visible transmittance (VT) is an optical property that
indicates the amount of visible light transmitted. The
NFRC's VT is a whole window rating and includes the
impact of the frame which does not transmit any visible
light. 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.