Mercedes-Benz will introduce a new breed of "switchable" glass roof as an optional extra on the new SLK next year. The panoramic vario-roof with MAGIC SKY CONTROL can – aside from being quite a mouthful – either be darkened or made transparent at the touch of a button.

Based on the physics of a plate condenser, when the glass structure of the roof is subjected to an electric voltage, particles in the structure position themselves so that light is able to pass through. If there is no voltage the particles position themselves randomly, darkening the glass and partially blocking the light. This allows the transparency of the glass to be altered at the touch of a button located in the vehicle’s overhead console which controls a converter integrated into the front section of the roof.

Mercedes says the roof is designed to provide an “open-air experience during cold weather” as well as offering shade to prevent the interior of the car from heating up in direct sunlight. Protecting the chrome domes of people like myself the roof will still block UV and infrared rays in transparent mode, while insulation reportedly increases significantly and noticeably in darkened mode, keeping interior parts such as armrests cooler by up to 10 degrees Celsius (50 F) compared with conventional green glass.

To test the technology, Mercedes-Benz headed to Death Valley in the U.S. with prototype vehicles fitted with the switchable glass roof. Summer temperatures there regularly hit over 50 degrees Celsius (122 F) in the shade and test engineers made the most of these conditions, heating up the SLK prototypes for four hours around midday.

Using a device known as a star pyranometer, which measures the intensity of exposure to sunlight, the team was able to measure the effectiveness of the roof. With an open roof the device measured a load of 1,000 to 1,100 W per square meter from the sun. With the transparent glass roof the reading dropped to 200 W, and when the roof was switched to darkened mode it dropped to 40 to 50W – one 20th of the original value.

As you’d expect, this also translated to relief from the heat, with thermal images showing that temperatures in the region of the head and shoulders were immediately and significantly reduced when the roof was switched to darkened mode.

But Mercedes says passengers won’t be the only ones to benefit from the technology. The switchable glass roof should also take some of the strain of the air-conditioning system, thereby cutting CO2 emissions.

Mercedes-Benz will debut the MAGIC SKY CONTROL roof on the new SLK, which is due out next year.

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