Light Transmittance Determination for New Materials: Standards, Methods, and Applications

Light transmittance determination is a core optical performance test for new materials such as transparent plastics, glass, optical films, and functional coatings used in the automotive, construction, electronics, and photovoltaic industries. This article explains the measurement principles, applicable standards such as ISO 13468, ASTM D1003, GB/T 2410, and ISO 9050, sample requirements, and typical applications of professional transmittance and haze testing. All tests described below can be performed by accredited third-party laboratories using calibrated spectrophotometers and hazemeters.

Technical Overview

Light transmittance (Tt), also referred to as luminous transmittance or visible light transmittance (VLT), is defined as the ratio of the luminous flux transmitted through a specimen to the luminous flux incident upon it, weighted by the photopic response of the human eye. The result is expressed as a percentage and is typically evaluated under a standard illuminant such as CIE Illuminant A or D65 across the visible wavelength range of approximately 380 nm to 780 nm.

For transparent and translucent new materials, including engineering plastics, optical films, functional glass, and coatings, light transmittance directly governs optical clarity, visual comfort, energy efficiency, and product aesthetics. To obtain a complete picture of optical quality, transmittance is usually measured together with related parameters such as haze, regular (specular) transmittance, diffuse transmittance, and full spectral transmittance.

Accredited third-party laboratories perform light transmittance determination in accordance with internationally recognized methods published by ISO, ASTM, EN, GB, JIS, and IEC. Calibrated UV-Vis-NIR spectrophotometers and integrating-sphere hazemeters ensure traceable, reproducible data suitable for research and development, incoming material inspection, quality control, and regulatory compliance.

Testing Samples

Light transmittance determination is applicable to a wide range of new materials in rigid sheet, molded plaque, or flexible film form. Typical sample categories include the following:

Transparent Plastics and Polymers

Polycarbonate (PC), polymethyl methacrylate (PMMA), general-purpose polystyrene (GPPS), styrene acrylonitrile (SAN), cyclic olefin copolymer (COC/COP), polyethylene terephthalate (PET), polyamide (PA), polypropylene (PP), PVC, TPU, and optical-grade silicones supplied as compression or injection-molded plaques, extruded sheets, or finished components.

Glass and Architectural Glazing

Clear float glass, tempered glass, laminated safety glass, low-E coated glass, tinted glass, and insulating glass units used in buildings, vehicles, and display panels.

Optical and Functional Films

PET optical films, window tint films, protective films, diffusion films, brightness enhancement films, transparent conductive films, agricultural greenhouse films, and transparent packaging films.

Coatings and Transparent Functional Layers

Anti-reflective (AR) coatings, hard coatings, transparent conductive oxides (TCO), and clear functional coatings applied to glass or polymer substrates, or measured as free-standing films.

Transparent Ceramics and Composites

Transparent ceramics such as YAG, ALON, and spinel for infrared windows and optical components, together with glass-fiber-reinforced translucent composites.

Standard specimens are typically flat sections of at least 50 mm x 50 mm, free of scratches, fingerprints, bubbles, and embedded dust. Specimens should be conditioned at 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or ASTM D618 before measurement. Because transmittance decreases with increasing thickness, the nominal thickness of each specimen must always be reported.

Testing Items

Common optical parameters determined during a light transmittance evaluation include:

• Total luminous transmittance (Tt / VLT) over the visible spectrum
• Regular (specular) transmittance, representing undeviated transmitted light
• Diffuse transmittance, representing scattered transmitted light
• Haze, defined as the percentage of transmitted light deviated by more than 2.5 degrees from the incident beam
• Clarity, describing narrow-angle scattering that affects image sharpness
• Spectral transmittance T(λ) across the 200 nm to 2500 nm UV-Vis-NIR range
• Ultraviolet transmittance (UV-A and UV-B blocking performance)
• Solar direct transmittance (Te), weighted against the reference solar spectral irradiance of ASTM G173

Test results are reported as a percentage to the applicable standard precision, together with specimen thickness, illuminant, instrument type, and the specific test method applied.

Testing Methods

Spectrophotometer Method (ASTM D1003 Method B, ISO 13468)

A double-beam UV-Vis spectrophotometer fitted with an integrating sphere measures spectral transmittance over 380 nm to 780 nm. The spectral data are then weighted by the CIE photopic luminosity function to calculate total luminous transmittance. ISO 13468-1 covers the single-beam procedure and ISO 13468-2 the double-beam procedure, offering high accuracy for transparent plastics.

Hazemeter / Integrating Sphere Method (ASTM D1003 Method A, ISO 14782)

A dedicated hazemeter with a broadband light source and an integrating sphere measures four luminous flux readings. Total transmittance and haze are calculated simultaneously, where haze equals the wide-angle scattered light as a percentage of total transmitted light. This method is the industry reference for transparent plastic sheets and films.

Architectural Glazing Method (ISO 9050, EN 410, GB/T 2680)

For building glass, coated glazing, and multilayer insulating glass units, spectral transmittance is measured at 5 nm intervals from 300 nm to 2500 nm. The data yield visible transmittance, solar direct transmittance, and derived quantities such as the solar heat gain coefficient (g-value), which are required for energy performance declarations of facade and window products.

Solar and UV Transmittance Method (ASTM E903, ASTM G173)

Spectral transmittance data are integrated against the standard solar spectrum to obtain solar transmittance and ultraviolet transmittance. This approach is essential for evaluating solar control films, greenhouse covers, and UV-blocking packaging materials.

Automotive Glazing Method (ISO 3538, GB/T 5137.2)

Vehicle safety glazing, including windscreens and window glass, is measured for regular transmittance to verify compliance with regulatory minimums, typically no less than 70 % for forward vision areas under requirements such as UNECE R43. JIS K 7361-1 and JIS K 7136 provide the corresponding Japanese national procedures for plastics.

Photovoltaic Encapsulant Method (IEC 62788-1-4)

UV-Vis spectroscopy determines the spectral transmittance of EVA, POE, and other encapsulant films used in photovoltaic modules, both in the as-received state and after UV or damp-heat aging, supporting qualification under the IEC 61215 series.

Applications

Light transmittance determination supports material selection, product certification, and quality control across many industries:

• Automotive: windscreens, side windows, sunroofs, headlamp lenses, and transparent interior trims requiring documented optical transmittance levels
• Building and construction: architectural glass, window films, polycarbonate roofing, and skylight panels for energy codes and green building certification
• Display and electronics: cover lenses, light guide plates, diffusion plates, and optical films where transmittance and haze jointly define display quality
• Photovoltaics: encapsulant films and cover glass for solar modules, including transmittance retention after weathering exposure
• Agriculture: greenhouse films engineered for high photosynthetically active radiation (PAR) transmittance and controlled UV transmission
• Packaging: transparent containers and films for food and medical products requiring visible clarity with UV protection
• Eyewear and healthcare: spectacle lenses, face shields, and transparent medical device components
• Aerospace and defense: aircraft transparencies, sensor windows, and transparent ceramics for demanding optical environments

FAQ

What is the difference between light transmittance and haze?

Light transmittance quantifies the total percentage of incident light that passes through a specimen, while haze quantifies the portion of that transmitted light that is scattered at angles greater than 2.5 degrees. A material can show high transmittance yet high haze, meaning it lets light through but blurs the view.

Which standard should be selected for transparent plastics?

ASTM D1003 and ISO 13468 (with ISO 14782 for haze) are the most widely accepted methods for transparent plastics, with GB/T 2410 and JIS K 7361-1 providing equivalent national procedures. For glass and glazing products, ISO 9050, EN 410, or GB/T 2680 are appropriate, while automotive glazing is typically tested to ISO 3538 or GB/T 5137.2.

How does specimen thickness affect the results?

Transmittance decreases and haze generally increases as thickness increases, following an exponential relationship with optical path length. Therefore, results are only comparable between specimens of the same nominal thickness, and thickness must always accompany the reported transmittance value.

How many specimens are required?

Most standards recommend testing at least three specimens per material lot, taken from different regions of the sheet or film, and reporting the average value along with the measurement range to characterize material uniformity.

What information is included in a professional test report?

A compliant report identifies the test standard, instrument model and calibration status, illuminant and observer geometry, specimen description and thickness, conditioning environment, individual and average transmittance and haze values, and the laboratory's accreditation information.

Can transmittance be measured after aging or weathering?

Yes. Specimens are frequently exposed to xenon-arc or fluorescent UV weathering per ISO 4892, ASTM G154, or ASTM G155, then retested for transmittance and haze to quantify optical degradation, for example yellowing of headlamp lenses or PV encapsulants.