Plastic Light Transmittance Testing: Standard Procedures, Equipment, and Technical Guidelines

Light transmittance testing measures how much visible light passes through transparent plastic materials, making it a critical quality control parameter for optical, automotive, packaging, and lighting applications. This article details the complete plastic light transmittance testing procedure, including sample preparation, instrument calibration, measurement steps, and applicable international standards such as ASTM D1003, ISO 13468, and GB/T 2410. Understanding these procedures ensures accurate, repeatable results for material certification and product development.

Technical Overview

Light transmittance, also known as luminous transmittance or total transmittance (Tt), is defined as the percentage of incident luminous flux that passes through a plastic specimen. It is measured using the CIE standard illuminant A (tungsten lamp, color temperature 2856 K) or D65 in combination with the CIE standard photopic observer function, which corresponds to the spectral sensitivity of the human eye. The measurement is typically performed on a hazemeter or a UV-Vis spectrophotometer equipped with an integrating sphere.

Transmittance is closely related to, but distinct from, haze. While transmittance quantifies the total light that passes through the material, haze measures the percentage of transmitted light that is scattered by more than 2.5 degrees from the incident beam direction. Both parameters are often determined simultaneously in a single test run, as required by ASTM D1003 and GB/T 2410. Typical transparent plastics such as polymethyl methacrylate (PMMA) can achieve transmittance values above 92%, approaching that of optical glass, while polycarbonate (PC) typically reaches 87-90%.

The principal standards governing this test include 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), GB/T 2410 (Chinese national standard for transmittance and haze of transparent plastics), and JIS K 7361-1. Accredited third-party laboratories routinely conduct these tests for material qualification and regulatory compliance.

Testing Samples

Rigid Transparent Sheets

Flat specimens cut from PMMA, PC, PS, SAN, PETG, or transparent PVC sheets. The recommended specimen size is 50 mm x 50 mm square or a circular disc of at least 40 mm diameter to fully cover the transmission port of the integrating sphere. Specimen thickness should be uniform and representative of the final product, typically ranging from 1 mm to 10 mm.

Plastic Films

Flexible films such as PET, BOPP, PE, and cellulose acetate films used in packaging and optical applications. Films must be mounted flat without wrinkles or air bubbles, as mechanical stress and creases artificially increase light scattering and distort haze readings. Thin films are often mounted on a rigid frame to maintain flatness.

Molded and Injection-Molded Parts

Finished components such as lighting diffusers, instrument lenses, protective covers, and display windows. Specimens must be sectioned so that a flat, representative area can be presented to the measurement port. Curved or textured surfaces require special reporting considerations, as curvature can introduce measurement bias.

Colored and Translucent Materials

Tinted or translucent plastics can also be measured, but spectral transmittance distribution should be recorded in addition to the total transmittance value, because heavily colored materials may exhibit strong wavelength dependence. Translucent specimens that scatter significant light may require the wide-angle correction procedures described in the standards.

Testing Items

A typical transmittance testing program includes the following measurable parameters:

• Total luminous transmittance (Tt): the ratio of total transmitted luminous flux to incident luminous flux, expressed as a percentage.
• Diffuse transmittance (Td): the portion of transmitted light scattered from the incident beam direction.
• Parallel transmittance (Tp): the unscattered portion of transmitted light, where Tp = Tt - Td.
• Haze (H): wide-angle light scattering expressed as a percentage of total transmitted light.
• Spectral transmittance curve: transmittance as a function of wavelength across the visible region (380-780 nm), useful for colored materials.
• Clarity: narrow-angle scattering affecting the sharpness of images viewed through the material.

Testing Methods

Method A: Hazemeter (ASTM D1003 Method A, GB/T 2410 Method A)

The hazemeter method uses a collimated light beam, an integrating sphere, and a regulated light trap that can be opened or closed to separate total transmittance from diffuse transmittance. The measurement sequence follows four standard readings:

• T1: Incident light flux with no specimen in place and light trap closed.
• T2: Total light flux transmitted by the specimen with the light trap closed.
• T3: Light scattered by the instrument with no specimen and the light trap open.
• T4: Light scattered by both the specimen and the instrument with the light trap open.

Total transmittance is calculated as Tt = (T2/T1) x 100%, and haze is calculated as H = [(T4/T2) - (T3/T1)] x 100%.

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

A double-beam UV-Vis spectrophotometer fitted with an integrating sphere measures spectral transmittance at each wavelength from 380 nm to 780 nm. The total luminous transmittance is obtained by weighting the spectral data with the CIE photopic luminous efficiency function V(λ) and the standard illuminant source distribution. This method provides full spectral information and is preferred when color analysis or wavelength-specific data is required.

ISO 13468-1 Integrating Sphere Method

ISO 13468-1 specifies the determination of total luminous transmittance using a single-beam or double-beam instrument with an integrating sphere, calibrated against certified reference standards rather than bare air, which improves accuracy for high-transmittance materials.

Detailed Testing Procedure Steps

Step 1: Sample Preparation

Cut at least three specimens from representative areas of the material. Verify that surfaces are clean, free of fingerprints, scratches, dust, and anti-static residues. If cleaning is necessary, wipe gently with a lint-free optical cloth and isopropyl alcohol if compatible with the plastic. Measure and record the specimen thickness at multiple points, as transmittance decreases with increasing thickness according to the Beer-Lambert relationship.

Step 2: Conditioning

Condition the specimens in a standard laboratory atmosphere of 23 ± 2°C and 50 ± 10% relative humidity for a minimum of 40 hours, in accordance with ISO 291 or ASTM D618. Conditioning stabilizes the moisture content and temperature of the specimens, both of which can influence optical properties, particularly for hygroscopic materials such as PMMA and cellulose-based plastics.

Step 3: Instrument Warm-Up and Calibration

Switch on the hazemeter or spectrophotometer and allow the light source and detectors to stabilize for the manufacturer-recommended warm-up period, typically 15 to 30 minutes. Perform the zero calibration with the light trap or beam block engaged. Then calibrate the instrument against certified reference standards with known transmittance values (for example, approximately 90% and 40% standards) that are traceable to national metrology institutes.

Step 4: Baseline Measurement

Measure the incident light flux (T1) and instrument scatter (T3) with no specimen at the transmission port, or perform the baseline scan for spectrophotometers using air as the reference. Verify baseline stability by repeating the reading until drift is within the specified tolerance, typically less than 0.1%.

Step 5: Specimen Measurement

Place the conditioned specimen directly against the transmission port, ensuring that the beam passes through the center of the specimen and that no ambient light enters the sphere. Hold the specimen firmly and flat without bending it. Record the total transmittance reading (T2) and the specimen-plus-instrument scatter reading (T4) with the light trap open. For spectrophotometers, record the full spectral scan across the visible range.

Step 6: Repeat and Average

Measure each specimen at least three times, repositioning it slightly between readings to account for local surface variations and material inhomogeneity. If multiple specimens are tested, report the arithmetic mean of all valid readings along with the standard deviation.

Step 7: Data Calculation and Reporting

Calculate Tt, Td, and haze according to the formulas in the governing standard. The test report should include the standard referenced, material identification and grade, specimen thickness, number of specimens, individual and mean transmittance and haze values, instrument model and calibration reference, test temperature and humidity, and the test date and operator identification.

Key Factors Affecting Accuracy

• Specimen thickness: thicker specimens reduce transmittance due to bulk absorption and scattering.
• Surface condition: scratches, dust, and fingerprints scatter light and inflate haze values.
• Internal structure: crystallinity, fillers, pigments, and voids increase light scattering within the material.
• Instrument state: lamp aging, dirty optics, and sphere coating degradation cause calibration drift.
• Edge effects: specimens smaller than the port or mounted with gaps allow stray light leakage.

Applications

Light transmittance testing supports quality control and material qualification across numerous industries:

• Automotive: headlamp lenses, glazing, instrument panel covers, and windshield interlayers requiring compliance with regulatory visibility requirements.
• Lighting and LED: diffusers, light guides, and lamp covers where a defined balance of transmittance and haze controls illumination quality.
• Building and construction: transparent roofing sheets, skylights, and window films evaluated for daylighting performance.
• Packaging: transparent films and containers where product visibility influences consumer acceptance and barcode scanning compatibility.
• Consumer electronics: display windows, touch panel overlays, and protective lenses for screens and sensors.
• Optical and medical devices: lenses, cuvettes, syringe barrels, and transparent housings requiring controlled optical clarity.

FAQ

What is the difference between transmittance and haze?

Transmittance measures the total amount of visible light passing through a specimen, regardless of direction, while haze measures only the portion of transmitted light scattered at angles greater than 2.5 degrees. A material can have high transmittance and high haze simultaneously, such as an opalescent diffuser that passes most light but scatters it widely.

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

The choice depends on the market and customer specification. ASTM D1003 is widely used in North America and provides both hazemeter and spectrophotometer methods. ISO 13468-1/-2 and ISO 14782 are preferred in international trade and European markets. GB/T 2410 is technically aligned with ASTM D1003 and is used in China. Always confirm the required standard with the end customer before testing.

What specimen size and thickness are required?

Specimens should be at least 50 mm x 50 mm to fully cover the standard transmission port, though smaller ports of 20-30 mm are acceptable for limited-size parts. Thickness is not fixed by the standard; instead, the tested thickness should match the nominal thickness of the end product, and it must be reported because transmittance is thickness-dependent.

How often should the instrument be calibrated?

Full calibration with certified reference standards should be performed daily before testing or whenever the instrument is moved or the light source is replaced. Mid-range verification standards should be checked after every 10 to 20 specimen measurements to confirm that drift remains within acceptable limits.

Can colored or translucent plastics be tested?

Yes. Colored plastics are best characterized with the spectrophotometer method, which provides spectral transmittance data across the visible range in addition to the single-number total transmittance value. Translucent materials that scatter light strongly can be tested, but the wide-angle correction and reporting requirements of the applicable standard must be followed carefully.

Why are my transmittance results inconsistent between laboratories?

Common causes include differences in specimen thickness, surface cleanliness, conditioning history, instrument geometry, and the reference standards used for calibration. Ensuring identical specimen preparation, conditioning at 23°C/50% RH, and testing according to the same standard method minimizes interlaboratory variation.