16-222-12 Me. Code R. § 2
In order for the installer of window tinting material to issued a certificate of compliance to the owner of a vehicle with previously tinted windows, the tinted windows must be measured for visible "light transmittance with a light meter accurate to + or - 3%.
Procedure: An untreated -window of the vehicle in question shall be tested (front windshield) for the amount of visible light transmittance. The tinted windows will then be tested to insure that the amount of visible light transmitted is at least 50% of the untreated window glass. In no case will a certificate of compliance be issued if the final application of tinting material combined with any factory original tinting manufactured in accordance with Federal Motor Vehicle Safety Standard 205 ( 49 CFR 571) transmits less than 35% of the visible light.
16-222 C.M.R. ch. 12, § 2