Darkness of tint is measured by Visible Light Transmission percentage (VLT%). In South Carolina, this percentage refers to percentage of visible light allowed in through the combination of film and the window.

Windshield Non-reflective tint is allowed along the top of the windshield above the manufacturer's AS-1 line.
Front Side Windows Must allow more than 27% of light in.
Back Side Windows Must allow more than 27% of light in.
Rear Window Must allow more than 27% of light in.