Lens for speckle free scanning mirror type head up displays and optical feedback for focus tunable lenses
Publication Date: 2014-Jun-18
The IP.com Prior Art Database
EP141168623.8: PATENT [+2]
This document describes the a passive solution to avoid speckles in a Head-up display. Furthermore in a second chapter an optical feedback solution to measure and control the focal length of focus tunable lenses is described.
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Laser Projection System for Reducing Speckle Noise
The present invention relates to a laser projection system for reducing Speckle noise according to claim 1.
Laser speckles are one of the major obstacles for laser projection systems. While area based projector technologies such as LCOS (for Liquid Crystal On Silicon) or DLP (for Digital Light Processor) based projectors can remove speckles using
moving diffusers, the integration time in scanning mirror type laser projection systems is not long enough to have an efficient despeckling using moving diffusers.
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 15
scattered from each point on the illuminated surface. If the surface is rough enough to create path-length differences exceeding for example one wavelength, giving rise to phase changes greater than 2, the amplitude, and hence the intensity, of the resultant light on a detector, where interference of different secondary spherical waves occurs 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 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 two-dimensional 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 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- 30
tern is the same regardless of the direction from which it is imaged, just as if it were a painted pattern.
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The "size" of the speckles is a function of the wavelength of the light, the size of the 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 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 5
the edge of the illuminated area changes by , 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 system images a coherently illuminated surface. The lenses of the imaging system focus light from different angles...