Different Laws in different States.
From:
http://www.tintlaws.com/
HOW DARK CAN WINDOW TINT BE IN WISCONSIN?
Darkness of tint is measured by Visible Light Transmission percentage (VLT%). In Wisconsin, 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 50% of light in.
• Back Side Windows Must allow more than 35% of light in.
• Rear Window Must allow more than 35% of light in.
HOW REFLECTIVE CAN THE TINT BE IN WISCONSIN?
Similar to sunglass lenses, some tinting film contain metallic elements that help in reflecting incoming light and reducing the glare and heat generated by visible light.
• Front Side Windows No reference to reflectivity in the current law.
• Back Side Windows No reference to reflectivity in the current law.
OTHER WISCONSIN RULES AND REGULATIONS
• Restricted Colors No colors of tint are explicitly banned.
• Side Mirrors Dual side mirrors are required if back window is tinted.
• Certificate Requirements Manufacturers of film do NOT need to certify the film they sell in the state.
• Sticker Requirements No sticker to identify legal tinting is required.
• Medical Exemption State allows medical exemptions for special tint. For more details about the specific terms of the exemption, consult your state law.