A window’s, door’s, or skylight’s ability to transmit sunlight into a home can be measured and rated according to the following energy performance characteristics:
* Visible transmittance (VT)
A fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye, that is transmitted through a window’s, door’s, or skylight’s glazing. A product with a higher VT transmits more visible light. VT is expressed as a number between 0 and 1. The VT you need for a window, door, or skylight should be determined by your home’s daylighting requirements and/or whether you need to reduce interior glare in a space.
* Light-to-solar gain (LSG)
The ratio between the SHGC and VT. It provides a gauge of the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light transmitted without adding excessive amounts of heat. This energy performance rating isn’t always provided.
Source: Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy