3 years ago when I lived in Minnesota they started to crack down on tint. I was pulled over twice in a year for 35% tint on my magnum (that had been tinted that way for the 6 years prior without a peep from the LEOs) I went back to the tint shop to have them remove it and they had a sign that said "Due to recent changes in tint law, we can no longer install illegal tint (even if you sign a waiver) unless you have a prescription...Ask US how to get a prescription!"
So I asked and they set me up with an eye doctor that would write and Rx without hassle. Of course I never got hassled after that so I was never able to test it. I moved to CA last year and the Rx had run out so I went to a new eye doctor. I showed him the slip and he said huh, ive never seen that before, well let me copy it verbatim to extend it a couple more years." Im still waiting for the day I need to test my prescription strength windows...but it is a loophole you can drive a car through.