection devices combined with glazing - Calculation of total solar energy transmittance and light transmittance - Part 2: Detailed calculation method

A much more complicated and presumably accurate method than sister standard EN 13363-1. If my memory is not playing tricks, this standard seems to be derived directly from ISO 15099. Calculations of the properties of slat shading systems from individual slat properties and combinations of glazing and shading-system layers are performed with an "energy balance" method. This is all to the good, because NFRC has resolved that Window5/6 shall be based insofar as possible on ISO 15099. W5/6 does not use the same formulations for the optical properties but I believe they are at least equivalent. We may not be able to use this standard directly, but at least we seem to be going in the same direction. An added benefit is that these algorithms are also used by the European analog of Window6, i.e.,WIS.

ISO

Not known at this time if there are ISO counterparts to the above CEN standards. Check back for updates on this section.

ASTM ASHRAE and NFRC

There have been no new standards from ASTM in this area for many years. ASTM E903 (and NFRC 300) has the potential to apply to diffuse materials using spectrophotometers. See the discussion above under EN 14500. ASTM E903 has officially lapsed but may be reinstated. There are three other ASTM standards for measurement of solar and visible properties of large samples with or without scattering. These standards need to be scrutinized for accuracy, and even if they were found to be sufficiently accurate, they probably do not provide enough information for current purposes.

ASTM E 1084-86

Standard test method for solar transmittance of sheet materials using sunlight
The directional-hemispherical transmittance of medium-sized samples of sheet material is measured by placing a pyranometer in direct sunlight and diffuse sky light on a relatively clear day and calculating the ratio of the sensor's outputs obtained when the sample is present and absent from the aperture of a test box specially designed for this purpose, respectively. This test procedure permits varying the angle of direct sunlight incident on the specimen from measurement to measurement.

ASTM E972-88

Standard test method for solar photometric transmittance of sheet materials using sunlight
This document specifies a photometric sensor to be used with the procedures of standards E 1084 and E 1175.

ASTM E1175.

Standard test method for determining solar photopic reflectance, transmittance, and absorptance of materials using a large diameter integrating sphere

This latter test method is applicable to large specimens of materials having both specular and diffuse optical properties. According to the standard's statement of scope: "It is particularly suited to the measurement of the reflectance of opaque materials and the reflectance and transmittance of semitransmparent materials including corrugated fiber-reinforced plastic,..., and nonhomogeneous materials such as woven wood, window blinds, draperies, etc." It also includes within this one standard the use of a photometric detector, therby permitting the measurement of luminous reflectance and transmittance. It permits use of either the sun as the source or a solar simulator.

ASHRAE 74-73

Method of Measuring Solar-Optical Properties of Materials
This standard is a compilation of ASTM standards including the three mentioned above and ASTM E903. This standard is often quoted when reporting properties of shading materials. Obviously the particular substandard should be specified although that is not always the case. When in doubt, ASTM E1084 is the likely choice.





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