Visible Transmittance (VT)

Visible Transmittance (VT) is a measure of the fraction of visible light (wavelengths 380–780 nanometers) that passes through a window. It ranges from 0 to 1: a window with VT 0.60 transmits 60% of the incident visible light. VT is reported on the NFRC label alongside U-factor and SHGC.

What Affects VT?

• Number of glass panes — each pane absorbs and reflects about 8–10% of light. Double-pane transmits about 82% of what single-pane does; triple-pane about 73%.
• Low-E coatings — most Low-E coatings reduce VT by 5–15% compared to uncoated glass. Spectrally selective Low-E coatings minimize this trade-off.
• Tinted glass — bronze, gray, or green tints significantly reduce VT (to 0.40–0.55) while reducing SHGC.
• Frame width — the frame does not transmit light; a narrow-frame window has higher effective VT for the same rough opening size.

VT and Room Daylighting

VT directly affects how bright a room feels. A room with low-VT windows (0.35–0.45) will require more artificial lighting, offsetting some of the energy savings from reduced solar heat gain. For most residential spaces, a VT of at least 0.50 is recommended to maintain comfortable daylighting.

Light-to-Solar Gain Ratio (LSG)

The LSG ratio is VT divided by SHGC. A high LSG (above 1.25) indicates a spectrally selective window that transmits more visible light relative to heat — the ideal for Ohio where you want daylight without excessive summer heat gain. Most quality Low-E windows achieve LSG values of 1.4–1.8.