Determination of Light Transmittance of Photovoltaic Module Glass: Testing Standards, Methods, and T

Light transmittance of photovoltaic module glass is a critical optical parameter that directly determines how much solar radiation reaches the encapsulated solar cells and, consequently, the power output of the module. This article provides a comprehensive technical guide to the determination of light transmittance in PV module glass, covering applicable international standards such as ISO 9050, ASTM E424, and GB/T 2680, sample requirements, measured parameters, spectrophotometric test methods, and typical industrial applications.

Technical Overview

Light transmittance is defined as the ratio of the luminous or solar radiant flux passing through a glass specimen to the incident flux striking its surface, expressed as a percentage. In crystalline silicon and thin-film photovoltaic modules, the front cover glass is the first optical interface in the light path, and any absorption, reflection, or scattering within the glass reduces the photon flux available for conversion into electrical energy. As a general rule of thumb in module design, every 1% gain in effective transmittance of the front glass translates into approximately 0.4% to 0.5% relative gain in module power output, which makes transmittance one of the most economically significant optical properties in PV manufacturing.

Modern PV modules typically use low-iron, ultra-clear tempered soda-lime-silica glass with a standard thickness of 3.2 mm (with thinner 2.8 mm and 2.0 mm variants increasingly common in lightweight designs). The low iron content, typically below 0.015% Fe₂O₃, minimizes the characteristic green tint and broadband absorption of ordinary float glass, allowing uncoated transmittance values around 91.5% at 550 nm. Anti-reflective (AR) coatings based on porous silica nanostructures further reduce Fresnel reflection losses and can raise effective transmittance to 93%–96%.

Transmittance determination is governed by several internationally recognized standards, including ISO 9050 (Glass in building — Determination of light transmittance, solar direct transmittance, total solar energy transmittance and ultraviolet transmittance), its Chinese counterpart GB/T 2680, ASTM E424 (Standard Test Methods for Solar Energy Transmittance and Reflectance of Sheet Materials), and ASTM D1003 for luminous transmittance and haze. Spectral weighting is performed against the AM1.5 global reference solar spectrum defined in ISO 9845 and ASTM G173. At the module level, IEC 61215 and IEC 61730 qualification sequences also rely on transmittance measurements to evaluate optical degradation of cover materials after damp heat, UV preconditioning, and other aging tests.

Testing Samples

Uncoated Tempered Front Glass

Standard ultra-clear tempered glass coupons, typically cut to dimensions of 50 mm × 50 mm or 100 mm × 100 mm to fit standard spectrophotometer sample compartments. Specimens must be free of edge chips, surface contamination, and handling scratches, and the tempering process must be completed before measurement because thermal treatment slightly alters the glass surface condition.

AR-Coated Glass

Glass specimens carrying single-layer or multilayer anti-reflective coatings. Because AR coatings are wavelength-dependent and can exhibit non-uniformity across large glass sheets, samples should be taken from multiple positions (center, corners, and edges) of the production sheet to assess coating uniformity and to detect coating defects or incomplete coverage.

Textured and Patterned Glass

Glass with embossed or rolled surface textures designed to trap light inside the module. Scattering surfaces require measurement with an integrating sphere to capture both direct and diffuse transmitted components, ensuring that total transmittance rather than only specular transmittance is characterized.

Aged and Weathered Specimens

Glass samples extracted before and after reliability test sequences such as damp heat exposure (85 °C / 85% relative humidity), UV preconditioning (15 kWh/m² total UV dose), thermal cycling, and humidity-freeze cycling per IEC 61215. Comparative transmittance measurements quantify optical degradation such as AR coating erosion, sodium migration haze, or surface corrosion.

Sample Preparation and Conditioning

• Clean both surfaces with anhydrous ethanol or isopropyl alcohol using lint-free optical wipes, then allow solvents to fully evaporate.
• Condition specimens at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 4 hours prior to testing.
• Mark the measurement area and orientation so that repeated and post-aging measurements are taken at identical positions.
• Record glass thickness, coating type, batch number, and surface condition as part of the test documentation.

Testing Items

The scope of a transmittance determination program typically includes the following measurable parameters:

• Spectral transmittance τ(λ): the fundamental measurement, normally acquired from 300 nm to 1200 nm for crystalline silicon applications, or extended to 250–2500 nm when full solar-energy characterization is required.
• Transmittance at 550 nm: a widely quoted single-wavelength acceptance parameter for production quality control, with typical specification limits of ≥91.5% for uncoated glass and ≥93% for AR-coated glass.
• Visible light transmittance (T_vis): photopically weighted transmittance over the 380–780 nm range using the CIE standard illuminant D65 and the CIE photopic observer function V(λ).
• Solar-weighted transmittance (T_sol): spectrally integrated transmittance weighted by the AM1.5 global reference spectrum (ISO 9845 / ASTM G173), representing the photon flux relevant to real outdoor energy production.
• PV-effective transmittance: transmittance weighted by the external quantum efficiency or spectral response of a specific cell technology, providing the most accurate input for module power simulation.
• Ultraviolet transmittance (300–380 nm): relevant both for assessing UV-induced degradation risk in EVA or POE encapsulants and for durability evaluation of AR coatings.
• Haze: the ratio of diffuse to total transmitted luminous flux, determined per ASTM D1003 or ISO 14782; excessive haze indicates coating degradation or surface corrosion.
• Transmittance uniformity: point-to-point variation across the glass sheet, important for verifying coating deposition quality.

Testing Methods

Spectrophotometric Measurement Procedure

The primary method employs a double-beam UV-Vis-NIR spectrophotometer equipped with a 150 mm or larger integrating sphere for total (specular plus diffuse) transmittance measurement:

• Establish a 100% transmittance baseline using an empty sample port and a 0% dark reference as specified by the instrument procedure.
• Verify system performance with certified reference materials such as neutral-density filters or known glass standards.
• Mount the specimen at the sample port with the coated or exterior surface facing the incident beam at normal (0°) incidence.
• Acquire spectra using a spectral bandwidth of 2–5 nm and an appropriate scan speed, ensuring wavelength accuracy better than ±1 nm and photometric accuracy better than ±0.5%.
• Perform a minimum of three measurements at different positions on each specimen and report the arithmetic mean together with the standard deviation.

Calculation of Weighted Transmittance

Weighted transmittance values are computed from the measured spectral data by numerical integration over defined wavelength limits, using the weighting function appropriate to the parameter. Visible transmittance applies the product of the D65 spectral distribution and the photopic response; solar transmittance applies the tabulated AM1.5 global spectral irradiance; PV-effective transmittance applies the measured spectral response of the target cell. The integrated result is normalized by the integral of the weighting function itself, yielding a dimensionless percentage directly comparable against specification limits.

Haze Determination

Haze is measured per ASTM D1003 using a hazemeter or integrating-sphere spectrophotometer by comparing total transmittance against the diffuse component obtained with a light trap. Four readings with 90° specimen rotations are averaged to compensate for any directional texture effects. Typical haze specification limits for PV glass are below 1.5% for smooth surfaces, while deliberately textured glass may exhibit higher diffuse fractions that must be characterized and reported separately.

Measurement Uncertainty and Quality Control

• Principal uncertainty contributors include wavelength accuracy, photometric linearity, stray light in the UV region, specimen positioning, and surface cleanliness.
• Participate in interlaboratory comparisons and use certified reference materials for periodic instrument verification.
• Achievable combined measurement uncertainty for transmittance is typically on the order of ±0.3% to ±0.5% absolute, which must be considered when setting acceptance limits.

Applications

• Incoming quality control: module manufacturers verify glass transmittance against supplier specifications before lamination, preventing yield and power-class losses caused by substandard raw material.
• Coating development and qualification: AR coating developers use spectral transmittance and haze data to optimize coating thickness, porosity, and durability performance.
• Reliability and certification testing: accredited third-party laboratories measure transmittance before and after IEC 61215 aging sequences to demonstrate long-term optical stability of cover glass and coatings.
• Energy yield simulation: PV-effective transmittance data feeds optical loss models used in module power rating, bankable energy assessments, and plant performance modeling.
• Bifacial module development: rear-side glass transmittance characterization supports optimization of bifacial energy gain, where rear covers must balance transmittance against mechanical strength.
• Supplier auditing and comparative benchmarking: standardized transmittance data enables objective comparison of glass products from different suppliers on an equal technical basis.

FAQ

What transmittance value should PV module glass achieve?

Uncoated ultra-clear tempered glass typically exhibits approximately 91.5% transmittance at 550 nm, while high-quality AR-coated glass reaches 93%–96%. Solar-weighted transmittance over the 300–1200 nm band is generally specified at ≥91% for uncoated and ≥93.5% for coated products, with exact limits set by the purchaser's technical specification.

Which standard should be selected for testing?

ISO 9050 and GB/T 2680 are the most commonly applied methods for solar and visible transmittance of glass, while ASTM E424 is preferred in North American supply chains and ASTM D1003 governs haze. All methods are spectrally based and yield comparable results when identical integration ranges and weighting functions are used.

Can transmittance be measured on a finished module?

Direct transmittance measurement requires an isolated glass specimen or an unencapsulated glass coupon, because the encapsulant, cells, and backsheet in a laminated module make in-situ optical separation impractical. For finished products, glass coupons processed alongside the module, or destructive extraction of the cover glass, are the accepted approaches.

Why do 550 nm values and solar-weighted values differ?

The single-wavelength value describes transmittance at one point of the spectrum, whereas the solar-weighted value integrates the entire response-relevant wavelength band, including the lower-transmittance blue and near-infrared regions. The solar-weighted figure is the more meaningful predictor of module current output, while the 550 nm value remains popular for rapid production screening.

How does AR coating degrade and how is this detected?

Porous silica AR coatings can erode under damp heat, UV exposure, and abrasive cleaning, reducing transmittance and increasing haze. Comparative measurements before and after standardized aging tests, combined with haze determination, provide a quantitative basis for evaluating coating durability in accordance with IEC 61215 test sequences.

What sample size is required for testing?

Specimens of approximately 50 mm × 50 mm are sufficient for standard spectrophotometer measurement, although larger coupons up to 100 mm × 100 mm allow multi-point uniformity assessment. Minimum documentation should include glass thickness, coating configuration, and sampling location on the production sheet.