Property of glazing that controls the amount of solar heat gains into the space It is typically represented as Solar Heat Gain Coefficient and will range from 0 – 1. The lower this figure is, the more effective the glazing will be at blocking solar heat eg. 0.1 will allow less Solar Heat through than 0.9.
Modern glass which blocks Solar Transmittance does not necessarily mean that light will also be blocked. Also single glazing may restrict solar transmission better than triple glazing even though its u-value is less desirable for heat loss purposes.