Measuring Performance: Visible Transmittance

The visible transmittance (VT) is an optical property that indicates the fraction of visible light transmitted through the window. This is separate from the Solar Heat Gain Coefficient (SHGC), since many modern windows include spectrally selective coatings that can allow different amounts of visible, infrared and ultraviolet light.

The NFRC’s VT is a whole window rating and includes the impact of the frame area. Since the frame does not transmit any light, the VT may be lower than expected; however, this is done to be consistent with the whole window ratings of U-factor and SHGC. While VT theoretically varies between 0 and 1, most values among double- and triple-pane windows are between 0.30 and 0.70. The higher the VT, the more light is transmitted. A high VT is desirable to maximize daylight.

How to maximize daylight?

Historically, only clear glass was used to maximize the amount of light entering through a window. Especially in cooling-dominated climate, this desirable daylight also came with undesirable solar heat gain. With the advancement of high-performance glazing systems, it is possible for low-E coatings to reject the solar heat gain while allowing the visible light to pass through the glass. The type of low-E coating that is appropriate for your specific house depends on location, orientation, window area, and shading strategies.