Optical: systems and elements – Holographic system or element – Hardware for producing a hologram
Reexamination Certificate
2000-10-16
2001-11-27
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Holographic system or element
Hardware for producing a hologram
C359S002000, C359S561000, C356S071000
Reexamination Certificate
active
06323972
ABSTRACT:
DEDICATORY CLAUSE
The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalties thereon.
BACKGROUND OF THE INVENTION
Traditionally, optical matched filters for pattern recognition applications were produced using two techniques. The first technique employed holographic interferometric architectures similar to the Mach-Zehnder arrangement., The optical Fourier transform, produced by a lens, was interfered with an off-axis plane carrier light wave. This technique has come to be called the Vander Lugt architecture.
In the xf-yf plane, the off-axis plane wave reference beam interferes with the Fourier transform of the input transparency. The phase profile of the reference wave projected onto the flat xf-yf plane is wedge shaped, so that straight fringes are recorded at the location of each spatial frequency component of the in put scene. These fringes behave as microscopic diffraction gratings when illuminated by the Fourier transform of a test scene, and indicate the presence of the original input scene. This type of filter was originally made using photographic emulsion to record the interference pattern. A high-resolution medium was required for recording the fringes that were typically a few wavelengths wide. Once the film was exposed and then developed, it would be placed back in the system. If the input now used to address the filter exactly matched the spatial frequency information in the filter, the reference beam would be recreated from each exposed area. This collection of plane waves was then typically Fourier-transformed by another lens, resulting in the correlation function.
The second technique for producing matched filters involved digitally computing the Fourier transform of the scene of interest, which in general contains both amplitude and phase information, then taking the complex conjugate of the function and displaying the result on an electrically addressable spatial light modulator. The correlation can be directed off-axis if a prescribed phase ramp can be included in the calculation. The most serious problem with this technique is in finding a suitable device for displaying the computed filter. Currently, a device having arbitrary amplitude and phase addressability does not exist. Researchers have been forced to use approximate filters or filters that modulate only phase with an uncontrollable amplitude.
An object of this invention is to perform the complex filter computation optically, and immediately test it—thereby avoiding the time consuming digital calculation and complications involved with photographic emulsions.
SUMMARY OF THE INVENTION
The figure of the drawing depicts a sketch of the coherent optical system used in creating and testing of holographic Fourier transform matched filters of this invention. The light source
12
is a laser for producing coherent monochromatic light that is suitably expanded and collimated with a pinhole
14
and lens
16
. Beamsplitter
18
divides the incident light beam into object and reference waves, shown as
20
and
22
respectively. The object wave passes through a spatial light modulator
24
that displays the input image
26
, supplied by a camera, computer, or other video equipment. This spatial light modulator may modulate the amplitude or the phase of the coherent lighter a combination of amplitude and phase. Lens
28
performs the Fourier transform of the encoded scene. This information falls on the detector
30
. The reference wave, labeled
22
in the figure, is also directed using mirrors
38
and
40
, along with beamsplitter
36
, onto detector
30
at a slight angle, thus providing the needed phase wedge for interference with the Fourier transform of the input scene. Detector
30
then records a series of interference fringes whose spacing is primarily determined by the reference beam angle and wavelength of the coherent light. The distance (d) between bright (or dark) fringes is given by:
d
=
λ
sin
θ
for two plane waves intersecting at an angle &thgr;. This produces a fringe spacing of 18.0 microns. This is within the detectable range of current state of the art CCD arrays. This array of fringe information is stored by the first computer
42
, via a framegrabber, and then becomes the filter displayed spatial light modulator
44
that is itself addressed using beamsplitter
34
. Lens
45
then Fourier transforms the field transmitted by spatial light modulator
34
thus producing the correlation at detector
46
(shown as point P or
48
in the figure), which is stored in the second computer
47
. In reality, the lens
45
creates three terms: the correlation (point P), the convolution, and the filter response. With an off-axis (by the amount of &thgr;) placement of detector
46
, the correlation alone can be detected.
REFERENCES:
patent: 5239595 (1993-08-01), Takemura et al.
patent: 5245402 (1993-09-01), Adachi
patent: 5710722 (1998-01-01), Wood
patent: 6002499 (1999-12-01), Corboline et al.
Erbach Peter S.
Gregory Don A.
Taylor Travis S.
Bush Freddie M.
Schuberg Darren
The United States of America as represented by the Secretary of
Tischer Arthur H.
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