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Reduction of Coherence-Related Noise in Laser Based Imaging Systems

IP.com Disclosure Number: IPCOM000114431D
Original Publication Date: 1994-Dec-01
Included in the Prior Art Database: 2005-Mar-28
Document File: 4 page(s) / 222K

Publishing Venue

IBM

Related People

Argyle, BE: AUTHOR [+3]

Abstract

Lasers are frequently used as illumination sources in imaging applications that require a particularly high brightness, stability, or monochromaticity of the illumination source. Applications include microscopy of magnetic domains in materials that are magneto-optically active and microbiological cells of a birefringent nature suspended in isotropic liquid nutrients. These are optical phase objects for which the most convenient method of conversion to amplitude images is wide-field polarized light microscopy. Typically, the phase objects of importance to frontier efforts in biology or technology exhibit phase shifts as small as a few ten-thousandths of visible light wave lengths or produce rotations of the plane of polarization on the order of millidegrees (1).

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Reduction of Coherence-Related Noise in Laser Based Imaging Systems

      Lasers are frequently used as illumination sources in imaging
applications that require a particularly high brightness, stability,
or monochromaticity of the illumination source.  Applications include
microscopy of magnetic domains in materials that are
magneto-optically active and microbiological cells of a birefringent
nature suspended in isotropic liquid nutrients.  These are optical
phase objects for which the most convenient method of conversion to
amplitude images is wide-field polarized light microscopy.
Typically, the phase objects of importance to frontier efforts in
biology or technology exhibit phase shifts as small as a few
ten-thousandths of visible light wave lengths or produce rotations of
the plane of polarization on the order of millidegrees (1).

      Conversion of microscopic phase objects to amplitude images is
usually accomplished in a microscope using nearly crossed polarizers,
sometimes with the introduction of an optical compensator (1-4).
These
conditions can diminish the intensity reaching the detector
below the detector's threshold of response.  Uncrossing the
polarizers to raise the intensity increases background light which
can saturate the detector or increase shot noise (4).  Such issues
are particularly acute at the highest optical magnification because
the intensity at the detector varies inversely as the square of the
optical magnification.  Background light arises also due to
non-magnetic reflections from the sample and the microscope lenses.
Spatially non-uniform amplitude and phase can arise when illuminating
through the objective (from ordinary Snell's law refraction).  This
non-uniformity causes image distortion.  The uniformity is improved
however, by sharply focusing the source image into the objective's
rear focal plane (1,2).  The diffuser plate normally present in the
Koehler illuminator (for the purpose of distributing the source image
over the entire rear focal plane) must first be removed to avoid
dispersing the illumination ray bundle over the rear focal plane (1).
Finally, residual background light reaching the detector is removable
via video analog-to-digital conversion, digital image averaging and
image enhancement (1-3).  Enhancement benefits greatly from
subtracting
a reference image containing the background 'light' sup 1.

      The desired result of obtaining images in real time (at video
frame rates) can be inhibited because of noise and the time needed
for the digital imaging processes (image accumulation, background
subtraction and averaging).  In order to decrease this time a strong
steady illumination source is needed.  (Detectors with increased
sensitivity could also help, but such detectors usually exhibit
larger shot noise.) The incoherent source considered to have the
greatest brightness is the 100 W Hg-arc lamp.  However, an arc is
unsteady in its light amplitude, phase and spati...