Optical Transmittance Measurement: Principles, Test Methods, Standards, and Applications

Optical transmittance measurement quantifies the fraction of incident light that passes through transparent and translucent materials, providing critical data for product design, quality control, and regulatory compliance across industries such as construction, automotive, displays, and photovoltaics. This technical guide explains the optical principles, standard test methods, sample requirements, and typical applications of optical transmittance testing performed by accredited third-party laboratories.

Technical Overview

Optical transmittance (also called luminous transmittance or light transmittance) is defined as the ratio of the luminous flux transmitted through a specimen to the luminous flux incident upon it, typically expressed as a percentage. It is one of the most fundamental optical properties used to characterize transparent and translucent materials such as glass, polymer films, sheets, coatings, and optical components.

Transmittance is governed by the combined effects of reflection at the material surfaces, absorption within the material bulk, and scattering by internal structures or surface roughness. According to the Beer-Lambert law, the internal transmittance of a homogeneous material decreases exponentially with its thickness and absorption coefficient, which is why specimen thickness must always be reported together with the measured transmittance value. Total transmittance is commonly decomposed into regular (specular) transmittance and diffuse transmittance; the diffuse component, expressed relative to total transmittance, is defined as haze.

Optical transmittance measurement is performed using UV-Vis-NIR spectrophotometers or dedicated haze meters, following internationally recognized methods published by ISO, ASTM, EN, JIS, and GB standard organizations. The measurement results directly determine whether products meet regulatory requirements such as minimum windshield light transmittance for vehicles, solar heat gain performance of architectural glazing, and UV protection categories for sunglasses.

Key Optical Quantities

• Total luminous transmittance (Tt): the percentage of incident luminous flux that passes through the specimen in all forward directions, weighted by the photopic response V(λ) of the human eye.
• Regular (parallel) transmittance: the portion of transmitted light that deviates by a small angle from the incident direction, used to assess image-forming clarity.
• Diffuse transmittance (Td): the portion of transmitted light scattered from the incident direction, typically caused by surface texture, inclusions, or internal haze.
• Haze: the ratio of diffuse transmittance to total transmittance, expressed as a percentage.
• Spectral transmittance τ(λ): the wavelength-dependent transmittance curve measured across the ultraviolet, visible, and near-infrared regions, typically from 200 nm to 2500 nm.
• Solar transmittance (Te): the transmittance integrated over the solar spectral irradiance distribution, used for building energy calculations.
• UV transmittance: the transmittance integrated over the UV-A (315-380 nm) and UV-B (280-315 nm) bands, used to evaluate ultraviolet blocking performance.

Testing Samples

Optical transmittance testing is applicable to a wide range of solid, transparent, and semi-transparent materials. Specimens are typically supplied as flat coupons or finished parts, and their surface condition, thickness, and orientation must be documented before testing.

Glass and Architectural Glazing

Clear float glass, body-tinted glass, coated low-emissivity glass, laminated safety glass, insulated glazing units, and patterned glass are tested to verify luminous transmittance, solar transmittance, and UV transmittance in accordance with ISO 9050, EN 410, and GB/T 2680 for building energy performance and daylighting design.

Polymer Films, Sheets, and Molded Plastics

Transparent engineering plastics and packaging films, including PET, PC (polycarbonate), PMMA (acrylic), PVC, TPU, PE, and EVA encapsulant films, are tested for luminous transmittance and haze using UV-Vis spectrophotometry with an integrating sphere, following ISO 13468, ASTM D1003, and GB/T 2410.

Display and Optical Components

Cover glass for touch panels, optical adhesives, polarizers, diffuser plates, light guides, and protective windows are characterized by spectral transmittance and haze to ensure display brightness, contrast, and viewing quality.

Automotive and Safety Glazing

Windshields, side windows, rear windows, and helmet visors are tested for regular luminous transmittance to demonstrate compliance with regulatory minimums, such as the 70 percent requirement for windshields and front side windows specified in ECE R43, GB 7258, and related national vehicle safety regulations.

Eyewear and Functional Coatings

Sunglass lenses, safety eyewear, blue-light filtering lenses, anti-reflective coatings, and transparent conductive films (such as ITO coatings) are evaluated for spectral transmittance, UV blocking, and category classification under ISO 12312-1, ANSI Z80.3, and applicable technical specifications.

Testing Items

Typical optical transmittance testing items offered by accredited third-party laboratories include:

• Visible light (luminous) transmittance, 380 nm to 780 nm, photopically weighted.
• Full spectral transmittance scanning from 200 nm to 2500 nm (UV-Vis-NIR).
• UV transmittance and UV blocking rate for UV-A and UV-B bands.
• Solar direct transmittance (Te) integrated over the reference solar spectrum.
• Total transmittance, regular transmittance, and diffuse transmittance separation.
• Haze determination for transparent plastics and films.
• Transmittance uniformity mapping across large-area samples.
• Thickness dependence studies and transmittance retention after weathering or abrasion testing.

Testing Methods

Two complementary instrumental approaches are used for optical transmittance determination: the spectrophotometric method with an integrating sphere and the dedicated haze meter method. Both methods require calibrated reference standards and traceable measurement conditions.

Spectrophotometric Method with Integrating Sphere

A double-beam UV-Vis-NIR spectrophotometer equipped with an integrating sphere (commonly 150 mm in diameter) is used to scan the spectral transmittance of the specimen at regular wavelength intervals. The integrating sphere collects both regularly transmitted and diffusely transmitted light, yielding total transmittance. When the specimen is placed at a distance from the sphere port, only the regular component is collected, allowing the diffuse component to be calculated by difference. This method follows ISO 9050, ISO 13468-1, ISO 13468-2, GB/T 2680, and JIS R 3106.

Haze Meter Method

A haze meter measures total luminous transmittance and haze in a single rapid operation using a broadband light source, a photopic detector, and a toggle between a regular-transmission and a diffuse-transmission optical geometry. This method, standardized in ASTM D1003 (Method B) and ISO 14782, is widely used for production quality control of films, sheets, and packaging materials.

Calculation of Weighted Transmittance Values

The spectral transmittance curve τ(λ) is weighted by the standard illuminant D65 spectral distribution S(λ) and the photopic spectral luminous efficiency V(λ) of the CIE 1931 standard observer to calculate luminous transmittance. Similarly, solar transmittance is obtained by integrating τ(λ) over the reference solar spectral irradiance defined in ISO 9845-1 or ASTM G173. This calculation framework is prescribed in ISO 9050 and EN 410 for glazing products.

Sample Preparation and Conditioning

Specimens should be flat, free of scratches, fingerprints, and dust, and are cleaned with lint-free wipes and suitable solvents such as isopropanol before measurement. Plastics are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291 prior to testing. Common specimen dimensions are approximately 50 mm × 50 mm for haze meters and 10 mm to 40 mm wide strips for standard spectrophotometer sample compartments; larger or curved parts may require customized fixtures. The measured thickness of each specimen is reported together with the result because transmittance is thickness dependent.

Applicable Standards

• ISO 9050: Glass in building — Determination of luminous and solar characteristics of glazing.
• ISO 13468-1 / ISO 13468-2: Plastics — Determination of the total luminous transmittance.
• ASTM D1003: Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics.
• ISO 14782: Plastics — Determination of haze for transparent materials.
• ASTM E424: Solar energy transmittance and reflectance of sheet materials.
• EN 410: Glass in building — Determination of luminous and solar characteristics of glazing.
• GB/T 2680: Determination of light transmittance, solar direct transmittance, and related properties of glass in building.
• GB/T 2410: Test method for light transmittance and haze of transparent plastics.
• JIS R 3106: Testing method on light transmittance, reflectance, and solar heat gain of flat glasses.
• ISO 3538: Road vehicles — Safety glazing materials — Test methods for optical properties.
• ISO 12312-1: Eye and face protection — Sunglasses — Transmittance specifications.
• ECE R43 and GB 7258: Regulatory transmittance requirements for automotive glazing.

Applications

Optical transmittance measurement supports design, certification, and quality control in numerous industries:

• Building and construction: verification of luminous and solar transmittance values declared in product datasheets for low-E glass, energy codes, green building certification, and daylighting simulations.
• Automotive: regulatory demonstration that windshields and front side glazing maintain a regular transmittance of at least 70 percent, and characterization of IR-reflective and privacy glazing.
• Photovoltaics: qualification of solar glass and EVA encapsulant transmittance, which directly affects module power output, including transmittance retention after damp heat and UV aging.
• Displays and consumer electronics: optimization of cover glass, optical adhesives, and touch sensor transmittance to maximize display brightness and reduce haze.
• Eyewear and personal protection: category classification of sunglasses, UV400 verification, and assessment of blue-light filtering lenses.
• Packaging: clarity and haze control of food and pharmaceutical packaging films, ensuring both product visibility and regulatory compliance.
• Aerospace and rail: verification of cockpit transparencies, instrument windows, and cabin glazing against strict optical specifications.

FAQ

What is the difference between transmittance and haze?

Transmittance measures how much light passes through a material relative to the incident light, while haze measures how much of the transmitted light is scattered away from the straight-through direction. A material can have high transmittance yet high haze (milky appearance), or high transmittance with very low haze (glass-clear appearance). Both parameters are usually reported together for transparent plastics.

Why do different thicknesses of the same material show different transmittance values?

Because absorption and scattering accumulate along the optical path length, transmittance decreases as thickness increases, following the Beer-Lambert exponential relationship. Specimen thickness must therefore always accompany a reported transmittance result, and comparisons should be made at equal thickness.

Which wavelength range is used for visible light transmittance?

Visible light transmittance is determined over the 380 nm to 780 nm spectral band, weighted by the CIE photopic photometric response V(λ) and typically referenced to standard illuminant D65, which approximates average daylight.

Can curved, textured, or large samples be tested?

Yes. Curved glazing such as windshields can be evaluated with complementary measurement geometries in accordance with ISO 3538, and textured or patterned materials can be measured with an integrating sphere that captures scattered light. Large panels may be cut into representative coupons or measured with adapted fixtures, provided the measurement location is documented.

How are transmittance and absorbance related?

Absorbance A is defined as the negative base-10 logarithm of transmittance, A = -log10(T). While transmittance is a linear ratio used for specification purposes, absorbance is proportional to concentration and path length, which makes it convenient for quantitative material analysis.

How should samples be submitted for testing?

Clients typically submit flat specimens of approximately 50 mm × 50 mm or larger, at least three pieces per condition for statistical validity, clearly identifying the test side, nominal thickness, and required standard. Accredited third-party laboratories clean, condition, and document each specimen before measurement and issue reports including spectral curves, weighted transmittance values, and instrument calibration information.