Transmittance
Light transmittance (LT, sometimes τv or VLT) is the fraction of visible light that passes through a glazing unit, on a 0–1 scale. It is the second material input a ray-tracing daylight simulation needs, alongside surface reflectance.
LT describes the glass only. The window frame is opaque and is accounted for separately — in LUX through the window frame ratio and frame reflectance, not through LT.
Do not confuse LT with the g-value (solar heat gain coefficient), which measures total solar energy transmission and is used for overheating and energy calculations, not for daylight. Low-e and solar-control coatings lower the g-value more than they lower LT, so the two numbers diverge for coated products.
Typical values
| Glazing type | Typical LT |
|---|---|
| Single clear glazing | 0.88 – 0.90 |
| Clear double glazing | 0.70 – 0.82 |
| Low-e double glazing | 0.60 – 0.75 |
| Triple glazing | 0.50 – 0.65 |
| Tinted / solar-control glazing | 0.30 – 0.55 |
Manufacturers publish the exact LT for each product in their glass datasheets; use that value when the specification is known.
How LUX uses transmittance
The backend prediction models are trained at a fixed LT = 0.67, representative of a standard low-e double-glazed unit. In IFC Daylight Factor the LT slider (0.45–0.89) rescales the prediction so you can explore how a different glass product would change the result; it does not re-run the underlying model.
LUX Certify uses a full Radiance simulation and takes the project glazing properties directly.