Glazings for the 21st Century - Windows and Daylighting Group

6Fenestration Facts“The most promising smartwindow technology may bedevices based onelectrochromic coatings.Although not yet commerciallyavailable, they appearto have a good chance tomeet performance, cost, andmanufacturing requirementsthat would result in a marketablewindow system.”F A L LW I N T E R1 9 9 7Residential WindowsJohn Carmody et al1996LAWRENCE BERKELEYNATIONALLABORATORYENVIRONMENTALENERGYTECHNOLOGIES DIVISIONBUILDING TECHNOLOGIESPROGRAMWINDOWS & DAYLIGHTINGGROUPGlazing exists because of its optical clarityand light transmission properties. Increasinglyimportant is the ability of coatedglazing to control infrared and ultraviolet radiation.Dynamic coatings such as electrochromics,anisotropic and light-redirecting materials, geometricallycomplex insulation and shading systemswill control optical radiation in new ways.Optical and radiative properties of glazing materialstherefore are primaryinputs for determiningthe energy performanceof buildings. Accordingly,LBNLmaintains oneof the most extensive facilitiesin the world forcharacterizing the opticalproperties of glazingmaterials, includingseven specialized instrumentsand the analyticaltools needed to interpretthe results and extractuseful information.The Glazing OpticsCharacterization LaboratorySpecialized Equipment Helps Accurately Determine the Energy and VisualPerformance of Existing Products and Experimental Glazing MaterialsThe optical properties of simple glazing typessuch as coated and uncoated glass and plastic areconventionally tested at normal incidence (lightstrikes the glass at 90° to the surface). The NationalFenestration Rating Council (NFRC), with thetechnical assistance of LBNL, established proceduresthat provide for accurate measurement andcalculation of several different optical properties.These properties are then used in a nationwideprogram to rate and label the energy performanceof windows. As a result, all major U.S. glazingmanufacturers now follow a uniform characterizationprocedure. LBNL has collected and validatedspectral data for over 750 glazing products, anddistributes the data on a diskette or by downloadfrom our web site. The 5th release occurred inSeptember and is the first to include internationaldata, although it is not yet NFRC certified.The integrity of the glazing database is guaranteedboth by peer comparison and by checks ofeach file at LBNL. Atypical operation involvesthe measurement of product parameters neededfor calculations of SHGF, U-factor, and even annualenergy performance. Standard measurementsFigure 1. Ellipsometer determination of opticalindices.of spectral transmittance and reflectance at normalincidence are made with highly automatedspectrometers. APerkin-Elmer Lambda 19 withLabsphere RSA-19 integrating sphere is used forsolar range measurements on glazing products.Emittance is measured with a Nicolet FTIR with aCsI beamsplitter for extended range in the thermalinfrared. Under the auspices of the NFRC, glazingmanufacturers can measure their own materialsusing similar spectrometers.Significant error inthis type of measurement,however, is common,as shown by roundrobin studies among theNFRC manufacturers togetherwith the IEATask 18. If any featureof a spectrum appears tobe incorrect, more versatileand accurateequipment is employedas described below.With the basic ratingsystem working smoothly, the issue of propertiesneeded to calculate annual energy performancebecomes primary. Sunlight often strikes glass atoblique angles for which the transmittance and reflectanceare significantly different from their valuesat normal incidence. Areliable procedure forextrapolating from normal properties to obliqueproperties is thus needed for accurate annual energyperformance calculations. Several instrumentsare available to measure angle-dependent propertieswith high accuracy. Asingle-beam goniophotometeremploying a feedback-stabilized laserdiode with a short coherence length in conjunctionwith a large-area silicon detector provides absoluteverification at 670 nm and any angle of incidence.We also have a goniospectroradiometer describedin more detail below in connection withcomplex glazing. In principle, this instrument canmeasure all required angle-dependent properties,but accuracy is still an open issue. No standardprocedures or reference materials yet exist. Finally,we have a variable-angle spectroscopic ellipsometer(Fig. 1) which is used to determine fundamentaloptical constants of the glazings. These capabilities,however, do not exist in most industrial