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Browse Prior Art Database

Perceived Speckle Reduction in Projection Display Systems

IP.com Disclosure Number: IPCOM000118774D
Original Publication Date: 1997-Jul-01
Included in the Prior Art Database: 2005-Apr-01
Document File: 4 page(s) / 134K

Publishing Venue

IBM

Related People

Kerigan, SC: AUTHOR [+2]

Abstract

Projection systems use small light sources in order to maximize the optical efficiency of the overall system. The image of the light source within the projection optics is sufficiently small in order to behave as a coherent source. The result produces the appearance of speckle in the image when observed by the human eye. The appearance of speckle is a random variation in the screen brightness, the resolution of this pattern being of the order of the spatial resolution of the eye. Speckle results from constructive and destructive interference of a coherent (or partially coherent) light source at an object plane because of 'temporally constant' phase differences in the incident light paths. Two forms of speckle can be observed: 1.

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Perceived Speckle Reduction in Projection Display Systems

      Projection systems use small light sources in order to maximize
the optical efficiency of the overall system.  The image of the light
source within the projection optics is sufficiently small in order to
behave as a coherent source.  The result produces the appearance of
speckle in the image when observed by the human eye.  The appearance
of speckle is a random variation in the screen brightness, the
resolution of this pattern being of the order of the spatial
resolution of the eye.  Speckle results from constructive and
destructive interference of a coherent (or partially coherent) light
source at an object plane because of 'temporally constant' phase
differences in the incident light paths.  Two forms of speckle can be
observed:
  1.  Subject Speckle
        This is observed when a screen is illuminated by a
       'uniform' coherent light source viewed with an imaging
       system.  The interference pattern is produced on the
       detector of the system.  In the case of the human eye,
       the pattern is produced on the retina and is a function
       of aperture of the eye.  The pattern will be seen to
       move with respect to the screen coordinate system if the
       eye position is changed.  A spatially resolvable detector
       placed at the screen surface will not detect any spatial
       variation due to subjective speckle.
  2.  Object Speckle
        This is observed when a secondary object plane is imaged
       onto a screen by a lens system.  The resultant speckle
       pattern is  produced on the screen and can be detected
       by a spatially resolvable detector placed on the screen
       surface.

      Certain conditions have to be met in order for speckle to be
observed.  These apply to both the physical system and the observer.
  1.  Physical Constraints
        The projected light must be perturbated by a rough surface
       or medium.  The roughness should be of the order of, or
       greater than, the wavelength of the light.  This criteria
       is easily met within most optical projection systems used
       for display applications.
        The light must have a coherence length greater than
       the amplitude of the roughness which is causing the
       speckle.  This is required for interference to take
       place.  This condition is also usually met within most
       optical projection systems.
        The light must be spatially coherent.  This requires that
       there is transverse coherence of the illuminating light
       source.  For most systems other than those employing lasers,
       the angular variation in illumination will give a coherence
       length that is substantially smaller than resolving powe...