Microlens arrays

A hot embossing technique that can fabricate doublesided arrays of glass microlenses with record levels of precision could provide an alternative to plastic lenses in projectors.

Researchers at Germany’s Fraunhofer Institute for Machine Tools and Forming Technology (IWU) claim the method is suitable for mass manufacturing and target applications, including home cinema or business projectors, and can be used without darkening the room.

All projectors contain lenses that gather light from the source and spread it evenly across a wall to produce a bright image.

One way of performing this task is to use a single component made up of thousands of microlenses. This saves space but to date it has only been possible to manufacture such lens arrays from plastic. This is due to the high temperatures and precision required to emboss glass in comparison to plastics.

Yet conventional projectors use bulbs rated in the region of 250W - the heat generated by the lamp can melt the plastic lens arrays.

Jan Edelmann and his team from IWU in Chemnitz claim to have developed a process for manufacturing glass lens arrays, that overcomes previous limitations.

Although the exact manufacturing process is proprietary, it involves heating the glass and tool system to temperatures in excess of 600°C, applying an embossing force, flowing the glass into the mould and then cooling the array to below the glass transition temperature.

Process chamber for hot
embossing of glass lensEdelmann explains, ‘Depending on the type of glass a temperature between 600 and 900ºC must be applied homogeneously with variations below five Kelvin. One challenge is to keep the glass exactly at a temperature where it is malleable but not yet molten. We also had to cope with different coefficients of expansion. Both the glass and the moulds will expand and contract at different rates’.

He continues, ‘Our moulds are made of tungsten carbide and machined with ultra-precise grinders to have a slightly different shape from the workpiece that we are looking to produce’.

To perform the technique, the IWU team has hot embossed glass with a refractive index of 1.8, into a part containing 1,700 single lenslets on each side of the array in one step.

Each lenslet measures 700x400μm giving an array diameter of 25mm. Alignment faults for each lens pair are said to be below 20 μm.

However, Endelmann was keen to stress that this does not represent the limits of the technology. ‘In our lab equipment, we are able to emboss glass parts up to 50mm,’ he adds.