Determining the optical properties of curtain wall glass is essential for verifying the energy efficiency, daylighting performance, and visual comfort of modern building envelopes. This technical article explains the key optical parameters of curtain wall glass—including visible light transmittance, solar transmittance, reflectance, solar factor (SHGC), and shading coefficient—along with the internationally recognized testing standards (ISO 9050, EN 410, GB/T 2680, ASTM E903) and laboratory methods used by accredited third-party laboratories to measure them.
Technical Overview
Curtain wall glass is one of the most influential components in a building envelope with respect to solar heat gain, daylighting quality, energy consumption, and exterior visual appearance. The optical performance of curtain wall glass describes how the glazing interacts with solar radiation across the ultraviolet (UV), visible, and near-infrared (NIR) regions of the spectrum, typically covering a wavelength range of 300 nm to 2500 nm.
Determination of optical properties is performed using spectrophotometric measurement combined with standardized weighted calculation methods. The spectral transmittance and reflectance of the glass are first measured with a UV-Vis-NIR spectrophotometer, and the measured spectral data are then integrated against the photopic luminous efficiency function and the solar spectral distribution to derive engineering parameters such as visible light transmittance, solar direct transmittance, total solar energy transmittance (solar factor, g-value), and shading coefficient.
The principal standards governing this determination include:
- ISO 9050 — Glass in building: determination of luminous and solar characteristics of glazing
- EN 410 — Glass in building: determination of luminous and solar characteristics of glazing
- GB/T 2680 — Determination of light transmittance, solar direct transmittance, total solar energy transmittance and ultraviolet transmittance of glass in building
- ASTM E903 — Solar absorptance, reflectance, and transmittance of materials using integrating spheres
- JIS R 3106 — Testing method on transmittance, reflectance of flat glasses and evaluation of solar heat gain
- GB/T 18091 — Standard for optical properties of glass curtain walls, which sets limits on reflected light to control glare and light pollution
These tests are routinely carried out by accredited third-party laboratories to support product certification, building energy compliance, coating quality control, and facade design verification.
Testing Samples
Monolithic Glass
Monolithic samples include clear float glass, body-tinted glass, reflective coated glass, and low-emissivity (low-E) coated glass. Specimens are typically cut to approximately 100 mm × 100 mm or to dimensions compatible with the spectrophotometer sample compartment. The coated surface must be clearly identified and its orientation documented, since reflectance and emissivity values differ significantly between the coated and uncoated surfaces.
Laminated Glass
Laminated glass samples consist of two or more glass plies bonded with interlayers such as PVB, EVA, or ionoplast. The complete laminate is measured as an assembly because interlayers strongly absorb UV radiation and modify the spectral curve. The interlayer type and thickness must be recorded in the test report.
Insulating Glass Units (IGUs)
For insulating glass units, each component lite (outer coated lite, intermediate lite if present, and inner lite) is measured individually, and the optical characteristics of the complete unit are calculated using the radiative transfer procedures specified in ISO 9050 or EN 410. Where an instrument can physically accommodate a finished unit, direct measurement may be performed as a cross-check.
Sample Preparation and Conditioning
All specimens must be free from scratches, stains, fingerprints, edge chips, and coating defects within the measurement area. Surfaces should be cleaned with lint-free wipes and an appropriate solvent that does not attack coatings. Specimens are conditioned at ambient laboratory temperature (typically 23 °C ± 5 °C) before testing, and the air-side versus tin-side or coating-side orientation is marked prior to measurement.
Testing Items
The determination of optical properties typically covers the following parameters:
- Visible light transmittance (Tv or τv): the fraction of incident visible radiation (380–780 nm) transmitted through the glazing, weighted by the D65 illuminant and the photopic observer function
- Visible light reflectance (ρv): measured on both the exterior and interior surfaces, critical for facade appearance evaluation and light pollution control
- Solar direct transmittance (τe): the fraction of solar radiation (300–2500 nm) directly transmitted through the glass
- Solar direct reflectance (ρe): the fraction of solar radiation reflected by the glazing
- Solar absorptance (αe): the fraction of solar radiation absorbed by the glazing, calculated as αe = 1 − τe − ρe for single glazing
- Total solar energy transmittance or solar factor (g-value / SHGC): the sum of direct solar transmittance and the inward-flowing fraction of absorbed energy
- Shading coefficient (SC): the ratio of the glazing solar factor to that of 3 mm clear glass, commonly expressed as SC = g / 0.87
- Ultraviolet transmittance (τuv): the transmittance integrated over the UV band as defined by the applicable standard, commonly 300–380 nm
- Light-to-solar-gain ratio (LSG): the ratio of visible transmittance to solar factor, used to evaluate daylighting selectivity
- Color coordinates (L*, a*, b*) and color difference (ΔE*ab): evaluated in transmission and reflection to verify color uniformity of coated glazing
- Normal and corrected hemispherical emissivity of coated surfaces: measured optically and used in the calculation of g-value and thermal transmittance (U-value)
Testing Methods
Spectral Transmittance and Reflectance Measurement
A double-beam UV-Vis-NIR spectrophotometer equipped with an integrating sphere is used to measure spectral regular transmittance and spectral reflectance over the range of 300 nm to 2500 nm. Reflectance is measured at an incidence geometry of approximately 8° using a calibrated reflectance standard or an absolute reflectance attachment. Baseline and dark corrections are performed before each measurement series, and spectral data are recorded at sufficiently fine intervals for numerical integration. For coated and lightly diffusing samples, the integrating sphere ensures that both specular and diffuse components are captured.
Determination of Luminous Characteristics
Visible light transmittance and reflectance are obtained by weighted integration of the measured spectral data against the D65 standard illuminant and the photopic luminous efficiency function V(λ) in accordance with ISO 9050, EN 410, or GB/T 2680. This converts the physical spectral curve into the photometric quantities relevant to human visual perception and daylighting design.
Determination of Solar Characteristics
Solar direct transmittance and reflectance are calculated by integrating the spectral data against the reference solar spectral irradiance distribution, such as that specified in ISO 9845 or ASTM G173. For multiple glazing systems, the transmittance and reflectance of each component are combined layer by layer using the radiative transfer calculation prescribed by the standard to obtain the values for the complete assembly.
Calculation of Solar Factor and Shading Coefficient
The total solar energy transmittance (g-value) is calculated as g = τe + qi, where qi is the fraction of absorbed solar energy transferred inward. The inward flow is computed from the solar absorptance of each pane and the standardized internal and external heat transfer coefficients specified in ISO 9050 or EN 410. The shading coefficient is then derived by dividing the g-value by 0.87, providing a familiar comparative index for facade and HVAC engineers.
Ultraviolet Transmittance Determination
UV transmittance is obtained by integrating the measured spectral transmittance over the ultraviolet band defined by the applicable standard, commonly 300–380 nm. This parameter is significant for evaluating fading protection for interior furnishings and for glazing systems incorporating UV-absorbing laminated interlayers.
Colorimetric Evaluation
Color coordinates in transmission and reflection are computed from the spectral data using the D65 illuminant and the 10° standard observer, expressed in the CIELAB color space. Color difference ΔE*ab between samples or against an approved reference panel is reported; project specifications for curtain wall glazing commonly limit ΔE*ab to a range of 2 to 3 to ensure visual uniformity across the facade.
Coating Emissivity Measurement
For low-E and solar-control coated glass, surface emissivity is determined by Fourier-transform infrared (FTIR) spectrometry of the coated surface, following the measurement principles referenced in ISO 10292 and EN 673. The corrected hemispherical emissivity is used both in the g-value calculation and in the determination of the U-value of the glazing.
Applications
Determination of optical properties of curtain wall glass supports a wide range of engineering and regulatory activities:
- Building energy compliance: verifying that glazing meets the solar factor, shading coefficient, and visible transmittance limits required by national building energy efficiency design codes
- Facade and HVAC design: providing optical and solar heat gain data as direct inputs for whole-building energy simulation and cooling load calculations
- Light pollution and glare assessment: demonstrating that visible reflectance of glass curtain walls satisfies the restrictions specified in GB/T 18091 for sensitive locations such as arterial roads, residential areas, and aviation corridors
- Coating production quality control: monitoring the consistency of low-E and reflective coating batches during manufacturing and incoming inspection prior to IGU fabrication
- Daylighting and visual comfort design: selecting glazing with appropriate visible transmittance and selectivity to balance daylight autonomy against glare risk
- Green building certification support: supplying declared optical performance values for energy modeling submissions under green building rating frameworks
- Contract verification and dispute resolution: providing independent test data when supplied glazing performance is challenged by developers, contractors, or facade consultants
FAQ
What is the difference between the solar factor (g-value) and the shading coefficient?
The g-value (total solar energy transmittance, also expressed as SHGC) is the physically defined fraction of incident solar energy that enters the building, including directly transmitted radiation plus inward-reemitted absorbed heat. The shading coefficient is a derived comparative index equal to the g-value divided by approximately 0.87, which represents the solar factor of standard 3 mm clear glass. A lower g-value or shading coefficient indicates stronger solar heat control.
Which standards should be specified for curtain wall glass optical testing?
ISO 9050 and EN 410 are the most widely referenced international methods, while GB/T 2680 is commonly specified for projects in China. ASTM E903 and JIS R 3106 are used where American or Japanese method references are required. Because the solar spectral data sets differ slightly between standards, the report should always state the exact standard and calculation basis used.
What sample size and quantity are required for testing?
Specimens of approximately 100 mm × 100 mm per glass component are typically sufficient for spectrophotometric measurement. For an insulating glass unit, each lite must be sampled. Testing at least three specimens from different positions or production runs is recommended to account for coating uniformity variations.
Can a complete insulating glass unit be tested directly?
In most cases, no. The thickness of an assembled IGU exceeds the sample compartment of standard spectrophotometers, so the individual lites are measured separately and the complete unit performance is calculated using the layer-combination procedures defined in ISO 9050 and EN 410. The results are equivalent to those of the physical assembly for optical purposes.
Are there regulatory limits on the reflectance of curtain wall glass?
Yes. Standards such as GB/T 18091 restrict the visible light reflectance of glass curtain walls to control reflected glare and light pollution, with general limits commonly capped at approximately 0.30 and stricter requirements applied near sensitive areas such as major roads and residential zones. The current edition of the applicable local standard should always be verified during design.
Why might two laboratories report slightly different g-values for the same product?
Small deviations can arise from differences in the reference solar spectrum, spectral measurement interval, surface heat transfer coefficients, calculation rounding, and whether values are reported for individual lites or complete IGU assemblies. Requiring all laboratories to cite the same standard and calculation basis minimizes such discrepancies.