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  • Proposed Doublet Lens Improves Depth of Focus

Photonics Spectra
Apr 2005
Richard Gaughan

One barrier to introducing imaging systems into new, practical industrial applications is the need for precise alignment. Outside of the laboratory, natural variations in object distance result in reduced resolutions or require real-time focus adjustment. The longer the depth of focus of a lens, the more the system will tolerate changes in object distance, so methods have been developed to extend depth of focus. Many of them, however, reduce the resolution of the image at the focus.

A doublet with one lens composed of conventional glass and another constructed from birefringent crystal has been proposed as a method for extending depth of focus. The system would include two polarizers, both oriented 45° from the vertical axis. The first lens in the doublet would be birefringent, with the crystal axis along the vertical direction, so that the doublet lens would produce two different foci for the two orthogonal polarizations; i.e., extraordinary and ordinary ray vibrations.

Xinping Liu and her colleagues at Canada's National Research Council in Ottawa had previously calculated that a birefringent lens could be designed with an extended depth of focus by manipulating the curvature, thickness and degree of birefringence. But those modifications also changed the focal length. Their new work, combining the birefringent lens with a conventional lens, enables the focal length to be set independently of the depth of focus.

The design rules developed by the group employ the additional degrees of freedom to flexibly control focal length and depth of focus. Liu used peak intensity on the optic axis to quantify the improvement in depth of focus and found that the doublet would remain above the half-maximum threshold for twice the distance from the nominal focal plane than would a conventional lens.

Although she admitted that the fabrication and measurement of this kind of lens would be difficult, she said it would be worthwhile because the approach does not reduce the resolution or modulation transfer function, unlike previous methods.

A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
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