When it comes to lenses for digital pico projectors, there’s currently something of a trade-off. Traditional lenses, where multiple glass magnifiers are placed one in front of the other, are long and bulky. Microlens arrays, in which many tiny lenses are assembled together on one flat surface, are a much more compact, lightweight alternative. However, so far such arrays have mostly been made out of plastic, which the bulbs in some projectors are capable of melting. Now, researchers from Germany’s Fraunhofer Institute for Machine Tools and Forming Technology have come up with what they say is a solution: microlens arrays made from glass, using a hot embossing technique.
The process starts with the formation of the die equipment, which is machined out of tungsten carbide using ultra-precise grinders. Because both the die and the glass will expand when heated, and at different rates, the lens pattern that is carved into the two die halves must be made to compensate accordingly – in other words, it does not look exactly like the finished product will look.
Next, in a vacuum chamber kept at a constant temperature between 600 and 900C (1,112 and 1,652F), the die halves are pressed together with the glass between them. “The main challenge is to keep the material exactly at the temperature where it is malleable but not yet molten,” explained project manager Jan Edelmann. “That is the only way to guarantee that components made from it will be within the prescribed tolerances to within a few micrometers.”
The glass must then be ejected from the mold before cooling begins, as the different cooling rates of the glass and the metal could cause the glass to shatter.
Using this technique, the Fraunhofer team have already produced high-refraction glass microlens arrays, in which alignment faults across all 1,700 microlenses were smaller than 20 micrometers. The researchers believe that it should be possible to apply the system to mass production, where it could serve to bring the price of projection lenses down to a tenth of their current cost. It is also thought that the glass arrays could be used to broaden and homogenize laser beams.