PC Sheet Light Transmittance Testing: Standards, Methods, and Technical Guidelines

Polycarbonate (PC) sheet light transmittance testing is a critical quality control procedure that measures how efficiently transparent PC glazing materials transmit visible light. This technical article explains the fundamental principles of luminous transmittance and haze measurement, the applicable international standards such as ASTM D1003 and ISO 13468, specimen preparation requirements, and typical performance values for solid, multiwall, corrugated, and diffuser-type PC sheets.

Technical Overview

Polycarbonate (PC) sheets are amorphous engineering thermoplastic glazing materials widely used as lightweight, impact-resistant alternatives to glass. Because most PC sheet applications—greenhouse coverings, skylights, canopies, machine guards, and LED diffusers—depend directly on optical performance, light transmittance is one of the most important quality indices in production control, incoming inspection, and product certification.

Light transmittance is defined as the percentage of incident luminous flux within the visible spectrum (380–780 nm) that passes through a specimen. The measurement is weighted by the photopic response of the human eye, typically using a CIE D65 standard illuminant and a 2-degree or 10-degree standard observer. High-clarity solid PC sheets can achieve a total transmittance of up to 88–90%, which is comparable to glass of equivalent thickness. However, the measured value is strongly influenced by sheet thickness, resin purity, color, surface texture, additives, and structural geometry.

In addition to transmittance, two related optical parameters are routinely evaluated: haze, defined as the percentage of transmitted light scattered more than 2.5 degrees from the direction of the incident beam, and clarity, which describes the ability of the material to preserve image resolution. For diffuser-grade PC sheets, high haze combined with high transmittance is actually a desired functional characteristic, whereas for optical-grade glazing, low haze is essential.

Testing Samples

Solid PC Sheets

Solid PC sheets range from 1 mm to 20 mm or more in thickness. Clear solid sheets typically exhibit total transmittance values of 88–90% for standard thicknesses between 2 mm and 6 mm. Due to bulk absorption described by the Beer–Lambert relationship, transmittance decreases gradually as thickness increases, so specimen thickness must always be reported together with the measured value.

Multiwall (Hollow) PC Sheets

Multiwall sheets—from 4 mm twin-wall to 16–25 mm multi-layer X-structure or honeycomb profiles—show lower transmittance values, typically 80–88%, because light crosses multiple polymer–air interfaces and is partially blocked by internal ribs. Multiwall sheets must be tested as complete assemblies, including all walls and ribs, since light transmission of the full cross-section is what matters in actual installation.

Corrugated PC Sheets

Corrugated and profiled sheets used for roofing present a non-flat geometry. Specimens should be cut and oriented so that the measurement beam passes through a representative region of the profile, and the direction of the corrugation relative to the beam must be documented. Larger apertures and repeated measurements at multiple positions are recommended to account for geometric variations.

Embossed and Diffuser PC Sheets

Embossed, prismatic, and opal diffuser sheets are engineered for controlled light scattering. Their haze values can range from 30% to above 90%, while total transmittance typically falls between 50% and 85% depending on the pattern depth and pigment loading. Both parameters must be measured together to properly characterize diffusion performance.

Coated and Functional PC Sheets

PC sheets with co-extruded UV-protective surface layers, hard coatings, anti-condensation treatments, or infrared-blocking additives require spectral scanning across a wider wavelength range (typically 300–2500 nm) to verify that the functional coating performs as intended without unnecessarily sacrificing visible transmittance.

Testing Items

A comprehensive optical evaluation of PC sheets generally includes the following test items:

• Total luminous transmittance (380–780 nm), weighted for the photopic observer
• Haze (wide-angle scattering), reported as the ratio of diffuse to total transmittance
• Ultraviolet transmittance (200–400 nm), used to verify UV-blocking performance
• Solar transmittance (300–2500 nm), relevant for energy-efficiency and thermal calculations
• Yellowness index (YI) per ASTM E313 or ISO 17223, indicating color degradation
• Transmittance retention after artificial weathering, verifying long-term optical durability
• Transverse uniformity testing at multiple positions across the sheet width to confirm coating consistency

Testing Methods

Spectrophotometer Method

A UV-Visible spectrophotometer equipped with a integrating sphere accessory measures the spectral transmittance of the specimen at regular wavelength intervals, and the photopic total transmittance is calculated by weighting the spectral data with the CIE source and observer functions. This method, corresponding to ASTM D1003 Method B and ISO 13468, provides full spectral information and is suitable for colored, tinted, and functional coated sheets.

Hazemeter Method

A dedicated hazemeter (per ASTM D1003 Method A, ISO 14782, and JIS K 7136) measures four luminous flux readings: incident flux, total transmitted flux, instrument-scattered flux, and combined instrument-plus-specimen scattered flux. Total transmittance and haze are derived from these four readings, with haze calculated as the ratio of diffuse transmittance to total transmittance. The hazemeter method offers fast, repeatable results ideal for routine production quality control.

Specimen Preparation and Conditioning

Accurate and reproducible results depend on disciplined specimen handling:

• Condition specimens at 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours prior to testing, in accordance with ISO 291 or ASTM D618.
• Cut specimens to at least 50 mm × 50 mm so they fully cover the instrument aperture; for corrugated and textured sheets, use oversized specimens.
• Clean both surfaces with a lint-free cloth and, if necessary, a neutral detergent or isopropyl alcohol; avoid touching the optical surfaces and exclude any specimen with scratches, embedded dust, or edge defects.
• Mark the orientation (machine direction and top surface) so results remain comparable between laboratories.
• Test a minimum of three specimens per sample and report the arithmetic mean, along with thickness, color, surface finish, and structural configuration.

Reference Standards

The principal standards governing PC sheet transmittance testing include ASTM D1003 (haze and luminous transmittance of transparent plastics), ISO 13468-1 and ISO 13468-2 (total luminous transmittance by single-beam and double-beam methods), ISO 14782 and JIS K 7136 (haze of transparent materials), GB/T 2410 (Chinese national standard for transmittance and haze of transparent plastics), and JIS K 7361-1. For durability assessment, specimens are exposed to xenon-arc or fluorescent UV lamps according to ISO 4892-2, ISO 4892-3, or ASTM G155, after which transmittance retention and yellowness index change are measured. Well-manufactured UV-protected PC sheets typically retain more than 90% of their initial transmittance after extended accelerated weathering.

Applications

Transmittance data directly governs material selection across the major PC sheet application fields:

• Greenhouse and agricultural roofing: transmittance levels of 80% or higher, combined with moderate haze, deliver uniform diffused light that penetrates deep into the plant canopy and improves crop yield.
• Architectural glazing and skylights: high transmittance maximizes daylighting, while low haze preserves visual clarity for overhead and vertical glazing in stadiums, stations, and commercial buildings.
• LED lighting and luminaire diffusers: opal and prismatic PC sheets balance transmittance against diffusion to hide light sources and eliminate glare.
• Signage and advertising displays: consistent transmittance ensures uniform backlighting across large panel areas.
• Safety and machine guards, noise barriers, and transportation glazing: optical grade verification confirms that impact-resistant panels remain transparent enough for visual supervision and safety compliance.

FAQ

What is the light transmittance of clear PC sheet compared with glass?

High-quality clear solid PC sheet achieves 88–90% total transmittance at typical glazing thicknesses, which is close to ordinary clear glass at 89–91%. Advanced low-absorption PC formulations can reach up to 91–92%, while anti-reflective coated products can exceed glass in effective transmission.

Why do multiwall PC sheets measure lower than solid sheets?

Multiwall sheets contain internal ribs and multiple polymer–air interfaces that reflect, refract, and block part of the incoming light. As wall count and total thickness increase, transmittance gradually declines, which is why 10 mm and thicker multi-layer structures typically measure between 76% and 84%.

Which standard should be used for the test report?

ASTM D1003 and ISO 13468 (with ISO 14782 for haze) are the most widely accepted international references. Chinese domestic projects commonly cite GB/T 2410, and Japanese markets frequently require JIS K 7361-1 and JIS K 7136. The chosen standard, instrument type, illuminant, and observer condition must be clearly stated in the report.

Does the co-extruded UV layer reduce transmittance?

The UV-protective co-extruded layer is extremely thin, typically under 50 micrometers, and absorbs almost entirely in the ultraviolet region below 380 nm. Its influence on visible transmittance is negligible, usually less than one percentage point, while it significantly extends the optical service life of the sheet.

Does PC sheet transmittance decrease over time?

Unprotected PC gradually yellows under UV exposure, reducing both transmittance and visual clarity. Sheets with properly applied UV surface protection retain more than 90% of initial transmittance after many years of outdoor service, and accelerated weathering tests are used to verify this durability before products reach the market.

How many specimens should be tested?

At least three specimens from different positions across the sheet width should be measured, and the average value reported. For wide production sheets, additional sampling points help detect uneven co-extrusion or coating defects along the transverse direction.