Visible Light Transmission (VLT)
Visible light transmission (VLT) is the percentage of visible light that passes through a glazing sample under a stated test method. Automotive standards often call the corresponding measurement luminous transmittance. ISO 13837:2021: Automotive glazing transmittance methods
Two uses of the term
- Pane VLT describes a monolithic or laminated glazing sample. A higher percentage means more visible light is transmitted; a lower percentage means less is transmitted and the pane generally appears darker.
- Installed-system VLT describes the result through the complete glass-and-film combination. An aftermarket film's nominal VLT is not automatically the same as the finished window's VLT because the original glass already transmits less than all incident light. Official film data therefore distinguish measurements for filmed glass. Compliance must be assessed against current rules for the installed pane in the relevant jurisdiction. 3M: Automotive window-film terminology UNECE Regulation No. 43: Safety glazing
What VLT does not measure
VLT is a visible-light metric. It does not by itself state ultraviolet transmission, infrared rejection, direct solar-energy transmittance, or total solar-energy transmittance. Those quantities cover different spectral ranges or different energy paths through the glazing.
This distinction matters when comparing privacy and thermal performance. Two panes can have similar VLT yet differ in solar-energy control, while a comparatively clear-looking pane can include an infrared-reflective treatment. See Solar-Control and Infrared-Reflective Glass for the solar-control distinction.
For the test context and the interaction between tint, solar load, and vehicle systems, see Solar-Control and Acoustic Automotive Glazing.