DENVER — Physics lessons may be needed when state lawmakers debate a bill about window tinting.

In Colorado, front windshields need to allow 70 percent of the light to come through to be legal. Side windows can be tinted dark enough to allow 27 percent of the light to come through.

Legislation being considered this year (HB19-1067) would allow vehicles registered out-of-state to have darker windows.

"We're just trying to get the lowest common denominator of all the states," said Rep. Barbara McLachlan (D-Durango).

McLachlan said law enforcement in her area wanted this change for safety and to not penalize out of state drivers for visiting Colorado.

Consider this a "New Mexico" law.

Of all the states that border Colorado, New Mexico has the loosest tinting laws. Side windows in New Mexico can have windows dark enough to allow just 20 percent of the light in.

In Oklahoma, vehicles made earlier than 1995 are allowed tint that has 10 percent of the light coming in.

"Most of the Sheriffs that I've talked to said that they know this tint so much that they can just randomly tell, at least in Colorado, if it's too dark," said McLachlan. "I have many gifts, but eyeballing window tints is not one of them."

"If one has not been trained in determining the percentage, it's a wild guess," said Metropolitan State University of Denver Physics Professor Dr. Grant Denn. "But, a light meter can always tell you the answer."

Using the light from a projector and even an Edison bulb, Denn demonstrated how to calculate tint. He put the light meter a set distance from the light to determine the intensity of the light. Then, he put a tint in front of the light and calculated the difference between the two.

"Without scientific equipment, an unskilled eye could not determine how much light is getting through," said Denn.

Aside from the visibility implications, Denn also described how a darker tint could help with keeping a vehicle cooler.

"When it tries to leave, it leaves as infrared light and that can get stopped by the windows themselves and cause cars to build up their heat quite a bit," said Denn. "By preventing that light coming in in the first place, by having heavy tint, you can reduce the effects of overheating in the interior of your car."

According the legislative council staff, which reviews the implications of a bill on the state, 31 people have been sentenced for illegal window tint in the last three years in Colorado.

You can reach political reporter Marshall Zelinger at marshall@9news.com

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