KNOXVILLE, TN – The Mercedes-Benz SLK roadster has served as a key test bed for new technologies from the luxury auto maker since the first generation launched 15 years ago, a product planner here says.

The ’97 SLK marked the debut of Mercedes’ retractable hardtop. The second-generation roadster in 2004 unveiled the patented “Air Scarf” feature, which warms the back of an occupant’s neck when driving with the top down.

The Air Scarf is a returning technology that has proven popular with buyers. Mercedes says the take rate is 98%.

But the ’11 SLK, now in its third generation, boasts what arguably is its most innovative feature yet – Mercedes’ “Magic Sky Control” roof.

With the push of a button, what appears to be an ordinary panoramic glass roof built into the retractable hardtop instantly darkens, blocking out nearly all light. The process may appear magical, but it’s based on science, says Scott Keilman, assistant product manager-SLK-Class.

The principal is similar to other high-tech automotive features, such as magnetic-ride suspension. In this case, the roof glass is composed of two layers. Sandwiched between them is a chemical film coated with randomly placed nanoparticles.

When the button is depressed, an electrical current causes the particles to arrange themselves uniformly, which allows light to pass through the glass panels.

When the current is removed, the particles scatter, blocking the light, Keilman tells Ward’s during a recent media event here. The system has been tested extensively in extreme climates, including the Arctic Circle and Death Valley.

For those who don’t want to hand over the $2,590 for the optional package that features Magic Sky Control, the SLK also is available with a solid retractable hardtop as well as one with the panoramic glass sans the nanoparticles.

Todd Grieco, product manager-SLK, says there is no projected take-rate for Magic Sky Control, noting it’s “more about being able to offer the customer three (roof) choices.”

While the technology currently is offered on the recently launched ’12 SLK350 only, it’s a good bet it will migrate to other vehicles in Mercedes’ lineup.

Magic Sky Control “kind of matches (the) SLK,” but it might soon be offered on “another roadster, which is coming in the near future,” says Bernhard Glaser, general manager-product management. “Then probably go to other cars like sedans.”

Another technology Mercedes hopes will strike a chord with consumers is the Attention Assist system. Standard on the ’12 SLK350, it uses a steering-movement sensor in conjunction with “intelligent” software to identify signs of drowsiness and erratic input.

A warning message then appears in the instrument cluster declaring “time for a rest!” Mercedes says the system, which measures 70 different parameters, is sophisticated enough to tell the difference between sporty and erratic driving.

Grieco says Attention Assist is not necessarily a feature customers have been asking about, but notes a lot of safety technologies launch with little fanfare.

“One of the core DNA attributes of our brand is safety, and we think this technology is core to the overall development of safety in autos,” he says.

Mercedes is hoping the SLK’s technologies and new, more “masculine” styling will help bolster sales, which have been on a downward spiral. Grieco says the luxury roadster segment has contracted by 100,000 units over the last decade. But the SLK still controls 6%-7% of the segment.

The SLK’s typical buyer is 55 years old, 55% are male with an annual salary of $160,000.

Grieco declines to reveal a sales target for the latest roadster, but says first-year deliveries likely will top sales of the last-generation SLK. Deliveries in 2010 totaled 1,980 units, down 22.8% from the prior year, according to Ward’s data.

“We’re optimistic,” he says. “We feel the car is all grown up now.”

The ’12 SLK350, powered by a 302 hp 3.5L V-6, starts at $54,800, excluding destination and delivery charges.

The SLK250 with a 1.8L turbocharged engine is scheduled to launch early next year. Pricing for the model has yet to be announced.