edge enhancement, the edges of objects (or, commonly, areas of rapid
intensity change) within an image are made to stand out. It is an
image-processing technique commonly used to help machine-vision systems
determine where and how many objects are in an image. The method
normally depends on computer calculation: in one approach, the
derivative of the digitized image-intensity field is calculated, and in
places where the derivative is high, an edge is assumed to exist. An
all-optical setup developed by researchers at the Nagaoka University of
Technology (Nagaoka, Japan) and the Himeji Institute of Technology
(Himeji, Japan) now allows edge enhancement to be done without
digitization or software.1 The setup relies on effects that occur within a photorefractive liquid crystal.
(Photo courtesy of Nagaoka U. of Technology)
material, termed a polymer-dissolved liquid-crystal composite (PDLCC),
contains a copolymer, a low-molar-mass liquid crystal, and a
sensitizing dye. Two glass substrates with transparent electrodes and
50-mm-thick spacer films form a cell into which the photorefractive
PDLCC is poured. The filled cell is part of an optical system
containing a Fourier-transform lens; the cell itself is placed at the
Fourier transform of the image to be analyzed. The beam from a
frequency-doubled diode-pumped Nd:YAG laser emitting at 532 nm is
split, with part providing a signal beam for the optical system and the
other part a reference beam that is shone directly on the PDLCC cell.
operation, the two beams interfere within the cell, producing gratings.
A holographic optical image produced by the gratings is reconstructed
with a 633-nm read beam. For low-frequency gratings at the center of
the Fourier transform, the ratio of reference to signal-beam intensity
is much less than one, producing gratings with poor diffraction
efficiency. For higher frequencies near the edge of the Fourier
transform, however, the intensity ratio of reference and signal beams
is approximately equal, creating efficient gratings. These higher
frequencies that give rise to edge enhancement are efficiently
reconstructed in real time by the read beam.
Adjusting the intensity ratio of the two
532-nm beams makes the edge-enhancement effect either disappear (top)
or appear (bottom). Researcher Hiroshi Ono says that an early use for
this technique will be as a preliminary operation to lighten the load
on traditional image-processing systems.
Laser Focus World October, 2001
- H. Ono et al., Appl. Phys. Lett. 79(7), 895 (Aug. 13, 2001).
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