Photovoltaic film transmittance testing measures how effectively encapsulant films, backsheets, and functional optical films transmit solar radiation across the UV, visible, and near-infrared spectrum. Accurate transmittance characterization directly affects module power output, encapsulant material qualification, and long-term reliability evaluation. This article explains the key testing items, applicable standards, sample requirements, instrumentation, and typical applications involved in professional PV film transmittance evaluation.
Technical Overview
Transmittance is one of the most critical optical performance parameters for photovoltaic films. Encapsulant films such as EVA and POE are positioned directly between the front glass and the solar cells, so any loss of optical transmittance immediately reduces the photon flux reaching the active junction and lowers module power output. For a typical crystalline silicon module, encapsulant films are generally expected to deliver an average transmittance above 90 percent across the cell response range, with minimal haze and stable optical properties over a 25-year service life.
Transmittance testing is not a single-point measurement. Optical performance must be evaluated as a function of wavelength, and it must be re-evaluated after environmental stress such as UV exposure, damp heat, and thermal cycling, because polymer yellowing, additive migration, and lamination-induced defects all degrade transmittance over time. Accredited third-party laboratories and in-house quality control teams therefore rely on spectrophotometric methods and internationally recognized standards to quantify both the initial and aged-state optical behavior of PV films.
Testing Samples
EVA Encapsulant Films
Ethylene-vinyl acetate copolymer films remain the most widely used encapsulants in crystalline silicon modules. Both fast-cure and conventional-cure grades are tested in the uncrosslinked state and after simulated lamination, since the curing process can alter optical clarity, UV-cut behavior, and yellowing index.
POE and EPE Encapsulant Films
Polyolefin elastomer films and EVA-POE-EVA coextruded films are increasingly used for high-efficiency modules, bifacial designs, and moisture-sensitive cell technologies such as PERC, TOPCon, and HJT. Their spectral transmittance, UV transparency, and post-aging optical retention are key qualification criteria.
PVB Films for BIPV Applications
Polyvinyl butyral films used in building-integrated photovoltaics and laminated safety glass are tested for luminous transmittance, haze, and solar transmittance to satisfy both energy-generation and architectural glazing requirements.
Fluoropolymer Front Films and Backsheet Films
Polyvinyl fluoride and polyvinylidene fluoride films, along with transparent backsheets for bifacial modules, are evaluated for spectral transmittance and weathering-related optical stability.
Functional Optical and Coating Films
Anti-reflective films, light-trapping films, UV-cut or UV-transparent films, and transparent barrier films for flexible and thin-film photovoltaics are characterized to verify wavelength-selective behavior and coating uniformity.
Testing Items
Typical transmittance-related test items for photovoltaic films include:
- Spectral transmittance distribution across the wavelength range of 280 nm to 2500 nm, covering the UV, visible, and near-infrared regions.
- Average visible light transmittance in the 380 nm to 780 nm band, used for general optical clarity assessment.
- Solar-weighted or photovoltaic-weighted transmittance calculated using the AM1.5 global reference spectrum, which reflects the actual energy-relevant transmittance of the film.
- UV transmittance in the 280 nm to 400 nm band, critical for distinguishing UV-cut films from UV-transparent films used with UV-sensitive or tandem cell architectures.
- Haze, defined as the ratio of diffuse to total transmitted light, measured in accordance with ASTM D1003 or ISO 14782.
- Yellowing index and color coordinates such as b*, which quantify polymer discoloration after thermal or UV aging.
- Transmittance retention after UV preconditioning, damp heat exposure at 85 degrees Celsius and 85 percent relative humidity, and thermal cycling.
- Transmittance verification after simulated lamination, to confirm that the curing process does not introduce cloudiness or yellowing.
Testing Methods
UV-Vis-NIR Spectrophotometry
The core method for transmittance measurement uses a ultraviolet-visible-near-infrared spectrophotometer to scan a conditioned flat specimen across the required wavelength range. The instrument records the ratio of transmitted to incident spectral irradiance at each wavelength, producing a full spectral transmittance curve. Sample conditioning is typically performed at 23 degrees Celsius and 50 percent relative humidity in accordance with ISO 291 before measurement.
Integrating Sphere Total Transmittance
Because polymer films scatter light, a bare-detector measurement would underestimate total transmittance. An integrating sphere, commonly 150 mm in diameter, collects both directly transmitted and diffusely scattered light, yielding true total transmittance in accordance with ISO 13468-1. This is essential for hazy, textured, or embossed encapsulant films.
Haze Measurement
Haze is quantified with a hazemeter or an integrating-sphere spectrophotometer by sequentially measuring total transmittance, diffuse transmittance, and instrument scattering corrections, following the procedures of ASTM D1003. Encapsulant films typically require haze values below approximately 2 percent to avoid glare and optical losses.
Solar-Weighted Transmittance Calculation
The measured spectral transmittance is mathematically weighted against the AM1.5 global solar spectral irradiance distribution defined in ASTM G173 and referenced by IEC 60904-3. The calculation method described in ISO 9050 and ASTM E424 converts the wavelength-dependent data into a single solar transmittance value that correlates with expected module photocurrent.
Aged-State Transmittance Evaluation
To support module qualification under IEC 61215 and IEC 61730, film specimens are subjected to UV preconditioning, damp heat, and humidity-freeze sequences, then re-measured. Comparing spectral transmittance before and after stress reveals optical degradation, UV absorber depletion, and polymer yellowing. Guidance for encapsulant-specific measurement procedures is provided in the IEC 62788 series of measurement standards for PV module materials.
Product-level material specifications, such as those in the GB/T 29848 standard for photovoltaic encapsulant films, define minimum transmittance requirements that manufacturers use for incoming material acceptance and batch release.
Applications
- Incoming quality control of encapsulant rolls and sheets at module manufacturing facilities, verifying that each batch meets specified transmittance and haze limits before lamination.
- New material development and formulation screening for EVA, POE, silicone, and ionomer encapsulants, including optimization of UV absorbers and antioxidants.
- Support for IEC 61215 and IEC 61730 module design qualification and type approval, where material optical stability under sequential stress testing is documented by accredited third-party laboratories.
- Bifacial module development, where transparent backsheets and dual-glass encapsulants must maintain high transmittance to capture rear-side irradiance.
- Building-integrated photovoltaics, where film and laminated glazing transmittance must simultaneously satisfy energy-yield targets and architectural daylighting codes.
- Advanced cell architectures such as perovskite-silicon tandem devices, which demand UV-transparent encapsulation to utilize the high-energy portion of the solar spectrum.
- Failure analysis and field-return investigation, comparing the transmittance of aged or discolored films against reference material to diagnose degradation mechanisms.
FAQ
What wavelength range should be scanned for photovoltaic film transmittance?
For crystalline silicon applications, scanning from 300 nm to 1200 nm covers the cell spectral response. Full solar characterization extends the range to 2500 nm, while UV-sensitive designs require reliable data starting at 280 nm.
Why is an integrating sphere necessary for film testing?
Polymer films scatter a portion of transmitted light through surface texture, crystallinity, and additive particles. An integrating sphere captures this diffuse component so that total transmittance is measured accurately rather than underestimated.
What is the difference between transmittance and haze?
Transmittance quantifies how much light passes through the film, while haze quantifies how much of the transmitted light is scattered rather than transmitted directly. A film can show high total transmittance yet still be unacceptable due to excessive haze.
What transmittance level do encapsulant films typically need?
Most specifications for EVA and POE encapsulants require average transmittance above 90 percent over the visible and near-infrared cell response range, with haze below approximately 2 percent, both before and after lamination and accelerated aging.
Which standards govern PV film transmittance testing?
Commonly referenced standards include ISO 9050, ISO 13468-1, ASTM D1003, ASTM E424, ASTM G173, the IEC 62788 series for PV module materials, IEC 61215 and IEC 61730 for module qualification, and GB/T 29848 for encapsulant film product requirements.
How does aging affect film transmittance results?
UV radiation, heat, and moisture can consume UV absorbers, generate chromophores, and cause polymer yellowing, all of which reduce visible transmittance and shift the UV cutoff. Measuring transmittance retention after standardized aging sequences is therefore essential for lifetime prediction.