Visible light transmittance (VLT) is the amount of light in the visible portion of the spectrum that passes through a glazing material. This is a key input to optimizing daylighting. And since good daylighting can help offset electric lighting and its associated cooling loads, understanding VLT is critical to designing high-performance buildings.

Visible light transmittance is influenced by the glazing type, the number of panes, and any glass coatings. Visible light transmittance of glazing ranges from above 90% for clear glass to less than 10% for glazing comprised of highly reflective coatings on tinted glass. 

In Sefaira, you have the ability to input a VLT value in the Properties section of the Plugin. Since Sefaira analyzes a single-plane model, it will apply a VLT to your glazing planes and will treat things like mullions or frames as shading. As a result, when considering VLT values informed by glazing options, you should apply the center-of-glass value rather than the assembly value.