Polycarbonate (PC) is one of the most widely used transparent engineering plastics, and accurate light transmittance measurement is essential for validating its optical performance in automotive, lighting, and optical applications. This article provides a comprehensive technical guide to PC plastic light transmittance testing, covering testing standards such as ASTM D1003 and ISO 13468, sample preparation, instrumentation, calculation methods, and typical application requirements. Whether you are a quality engineer, materials scientist, or product designer, this guide will help you understand how transmittance data for PC materials is generated, interpreted, and controlled.
Technical Overview
Polycarbonate (PC) is an amorphous engineering thermoplastic recognized for its exceptional combination of optical clarity, impact resistance, and dimensional stability. Optical-grade PC can achieve a total luminous transmittance of 88% to 92% for thin sections, approaching the performance of glass while offering significantly lower weight and higher fracture resistance. Because transmittance directly determines the suitability of PC for glazing, lens, and lighting applications, precise and repeatable measurement is a critical element of incoming material inspection, production quality control, and product certification.
Light transmittance, also referred to as luminous transmittance, is defined as the ratio of the luminous flux passing through a specimen to the luminous flux incident upon it, expressed as a percentage. The measurement is performed over the visible spectral range (380 nm to 780 nm) and weighted according to the photopic response of the human eye, typically using CIE Standard Illuminant D65 and either the 2-degree or 10-degree standard observer. For transparent materials such as PC, the measured transmittance is influenced by bulk absorption, surface reflections, molecular structure, and any scattering caused by additives, degradation products, or processing defects.
Why Transmittance Testing Matters for PC
PC materials undergo thermal and mechanical stress during extrusion, injection molding, and thermoforming. Improper drying, excessive melt temperatures, or long residence times can cause yellowing and a measurable drop in transmittance. Additionally, UV stabilizers, mold release agents, colorants, and recycled content all reduce optical clarity in measurable ways. Transmittance testing therefore serves three key purposes:
- Verifying that raw resin and finished parts meet optical specifications and customer drawings.
- Detecting processing degradation, contamination, or formulation errors before parts reach the field.
- Quantifying the effects of UV weathering, thermal aging, and accelerated life testing on long-term optical performance.
Testing Samples
Sample selection and preparation have a direct impact on transmittance results. Because transmittance decreases with increasing thickness and surface quality affects both regular and diffuse components, specimens must be clearly defined, documented, and consistently prepared.
Extruded and Cast PC Sheets
Flat PC sheets and films are the most straightforward specimens to test. Samples should be cut to the size required by the instrument aperture, commonly 50 mm × 50 mm squares or 50 mm diameter discs. Edges do not affect the measurement, but both surfaces must be free of scratches, fingerprints, dust, and embedded debris. Common reference thicknesses are 1 mm, 2 mm, and 3 mm, and the tested thickness must always be reported alongside the transmittance value.
Injection-Molded Plaques and Finished Parts
For molded components such as lamp lenses, light guides, and transparent covers, testing is typically performed on standardized plaques molded under production-representative conditions, or directly on flat regions of the finished part. Curved or textured surfaces introduce measurement error because they redirect light away from the detection geometry; therefore, only flat, polished areas should be measured, or dedicated flat plaques should be molded for qualification purposes.
Coated and Weathered Specimens
PC products frequently carry hard coatings, anti-reflective coatings, or UV-protective layers, each of which modifies transmittance. Coated specimens should be tested in the as-delivered condition to reflect real-world performance. For weathering studies, specimens are exposed to xenon-arc or UV fluorescent lamps per ASTM G155 or ISO 4892, then periodically removed for transmittance measurement to track degradation over time.
Sample Conditioning
All specimens should be conditioned prior to testing in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours, in accordance with ASTM D618 or ISO 291. Immediately before measurement, both surfaces should be cleaned with a lint-free cloth and, if necessary, optical-grade isopropyl alcohol, followed by handling with gloves to prevent contamination.
Testing Items
A complete optical characterization of PC typically includes the following measured and derived quantities:
- Total luminous transmittance (Tt): The percentage of incident visible light that passes through the specimen in all directions, combining direct and scattered light. This is the primary specification value for transparent PC.
- Regular (specular) transmittance: The portion of transmitted light that passes through without deviation from the original optical path, which is critical for lens and glazing applications.
- Diffuse transmittance (Td): The scattered portion of transmitted light, calculated as the difference between total and regular transmittance.
- Haze: The percentage of transmitted light that is scattered more than 2.5 degrees from the incident beam direction, calculated as Td divided by Tt and multiplied by 100. Haze is essential for evaluating light-diffusing PC grades used in LED luminaires.
- Yellowness index (YI): Often measured in the same session per ASTM E313, yellowness index correlates with thermal or UV degradation and frequently accompanies transmittance reporting.
Testing Methods
Two internationally recognized standards govern the measurement of luminous transmittance in transparent plastics: ASTM D1003 and ISO 13468. Both rely on an integrating sphere photometer or spectrophotometer to capture all forward-transmitted light.
ASTM D1003 Method
ASTM D1003, Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, defines two techniques. Method A uses a dedicated hazemeter with a broadband light source and filters that approximate the photopic response of the eye. Method B uses a scanning spectrophotometer equipped with an integrating sphere, measuring transmittance wavelength by wavelength across the visible spectrum before computing the luminous value. Method A is faster and well suited to routine production control, while Method B provides full spectral information useful for research and failure analysis. The standard specifies CIE Illuminant D65 with the 2-degree standard observer for Method B calculations.
ISO 13468 Method
ISO 13468, Plastics — Determination of the total luminous transmittance of transparent materials, is specified in two parts. Part 1 describes the single-beam method, in which measurements with and without the specimen are taken sequentially and any instrument drift is corrected through a second baseline check. Part 2 describes the dual-beam method, which uses a reference beam to compensate continuously for source fluctuations, offering superior precision for high-transmittance materials such as PC. ISO 14782 is the companion standard for haze determination when both properties are required.
Instrumentation and Measurement Sequence
The core instrument consists of a stable light source, a specimen port coupled to an integrating sphere coated with a highly reflective diffuse material, a photodetector, and a light trap for excluding regular transmitted light when required. A typical measurement sequence following ASTM D1003 involves four readings:
- T1 — baseline reading with no specimen installed and no light trap (defining 100% transmission).
- T2 — total transmittance reading with the specimen installed and all transmitted light collected.
- T3 — reading with the specimen installed and the light trap engaged, capturing diffuse light only.
- T4 — instrument reading with the light trap engaged and no specimen, used to correct for sphere wall effects.
Calculation of Results
The measured readings are converted into reported optical properties using the following relationships:
- Total luminous transmittance: Tt = 100 × T2 / T1
- Diffuse luminous transmittance: Td = 100 × (T2 − T3) / (T1 − T4)
- Haze = 100 × Td / Tt
- Regular transmittance = Tt − Td
Each specimen should be measured at a minimum of three positions, and the average value reported. Well-maintained instruments operated by trained personnel typically achieve repeatability within ±0.3% and reproducibility within ±1.0% between laboratories for clear PC. Instrument calibration should be verified at least daily using certified reference standards traceable to national metrology institutes.
Factors Influencing Measured Transmittance
- Thickness: Transmittance declines predictably as thickness increases due to bulk absorption; comparing values requires matching thicknesses.
- Surface condition: Scratches, texture, and dust convert regular transmittance into diffuse transmittance and inflate haze.
- Moisture and processing history: Hydrolytic or thermal degradation during molding causes yellowing and reduced transmittance.
- Additives: UV absorbers, flame retardants, and impact modifiers measurably reduce clarity depending on loading and dispersion quality.
- Temperature: Measurements should be performed at standard laboratory temperature, as refractive index and dimensional changes can shift results.
Applications
Transmittance data for PC materials directly supports qualification and quality control across a wide range of industries:
- Automotive: Headlamp lenses, tail lamp covers, panoramic roof panels, and side glazing require high total transmittance with low haze and long-term UV stability.
- LED and general lighting: PC diffusers, lenses, and light guides are engineered to specific combinations of transmittance and haze to balance brightness with glare control.
- Optics and eyewear: Ophthalmic lenses, safety goggles, and camera windows demand maximum regular transmittance with minimal scattering.
- Construction and agriculture: Transparent roofing, skylights, and greenhouse glazing rely on sustained high transmittance after years of outdoor exposure.
- Consumer electronics: Display windows, touch panel substrates, and transparent housings are specified with minimum transmittance thresholds.
- Medical devices: Transparent PC used in filtration housings, IV components, and incubator doors must maintain clarity after sterilization cycles such as gamma irradiation or autoclaving.
FAQ
What is the typical light transmittance of clear PC plastic?
Optical-grade PC typically exhibits a total luminous transmittance of 88% to 92% at 2 mm to 3 mm thickness. General-purpose grades generally fall between 85% and 89%. Values above 92% at these thicknesses are uncommon without anti-reflective surface treatments.
What specimen thickness should be used for testing?
The most common reference thickness is 3 mm for general qualification and 1 mm to 2 mm for optical and lighting grades. Because transmittance decreases with thickness, results are only comparable when thickness is identical. The tested thickness must always accompany the reported value.
What is the difference between transmittance and haze?
Transmittance measures how much light passes through the specimen regardless of direction, while haze measures the fraction of that transmitted light that has been scattered by more than 2.5 degrees. A diffuser can have moderate transmittance with very high haze, whereas a lens-grade PC aims for high transmittance with haze below 1%.
Which standard should I choose, ASTM D1003 or ISO 13468?
ASTM D1003 is the dominant specification in North American supply chains and simultaneously yields transmittance and haze. ISO 13468 is preferred in international and European contexts and offers a dual-beam variant with improved precision. Results from the two standards generally agree within approximately 1% when specimens and conditions are matched, but specifications should name one standard explicitly.
Why do my transmittance results vary between tests or laboratories?
Common causes include differences in specimen thickness, surface cleanliness, conditioning history, instrument calibration standards, and beam geometry. Following standardized conditioning, careful cleaning, a minimum of three measurement positions per specimen, and regular calibration verification typically reduce variability to well within ±1%.
How does UV weathering affect PC transmittance over time?
Prolonged UV exposure photo-oxidizes the polymer surface, producing yellow discoloration and micro-cracking that reduce transmittance and increase haze. UV-stabilized and hard-coated PC grades are specifically formulated to slow this process, and transmittance retention measured after accelerated weathering per ASTM G155 or ISO 4892 is a key durability acceptance criterion.