Ink-Jet Technique Yields Microlenses of Various Focal Lengths
Microlenses, which are used for technologies such as CCD cameras and laser beam collimators, can be difficult and, thus, expensive to produce. Some researchers have applied ink-jetting methods to deposit polymer solutions and UV-curable adhesives to form lenses. Now investigators at Max Planck Institute for Polymer Research and at the University of Mainz, both in Mainz, Germany, have developed an ink-jetting technique that instead uses a simplified two-step process to form spherical lenses with a range of focal lengths.
To form convex microlenses with various focal lengths, researchers first constructed concave lenses using an ink-jet printing technique. In each array of lenses, they increased the number of deposited drops in the Y-direction (A). They then obtained convex microlenses by template-casting from the concave ones (B). Reprinted with permission of Optics Express.
First, the researchers used a drop-on-demand ink-jetting system made by GeSiM GmbH of Großerkmannsdorf, Germany, to deposit droplets of toluene onto extruded plates of polystyrene. The droplets were arranged in a square grid pattern with a 250-μm pitch, forming concave lenses in the diameter of the deposited droplets. Along the X-axis, they made lenses using the same number of droplets but increased the number of droplets per deposition along each row going in the Y-direction.
In the second step, they formed convex lenses by using the concave forms as molds for casting an elastomeric polymer made by Dow Corning. They spread the polymer over the convex molds at room temperature, then baked it for 30 min. at 80 °C, cooled it and separated the array of lenses from the mold mechanically.
The scientists controlled the focal length of each lens by counting the number of droplets per deposition and by varying the depth by delaying subsequent droplets of equal volume in the same location. By doing this, they generated lenses with focal lengths of 0.2 to 4.5 mm. The technique is described in the July 23, 2007, issue of Optics Express.
They characterized the surface quality of the lenses using the intermittent-contact mode of an atomic force microscope made by JPK Instruments AG of Berlin. They found that the roughness of the lenses was 7 nm rms and that the peak-to-peak surface value was 380 nm rms. The latter value is comparable to that achieved with other microlens-forming techniques. Laser scanning confocal microscopy, using equipment from Carl Zeiss AG, proved the optical qualities of the lenses.
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