Photovoltaic glass light transmittance determination testing quantifies how effectively solar cover glass transmits sunlight across the visible and near-infrared spectrum, a property that directly determines photovoltaic module power output and long-term energy yield. This article explains the applicable standards such as ISO 9050, GB/T 2680, and ASTM E424, the typical sample types, testing items, laboratory procedures, and practical applications of transmittance measurement for PV glass.
Technical Overview
Photovoltaic glass is the optical window of a solar module, and its transmittance governs how much incident solar radiation actually reaches the encapsulant and solar cells. Modern front cover glass is a low-iron, high-clarity soda-lime-silica glass, usually tempered for mechanical strength, with common thicknesses of 1.6 mm, 2.0 mm, and 3.2 mm. Because standard glass absorbs light in the short-wavelength region due to iron content and reflections at the two air-glass interfaces, manufacturers reduce iron oxide content, apply anti-reflective (AR) coatings, or texture the surface to maximize transmitted photon flux.
Spectral transmittance is defined as the ratio of spectral radiant flux transmitted through the specimen to the spectral radiant flux incident upon it, expressed as a function of wavelength. Raw spectral data alone are not sufficient for engineering evaluation, so the transmittance determination test converts measured spectra into weighted values such as luminous transmittance (visible light transmittance) and solar direct transmittance, calculated by weighting with the D65 standard illuminant, the photopic response of the human eye, and the AM1.5 solar spectral irradiance distribution as specified in ISO 9050 and GB/T 2680. Typical uncoated low-iron tempered glass achieves approximately 91% to 91.8% visible transmittance, while AR-coated glass can reach 93% to 96% peak transmittance in the cell-responsive wavelength band of roughly 380 nm to 1100 nm.
Testing Samples
Low-Iron Tempered Front Cover Glass
Clear, ultra-clear tempered glass samples in production thickness are tested to verify base transmittance, iron-content uniformity, and consistency between production batches. Specimens are typically cut to dimensions compatible with the spectrophotometer aperture, commonly 50 mm × 50 mm or 100 mm × 100 mm coupons.
Anti-Reflective (AR) Coated Glass
AR-coated samples are tested to quantify the transmittance gain delivered by the interference coating and to evaluate transmittance retention after environmental exposure such as damp heat, thermal cycling, or abrasion, which is critical for verifying coating durability as referenced in IEC 61215 and IEC 61730 module qualification sequences.
Textured and Patterned Glass
Glass with prismatic, inverted-pyramid, or matte surface textures scatters transmitted light and traps more photons inside the module. These specimens are light-diffusing and therefore require measurement with an integrating sphere to capture both regular and diffuse transmittance components.
Rear Glass for Bifacial and Dual-Glass Modules
Rear-side glass, often with laser-etched or transparent regions and possibly transparent backsheet replacement constructions, is tested to determine the rear-side optical gain that contributes to bifacial energy yield.
Testing Items
Depending on the purpose of the evaluation, a typical photovoltaic glass transmittance determination program includes the following items:
- Spectral transmittance curve from 300 nm to 2500 nm (or at minimum 300 nm to 1200 nm covering the silicon cell response range).
- Luminous (visible light) transmittance weighted with the D65 illuminant and photopic observer function.
- Solar direct transmittance weighted with the AM1.5 global solar spectral irradiance distribution.
- Effective transmittance for module design, evaluated in the wavelength band relevant to the encapsulant and cell technology.
- Haze, characterizing the diffuse-to-total transmitted light ratio for textured surfaces.
- Transmittance retention after damp heat, thermal cycling, UV exposure, or abrasion testing of AR coatings.
- Transmittance uniformity mapping across the glass surface to detect coating or composition inhomogeneity.
Testing Methods
Sample Preparation and Conditioning
Specimens are conditioned at 23 °C ± 2 °C and 50% ± 10% relative humidity for a defined equilibrium period before measurement. Both surfaces are cleaned with anhydrous ethanol and lint-free wipes to remove dust, fingerprints, and processing residues, since surface contamination can depress the measured transmittance by several tenths of a percent, which is significant relative to the acceptance thresholds used in the industry.
Spectral Transmittance Measurement
Measurements are performed with a double-beam UV-Vis-NIR spectrophotometer in accordance with ISO 9050, GB/T 2680, or ASTM E424. Clear specimens may be measured in the normal-incidence configuration. Textured, patterned, or light-scattering specimens must be measured using an integrating sphere, typically 150 mm in diameter, positioned so that the specimen port captures all forward-scattered radiation within the hemispherical solid angle.
Weighted Luminous and Solar Transmittance Calculation
The measured spectral transmittance values are numerically integrated using defined weighting functions. Luminous transmittance is calculated over 380 nm to 780 nm using the CIE photopic response and D65 illuminant, while solar direct transmittance is calculated over approximately 300 nm to 2500 nm using the reference solar spectral irradiance. The results are reported to 0.1% resolution together with the measurement wavelength interval, which is normally 5 nm or 10 nm.
Haze Measurement
Haze is determined according to ASTM D1003 using a hazemeter or spectrophotometer with an integrating sphere, comparing total transmittance with regular (specular) transmittance after exclusion of diffuse components by a light trap. For PV glass, moderate haze values often indicate beneficial light-trapping texture rather than a defect, so haze results are interpreted together with total transmittance.
Quality Assurance of Results
Laboratories verify instrument performance before each session using calibrated reference transmittance standards traceable to national metrology institutes. Baseline and dark corrections are recorded, and replicate measurements at multiple positions on each specimen are averaged to account for spatial non-uniformity. Measurement uncertainty, typically on the order of ±0.3% to ±0.5% absolute transmittance, is reported with the final result. These procedures are routinely performed by accredited third-party laboratories within ISO/IEC 17025-accredited scopes.
Applications
Transmittance determination testing supports the photovoltaic industry at multiple stages of the product lifecycle:
- Incoming quality control at module manufacturers, where glass lots are accepted or rejected against minimum transmittance specifications such as 91.5% for uncoated glass.
- Verification and rating of AR coating performance, including transmittance gain relative to uncoated reference glass from the same production run.
- Durability evaluation, where transmittance is re-measured after damp heat, humidity-freeze, UV, or abrasion conditioning to demonstrate coating robustness in support of IEC 61215 and IEC 61730 qualification.
- Optical input data for module power simulation and yield modeling, where every percentage point of weighted transmittance translates directly into module short-circuit current and rated power.
- Research and development of new glass compositions, coating chemistries, surface textures, and lightweight thin-glass designs for high-efficiency and bifacial module architectures.
FAQ
What is the typical visible transmittance of PV cover glass?
Uncoated low-iron tempered glass of 3.2 mm thickness typically exhibits approximately 91% to 91.8% visible transmittance. High-quality AR-coated glass reaches approximately 93% to 94% weighted transmittance, with peak spectral values up to 96% in the cell-responsive wavelength band.
Which standards govern PV glass transmittance testing?
The principal standards are ISO 9050 and GB/T 2680 for spectrophotometric determination of luminous and solar transmittance of glass, ASTM E424 for solar energy transmittance of sheet materials, ASTM D1003 for haze, and EN 410 for luminous and solar characteristics of building glass. IEC 61215 and IEC 61730 reference optical performance indirectly through module power and safety qualification.
Why is an integrating sphere required for textured glass?
Textured surfaces scatter transmitted light over a wide angular range. A normal-incidence detector would intercept only the regular component and underestimate total transmittance. An integrating sphere collects the entire forward-scattered flux, producing a physically correct total transmittance value.
How does glass transmittance affect module power output?
Module short-circuit current is approximately proportional to the photon flux reaching the cells. A transmittance improvement of one percentage point typically yields a module power gain on the order of 0.4% to 0.6%, which is why AR coatings delivering roughly 2% to 3% additional transmittance are standard in high-efficiency modules.
What sample size and quantity are needed for testing?
Coupons of approximately 50 mm × 50 mm to 100 mm × 100 mm fit standard spectrophotometer apertures. Laboratories generally request at least three specimens per glass type to characterize batch uniformity and to report a statistically meaningful mean value.
How is AR coating durability verified?
Coated specimens are subjected to environmental conditioning such as damp heat at 85 °C and 85% relative humidity, thermal cycling, or standardized abrasion, after which transmittance is re-measured. A common acceptance criterion requires that weighted transmittance retention remain above a defined threshold, for example no more than a 1% absolute decrease relative to the initial value.