Some of the cops here in the US carry light meters to measure car tinting. I have only heard of two times that they ever use the meters.
1) When they stop the car for speeding or something else and the driver is being a pain. Then they start looking for things to tack onto the ticket, and window tinting is normally a easy target for the generally young men with an attitude problem.
2) The window is so dark that the cop can not see the driver in broad daylight with a bright back lightning from the far side window.
In general, they are not going to waste the time pulling someone over when they are not committing a moving violation just to write them a ticket for dark windows.