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Microlens Arrays Emerge as Low-Cost Option.

Microlens arrays are small replicated optical elements consisting of arrays of microscopic lenslets tens to hundreds of micrometers in diameter. Each lenslet presents a high-quality optical surface that is equivalent to an ordinary single-element lens. Such arrays have been used in a wide variety of spatial-filtering applications ranging from lentographs to confocal microscopes.

When used as a light source, the microlens array collects light from a collimated beam and focuses it to an array of small, bright points at the focal plane. Each lenslet focuses the light it intercepts to a separate point. These points can be extremely small because the microscopic lenslets can have very short focal lengths while presenting reasonable numerical apertures.

The numerical aperture for a lens in air is

NA = sin a/2f

where a is the lens' diameter and f is its focal length. In practice, it is nearly impossible to make a lens with a numerical aperture larger than about 0.95. That means that the focal length of a practical lens can be no less than about 40% of its diameter. Thus, the shortest focal length a macroscopic lens 5 mm in diameter can have is about 2 mm. A microlens array with lenslets 50 [micro]m in diameter can focus as close as 0.02 mm.

Alternatively, lenslets for such an array could be made with a much lower numerical aperture (which automatically reduces their optical aberrations) while still having a focal length much smaller than would be possible with macroscopic lenses.

Microlens arrays can also be used to provide structured-light lines for applications involving line-scan CCD cameras. Instead of using axially symmetric lens shapes, the lenslets for this application resemble miniature cylinder lenses, which focus light in one direction only. Thus, an array of cylindrical lenslets oriented with their long axes parallel to the x-axis, will spread a laser beam into a line oriented parallel to the y-axis. In the x/z plane, no focusing occurs, and the laser beam keeps its tight collimation. In the y/z plane, however, the lenslets bring rays to a sharp focus close to the array. Past that focal plane, however, the rays begin to diverge rapidly. The result is a broad, fan-shaped beam that is only as thick as the original laser beam.

One advantage that microlens arrays have over conventional optics is that they can be made extremely inexpensively. Each array consists of a set of identical microscopic bumps or depressions on the surface of a clear plastic sheet.

The process begins with creating a high-quality master and transferring it to a template containing the inverse of the desired surface shape. If, for example, an array of axially symmetric positive lenses are to be created, the template's surface will be covered with an array of depressions or wells. Once the template is created, an unlimited number of copies can be made at very low cost by casting or thermoforming methods.

A microlens array can be used to selectively illuminate sample points with much finer resolution than the size of the lenslets. All of the light entering the lenslets focuses into an array of points in the object plane of the objective lens. The objective lens, in turn, forms an image of the array on the face of the sample. Each lenslet selectively illuminates one of the sample picture elements (pixels), which are only one quarter of the size of the lenslets. By scanning the microlens array horizontally, the system can illuminate four sets of points in turn.

Researchers at Los Alamos (N.M.) National Laboratory have used this scanned-array technique to increase the imaging speed of a confocal microscope they have developed. The system uses a microlens array to illuminate their sample through the microscope's objective. The microscope then focuses the light returning through the objective on a CCD array. Scanning the array as described above scans the image points across the CCD face.

Each lenslet forms a separate imaging channel, complete with its own source (the lenslet), a separate path through the microscope objective, target of interest (the place where the objective focuses that spot of light on the specimen), and return path through the objective lens to an area of the CCD. The lenslets are made to be large enough so that their optical channels are separated well enough to avoid crosstalk.

In effect, the microscope is really many (equal in number to the number of lenslets) confocal microscopes working in parallel. The Los Alamos team estimates that their microscope will cost less than a fifth what conventional confocal microscopes cost, due in no small part to the cost savings from using an inexpensive replicated microlens array.
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Comment:Microlens Arrays Emerge as Low-Cost Option.
Author:Masi, C.G.
Publication:R & D
Article Type:Brief Article
Geographic Code:1USA
Date:Mar 1, 2000
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