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Optical system for avoiding speckle patterns

IP.com Disclosure Number: IPCOM000240137D
Publication Date: 2015-Jan-06
Document File: 36 page(s) / 1M

Publishing Venue

The IP.com Prior Art Database

Abstract

The invention relates to an optical system, comprising: a light source, particularly a laser, designed to emit a light beam, a light deflector, wherein the light source is configured such that said light beam impinges on the light deflector, an exit pupil expander in the form of a lens or mirror array, wherein the light deflector is designed to deflect the light beam onto the exit pupil expander so as to generate a projection image comprising image pixels on the exit pupil expander, wherein the exit pupil expander is designed to diverge the light beam, particularly such that the projection image can be seen by an observer from multiple viewing angles. According to the invention, the optical system comprises an optical device being designed to interact with said light beam such that the light beam comprises a uniform light intensity distribution on the exit pupil expander.

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Specification

The invention relates to an optical system, particularly a laser projection system, for avoiding speckle patterns as well as Moiré patterns.

    Laser speckles are one of the biggest obstacles for laser projection systems. The speckle effect is a result of the constructive and destructive interference of many waves of a coherent laser light resulting in a randomly varying intensity profile of a 10

light projection.

    When a surface is illuminated by a light wave, according to diffraction theory, each point on an illuminated surface acts as a source of secondary spherical waves. The light at any point in the scattered light field is made up of waves which have been scattered from each point on the illuminated surface. If the surface is rough enough 15

to create path-length differences exceeding for example one wavelength, giving rise to phase changes greater than 2π, the amplitude, and hence the intensity, of interfering light varies randomly.

    In a projection system, two types of speckles can be distinguished, namely subjective and objective speckles. The objective speckles are interference patterns which are 20

    generated on a surface. In particular, objective speckles can be seen very well, when laser light has been scattered off a rough surface and then falls on another surface. For example, if a photographic plate or another 2-D optical sensor is located within the scattered light field without a lens, a speckle pattern is obtained whose characteristics depends on the geometry of the system and the wavelength of the 25

laser. The light at a given point in the speckle pattern is made up of contributions from the whole of the scattering surface. The relative phases of these waves vary across the surface, so that the sum of the individual waves varies randomly. The pat- tern is the same regardless of how it is imaged, just as if it were a painted pattern.

    The "size" of the speckles is a function of the wavelength of the light, the size of the 30

laser beam which illuminates the first surface, and the distance between this surface and the surface where the speckle pattern is formed. This is the case because when

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Optical system for avoiding speckle patterns

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the angle of scattering changes such that the relative path difference between light scattered from the center of the illuminated area compared with light scattered from the edge of the illuminated area changes by λ, the intensity becomes uncorrelated.

   The second type of speckles is the so called subjective speckles. Subjective speckles are created when an observer, for example an eye or another imaging 5

system images a coherently illuminated surface. The lenses of the imaging system focus light from different angles onto an imaging point (pixel), resulting in the interference of the light on this point. When the light has a disturbed wavefront, or the imaging system itself introduces a large disturbance of the wavefront, the lig...