Plastic Light Transmittance Analysis: Standards, Test Methods, and Applications

Plastic light transmittance analysis quantifies the amount of visible light that passes through transparent and translucent plastic materials, making it a critical quality parameter for automotive glazing, optical components, packaging, and LED lighting products. This article explains the key optical properties involved, the governing international standards such as ASTM D1003, ISO 13468, and GB/T 2410, the accepted test methods, and practical considerations for sample preparation and result interpretation.

Technical Overview

Light transmittance, also referred to as luminous transmittance, is defined as the ratio of the luminous flux transmitted through a specimen to the luminous flux incident upon it, expressed as a percentage. The measurement is weighted according to the photopic response of the human eye over the visible wavelength range of approximately 380 to 780 nanometers, typically using the CIE Standard Illuminant A with the CIE 1931 standard colorimetric observer (2-degree field of view). Together with haze and clarity, transmittance forms the core set of optical quality indicators for transparent plastics.

The light transmission behavior of a plastic is governed primarily by its molecular structure. Amorphous polymers such as polymethyl methacrylate (PMMA), polycarbonate (PC), and general-purpose polystyrene (PS) allow light to pass with minimal scattering, while semi-crystalline polymers such as polyethylene, polypropylene, and polyamide scatter light at crystallite boundaries and generally appear translucent or opaque unless their crystallinity is carefully controlled or the specimen is very thin. Additives, colorants, surface texture, internal stress, degradation products, and specimen thickness all influence the measured transmittance value.

Typical total light transmittance values for common transparent plastics are as follows:

• PMMA (acrylic): approximately 92 to 93 percent, comparable to optical glass
• Amorphous polycarbonate (PC): approximately 86 to 90 percent
• General-purpose polystyrene (PS): approximately 88 to 92 percent
• Amorphous PET: approximately 88 to 90 percent
• Rigid clear PVC: approximately 86 to 90 percent
• Cyclic olefin copolymer (COC/COP): approximately 91 percent

Transmittance analysis is essential for material qualification, incoming inspection, production quality control, failure investigation, and the verification of optical performance after processing or environmental aging.

Testing Samples

Specimen selection and preparation directly affect the accuracy and comparability of transmittance results. Accredited third-party laboratories generally accept the following sample forms:

Films and Sheets

Flat film or sheet samples with a minimum test area of approximately 50 mm by 50 mm are ideal. Specimens must be free of wrinkles, creases, scratches, fingerprints, and embedded contamination, and the nominal thickness must be reported because transmittance decreases as thickness increases.

Molded Plaques and Plates

Granular resins cannot be tested directly and must first be converted into defined plaques by injection molding or compression molding in accordance with the relevant material standard, for example ISO 293 or ISO 294, at the specified thickness (commonly 1 mm, 2 mm, or 3 mm).

Finished Parts and Optical Components

Lenses, display windows, covers, and glazing panels can often be tested as received, provided the test area is sufficiently flat and uniform. Curved or textured surfaces may require special fixtures or may only permit comparative results.

Conditioning Requirements

Unless otherwise specified, specimens should be conditioned at 23 ± 2 °C and 50 ± 10 percent relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618 before measurement.

Testing Items

A comprehensive plastic optical analysis program typically includes the following measurement items:

• Total luminous transmittance (Tt): the overall percentage of incident visible light passing through the specimen
• Haze: the percentage of transmitted light scattered more than 2.5 degrees from the incident beam direction, quantifying milkiness and loss of contrast
• Clarity: the ability to resolve fine detail through the specimen, related to narrow-angle forward scattering
• Spectral transmittance curve: transmittance as a function of wavelength across the ultraviolet and visible regions, typically 200 to 800 nm
• Yellowness index and color coordinates: determined in accordance with ASTM E313 or ISO 11664 to quantify discoloration and color shift
• Ultraviolet cutoff wavelength: relevant for UV-blocking grades used in eyewear, glazing, and packaging
• Transmittance retention after weathering: comparative measurement before and after xenon-arc or UV fluorescence exposure per ISO 4892 or ASTM G154

Testing Methods

Two principal instrumental approaches are used, both of which are routinely performed by accredited third-party laboratories.

Hazemeter Method (ASTM D1003 Method A)

A hazemeter uses a collimated light beam, a CIE Standard Illuminant A source, and an integrating sphere photometer. A four-step measurement sequence (incident flux, total transmitted flux, and two scatter corrections) yields both total transmittance and haze in a single rapid test. This method is the industry workhorse for quality control of transparent plastics and is harmonized with GB/T 2410.

Spectrophotometer Method (ASTM D1003 Method B, ISO 13468-2)

A double-beam UV-Vis spectrophotometer equipped with an integrating sphere measures spectral transmittance at each wavelength, from which photopically weighted total transmittance is calculated. ISO 13468-1 covers the single-beam procedure and ISO 13468-2 covers the double-beam procedure, while ISO 14782 specifies the determination of haze for transparent materials. This approach also enables color, yellowness index, and UV cutoff to be derived from the same spectral dataset.

Key Applicable Standards

• ASTM D1003: Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics
• ISO 13468-1 and ISO 13468-2: Determination of the total luminous transmittance of transparent materials
• ISO 14782: Determination of haze for transparent materials
• GB/T 2410: Chinese national standard for light transmittance and haze of transparent plastics
• JIS K 7361-1, JIS K 7361-2, and JIS K 7136: Japanese industrial adoptions of the corresponding ISO methods

Instrument calibration is verified using traceable reference standards, and results are reported together with specimen thickness, illuminant, observer angle, and conditioning state. Measurement repeatability of better than ±0.5 transmittance units is normally achievable on clean, flat specimens.

Applications

Light transmittance analysis supports product development and compliance verification across a wide range of industries:

• Automotive: headlamp lenses, glazing, instrument cluster covers, and panoramic roof panels, where regulatory minimum light transmission and haze limits apply
• Displays and optics: touch panel covers, light guide plates, prismatic films, and window components for consumer electronics
• Lighting: LED diffusers and lenses engineered to balance total transmittance against hiding power and glare control
• Packaging: blister films, bottles, and window films where product visibility and shelf presentation are critical
• Medical devices: transparent IV components, syringe barrels, and diagnostic consumables requiring visual fluid inspection
• Construction and agriculture: greenhouse films, polycarbonate sheets, and transparent roofing evaluated for daylight transmission and long-term weathering resistance
• Material development: comparison of resin grades, anti-fog and anti-reflective coatings, and optical silicones or hybrid polymers

FAQ

What is the difference between light transmittance and haze?

Transmittance measures how much visible light passes through the specimen, while haze measures how much of that transmitted light is scattered away from the direct beam. A material can have high transmittance yet high haze, meaning it lets light through but blurs the view through it.

Which standard should I choose, ASTM D1003 or ISO 13468?

Both methods produce photopically weighted total transmittance values that are broadly comparable. ASTM D1003 is widely specified in North American supply chains and yields haze in the same measurement, whereas ISO 13468-2 is preferred in international and European contexts and offers robust correction for instrument scatter. The choice usually follows the specification invoked by the end customer or product regulation.

How does specimen thickness affect the result?

Transmittance decreases approximately exponentially with thickness due to bulk absorption and scattering, consistent with the Beer-Lambert relationship. Results are only comparable between specimens of identical nominal thickness, which is why thickness must always accompany the reported value.

Can finished parts with curved surfaces be tested?

Flat test areas are strongly preferred. Mildly curved lenses can often be measured with appropriate aperturing, but strongly curved, textured, or embossed surfaces can produce artificially low transmittance and elevated haze, so comparative testing against a reference part is recommended in such cases.

Why measure transmittance after weathering?

UV exposure, heat, and humidity can cause yellowing, haze development, and loss of transmission in polymers such as PC and PET. Comparing transmittance and yellowness index before and after accelerated weathering per ISO 4892 or ASTM G154 is a standard way to validate UV stabilizer packages and predict service life.

How quickly can results be obtained?

Standard transmittance and haze measurement is a rapid, non-destructive test, and most accredited third-party laboratories can report results within a few working days once properly prepared and conditioned specimens are received.