What Is Visible Light Transmittance?
Reducing the amount of solar radiation transmitted through glazing can reduce the amount of daylight entering your home. The amount of light transmitted by glazing is specified by the visible light transmittance, which is recorded as either TVw (visible transmittance of the window) or VLT (visible light transmittance). Both mean exactly the same. While the VLT rating varies between 0 and 1, most double glaze3d windows rate between 0.3 and 0.7 . The higher the value, the better
A simple way to consider this is:
Low TVw = too dark inside = lights on = higher energy costs.