SAE J1796–2020: Spectral Transmission Test for Automotive Safety Glazing

The SAE J1796–2020 standard defines a test method for measuring the direct solar and visible transmittance of safety glazing materials used in road vehicles. It provides a consistent procedure to evaluate clear or tinted, monolithic or laminated glazing. This standard is critical for compliance with regulations such as ECE R43 and for ensuring performance in automotive applications.

Scope and Application

The standard applies to essentially flat sections of safety glazing, including curved samples that can be flattened for testing. It covers measurements from 300 nm to 2500 nm, encompassing the solar spectrum (ultraviolet, visible, and near-infrared). The test yields three key parameters: Solar UV Transmittance (TUV), Solar Direct Transmittance (TDS), and Visible Luminous Transmittance (LTA). Additionally, color transmittance is reported as CIELAB coordinates (L*, a*, b*) computed from D65 illuminant and the 10° observer.

Measurement Procedure and Calculations

The test requires a spectrophotometer, preferably with an integrating sphere, capable of scanning from 300 nm to at least 2300 nm (2500 nm preferred). Samples must be clean and oriented normal to the beam. For curved specimens, the flattest area is used, and curvature orientation is noted.

Spectral data are recorded at specific intervals: 5 nm from 300–400 nm, 10 nm from 400–800 nm, and 50 nm from 800–2500 nm. Calculations involve weighting transmittance by normalized solar spectral irradiance (AM 1.5 global) for TUV and TDS, and by CIE illuminants and color matching functions for LTA and color.

Parameter	Wavelength Range	Weighting Source	Measurement Interval
TUV	300–400 nm	Modified ISO 9845-1 (AM1.5 global)	5 nm
TDS	300–2500 nm	Modified ISO 9845-1 (AM1.5 global)	5/10/50 nm
LTA	380–780 nm	Illuminant A × V(λ)	10 nm
Color (CIELAB)	380–780 nm	D65 × 10° observer	10 nm

Engineering Design Insights and Common Pitfalls

The standard’s design reflects careful consideration of real-world conditions. The use of air mass 1.5 global solar spectrum matches average terrestrial solar exposure. The trapezoidal integration method ensures accurate weighting for irregular intervals. The inclusion of color transmittance via CIELAB allows precise specification of tinted glazing.

Common mistakes include:

• Not measuring to 2500 nm or failing to apply tail correction.
• Using direct normal instead of global solar irradiance weighting.
• Forgetting to convert XYZ to CIELAB coordinates.
• Using outdated ASTM or CIE references.

Frequently Asked Questions

What is the required wavelength range for measurement?

The standard requires measurement from 300 nm to at least 2300 nm. The preferred range is 300–2500 nm to avoid tail correction.

How should curved samples be prepared?

Cut and clean the flattest area of the specimen. Mount it at normal incidence and note the curvature orientation, as it can affect transmittance.

How is luminous transmittance (LTA) calculated?

LTA is computed by summing the product of spectral transmittance at 10 nm intervals with the Illuminant A spectral power distribution and the photopic luminosity function V(λ), then normalizing.

What color space is used for reporting transmittance?

The standard uses the CIELAB color space (L*, a*, b*), computed from tristimulus values X, Y, Z using Illuminant D65 and the 10° observer.

🔍 For full details, including the weighting tables, refer to the SAE J1796–2020 document.