Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-07-02
2004-06-08
Spector, David N. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S316000, C359S559000
Reexamination Certificate
active
06747781
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of laser illumination. More particularly, this invention relates to the field of laser illumination where an intensity detector observes a surface illuminated by the laser illumination and where it is desirable to reduce speckle observed by the intensity detector.
BACKGROUND OF THE INVENTION
A human eye has finite resolution. When the eye views an object, the eye quantizes the object into resolution spots, each of which are point spread functions of the eye. For example, if a person stands about 3 meters from a surface, the eye resolves the surface into the resolution spots with each of the resolution spots having a diameter of about 1 mm.
FIG. 1
illustrates the eye
12
viewing a diffuse surface
14
. A laser illumination
16
illuminates the diffuse surface
14
. A particular resolution spot
18
is imaged onto a retina of the eye
12
. Features of the diffuse surface
14
that are within the resolution spot
18
are not resolvable by the eye
12
. The diffuse surface includes many scattering centers within the resolution spot
18
. The scattering centers scatter the laser illumination
16
which is illuminating the resolution spot
18
. Because the laser illumination
16
is coherent, the scattering centers create interference within the eye
12
. The interference causes the eye
12
to perceive the resolution spot on a brightness scale ranging from a bright spot to a dark spot.
Each scattering center forms a source of lightwaves. The lightwaves constructively interfere; or the lightwaves partially constructively interfere and partially destructively interfere; or the lightwaves destructively interfere. If the lightwaves constructively interfere, the resolution spot
18
is the bright spot. If the lightwaves partially constructively interfere and partially destructively interfere, the resolution spot
18
has an intermediate brightness forming an intermediate brightness spot. If the lightwaves destructively interfere, the resolution spot
18
is the dark spot.
Thus, the eye
12
images the diffuse surface
14
into surface resolution spots in a random pattern of bright spots, intermediate brightness spots, and dark spots. This is speckle. More generally, an optical system which employs an intensity detector will also detect the speckle. One skilled in the art will recognize that the eye
12
is a biological optical system in which the retina functions as the intensity detector. A camera employs a type of intensity detector, which is film for a conventional camera or, typically, a charge coupled device for a digital camera. Thus, a photo of the diffuse surface
14
will show the speckle.
FIG. 2
is a photo of speckle
19
which shows a granular pattern of the bright spots, the intermediate brightness spots, and the dark spots.
A measure of the speckle is contrast (C). The contrast, in percent, is given by C=100*I
RMS
/
where
is a mean intensity and I
RMS
is a root mean square intensity fluctuation about the mean intensity.
Goodman in “Some fundamental properties of speckle,” J. Opt. Soc. A., Vol. 66, No. 11, November 1976, pp 1145-1150, teaches that the speckle can be reduced by superimposing N uncorrelated speckle patterns. This reduces the contrast by a speckle reduction factor of
provided that the N uncorrelated speckle patterns have equal mean intensities and contrasts. If the N uncorrelated speckle patterns have non-equal mean intensities or non-equal contrasts, the speckle reduction factor will be less than
. Thus, the speckle reduction factor of
is a best case for the speckle reduction for the N uncorrelated speckle patterns. Goodman further teaches that the uncorrelated speckle patterns can be obtained by means of time, space, frequency, or polarization.
A speckle reduction method of the prior art creates multiple speckle patterns by moving a viewing screen in an oscillatory motion, which employs the time means taught by Goodman. The oscillatory motion typically follows a small circle or a small ellipse about the optic axis. This causes the speckle pattern to shift relative to the eye
12
viewing the viewing screen and, thus, forms multiple speckle patterns over time. Though the amount of the speckle at any instant in time is unchanged, the eye
12
perceives the reduced speckle provided that the speed of the oscillatory motion is above a threshold speed. Stated another way, the eye
12
detects reduced speckle if an integration time for the eye
12
is sufficiently long that the oscillatory motion produces the uncorrelated speckle patterns within the integration time.
In the art of laser illuminated display systems, it is known that an active diffuser can be added to a laser illuminated imaging system to reduce laser speckle. The active diffuser is placed in an intermediary image plane or near the intermediary image plane. The active diffuser is moved in the intermediate image plane in a rotation or toroidal pattern about a display system optic axis in order to create a shifting phase at a display screen. The shifting phase creates uncorrelated speckle patterns over time, thus employing the time means, taught by Goodman.
Wang et al. in “Speckle reduction in laser projection systems by diffractive optical elements,” Applied Optics, Vol. 37, No. 10, April 1998, pp 1770-1775, teach a method of laser speckle reduction in a laser projection system such as a laser television system. In the laser projection system a laser spot forms an image on a display screen by a raster scan similarly to how an electron beam forms an image in a CRT (cathode ray tube) display. The method taught by Wang et al. is accomplished by expanding a laser beam, placing a diffractive optical element in the expanded laser beam to form multiple beamlets, and then focusing the laser beamlets to form the laser spot on the display screen. The multiple beamlets shift slightly as each pixel is formed on the display screen. This provides a time varying speckle pattern and consequently a speckle reduction. Wang et al. further teach that the diffractive optical element can be rotated to slightly improve the speckle reduction.
Bloom et al. in U.S. Pat. No. 5,982,553 issued on Nov. 9, 1999, incorporated herein by reference, teach a display system including a grating light valve, red, green, and blue lasers, various lens arrangements, a scanning mirror, a display screen, and electronics. The electronics control the grating light valve, the lasers, and the scanning mirror to form a two dimensional image on the display screen.
In the display system taught by Bloom et al., the grating light valve forms a line image composed of a linear array of pixels on the display screen. The scanning mirror repeatedly scans the line image across the display screen in a direction perpendicular to the line image as the grating light valve modulates the linear array of pixels thereby forming the two dimensional image.
Because the two dimensional image taught by Bloom et al. is formed by laser illumination, the two dimensional image exhibits laser speckle, which degrades an image quality. It would be desirable to improve the image quality by reducing the laser speckle.
What is needed is a method of reducing laser speckle in a laser illuminated display system where a two dimensional image is formed on a display screen.
What is needed is a method of reducing laser speckle in an optical system where a laser illumination illuminates a diffuse surface.
SUMMARY OF THE INVENTION
The present invention is a method of reducing speckle, an apparatus for reducing speckle, a display apparatus featuring reduced speckle, and a diffuser for reducing speckle. The method of the present invention includes dividing a laser illuminated area into phase cells, subdividing the phase cells into a number of cell partitions, and applying a temporal phase variation to the cell partitions within an integration time of an intensity detector viewing the laser illuminated area. If the temporal phase variation is optimally applied, the intensity detector detects an optimum speckle reduc
Haverstock & Owens LLP
Silicon Light Machines Inc.
Spector David N.
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