When laser light is reflected from a surface such as paper, a wall or a projection screen, a high contrast, fine scale granular pattern called laser speckle is seen by an observer looking at the illuminated spot. From the earliest days of lasers, the cause of speckle was recognized to be the fact that most materials are randomly rough on the scale of an optical wavelength, with the exception of highly polished surfaces such as mirrors. Upon reflection from a rough surface, different facets contribute elementary wavelets with slightly different optical path lengths that interfere with each other upon propagation, producing the high contrast, fine scale speckle pattern. The statistics of the light intensity in the speckle pattern can be related to the statistics of the rough surface, the size of the scattering spot, the wavelength of the light, and other parameters of the illuminating beam. The speckle phenomenon is not confined to visible light, and is well known in radar and other regions of the electromagnetic spectrum. In fact, speckle is a fundamental part of any wave or field phenomenon, and is also well known in applications such as ultrasound, and even in the study of gravitational fields in astrophysics.
Because of the sensitivity of laser imaging systems to speckle, it is useful in the metrology or characterization of rough surfaces, or other scattering objects such as small particles. In image formation, however, speckle is a noise source that interferes with the image or signal information, and must be reduced or eliminated. In particular, in laser projection systems used in near-to-eye, office or theatre environments, it is critical that speckle be minimized so that it does not interfere with the clarity or enjoyment of the image. Many methods have been suggested in the prior art to reduce speckle in projection imaging systems. They can be divided roughly into two classes: 1) methods pertaining to reduction of speckle at the projection source and 2) methods pertaining to reduction of speckle at the projection screen or surface where the final image is formed. All classic methods of speckle reduction involve an averaging process, as explained in the text “Speckle Phenomena in Optics”, by J. W. Goodman, Roberts and Co., Englewood Colo., 2007. That is, a number of statistically independent speckle patterns must be either simultaneously averaged, or averaged within a time period shorter than the response time of the human eye. Methods reviewed by Goodman include averaging over wavelength (increasing the bandwidth of the optical system), averaging over time (using a moving diffuser or changing phase mask in the projector) and overdesign of the projection optics relative to the eye (averaging many projector blur spots within a single eye blur spot).
A successful laser projection system can apply multiple methods of speckle reduction, at both source and screen, since most methods fail to eliminate speckle entirely in a single step. Speckle is characterized by the mean intensity <I> and standard deviation σI of its intensity (I), which are combined into a single metric of speckle contrast C:
                              C          =                                    σ              I                                      〈              I              〉                                      ,                            (        1        )            where 0≤C≤1. This can also be expressed as a percent. Each method of speckle reduction is as good as the effective number of statistically independent speckle patterns it generates. Here “effective” means the number of statistically independent patterns generated within the integration or response time of the eye, detector or camera used to observe them. If reduction method i generates Ni independent patterns, it contributes a factor 1/√{square root over (Ni)} to the overall speckle reduction. If M methods are used in total, the overall speckle reduction factor R is
                              R          =                                    [                                                ∏                                      i                    =                    1                                    M                                ⁢                                  N                  i                                            ]                                      1              /              2                                      ,                            (        2        )            so that the reduced speckle contrast isC′=C/R.  (3)As mentioned earlier, one method of generating multiple speckle patterns is to use a moving diffuser or changing phase screen in projector, or in the optical beam between the projector and screen. In a well-known method, a rotating diffuser or phase plate is placed in the beam of a projection system either in the projection optics, or between the projector and the screen, so that the imaging beam suffers random phase delays across its extent. A fixed diffuser would merely lead to a slightly different speckle pattern than if it were absent. However, by moving the diffuser plate or phase plate, a series of speckle patterns is created. If the plate is moved rapidly and through a sufficient distance, enough independent speckle patterns are generated to reduce the overall noise.
Other methods of speckle reduction between the projector and screen are directed towards reducing the bulk of the diffuser mechanism. For example, U.S. Pat. No. 8,500,287 (Moussa) describes a device based on a piezoelectric actuator, in which a diffuser is fixed inside a vibrating metallic frame. The frame moves the diffuser laterally, i.e. in the plane of the frame. In another example, U.S. Pat. Nos. 8,553,341 and 8,902,520 (both to Aschwanden) describe an electroactive optical device suitable for inclusion in the optical beam between the projector and the screen. The device is comprised of a pre-stretched polymer film with electrodes on both surfaces, and a rigid optical element (such as a diffuser) connected to either surface or the polymer film. The application of a voltage to the electrodes displaces the optical element along the plane of the polymer film, due to Coulomb forces. In-plane displacements of a diffuser or phase plate are used to create a series of independent speckle patterns. The in-plane displacements by be rotational or lateral in the x and y directions. Out-of-plane movements in the z direction are disclosed in U.S. Pat. No. 8,500,287 as bending the plate across the full width of the image beam causing image distortion and loss of image sharpness as the speckle reduction averaging process also averages the distortions. Problems with the placement of moving diffusers into the beam optics include scatter from the diffuser reducing the sharpness of the focused image, or creating haze in the image. Reduction of the diffusive properties of the diffuser to reduce haze in the image can render the diffuser ineffective in reducing speckle as demonstrated in comparative EXAMPLE 1. While the loss of image sharpness can be eliminated by positioning of the diffuser and optical modulator before the beam-shaping element of an imaging system and focusing the dispersed beam onto the beam-shaping element, the effect of the optical modulator is reduced or eliminated as demonstrated in comparative EXAMPLE 2. The loss of image sharpness and hazing cause by diffusers can be reduced by placing a diffuser closer to the screen, or nearly in contact, but then the diffuser becomes very large and hard to move laterally.
This leads to the second class of speckle reduction methods, directed towards improvements to the projection screen. In one example, U.S. Pat. No. 6,122,023 (Chen et. al.) describes a liquid crystal projection display screen constructed in a highly scattering state. The display includes a plurality of liquid crystal spheres. When no voltage is applied, the medium is highly scattering and the screen is opaque. When a voltage is applied, the liquid crystal molecules are aligned and the light is transmitted. When the voltage is varied with a 60 Hz signal, the spheres vibrate, causing a varying speckle pattern which the eye averages. In another example, U.S. Pat. No. 8,724,218 (Curtis et. al.) describes speckle reduction using mechanical vibration of the screen. Devices near, but not in contact with, the screen generate acoustic or electromagnetic waves that couple to the screen and produce mechanical vibrations, creating a changing speckle pattern. The screen vibration may occur in all of the x, y and z directions, that is to say movement in the x and y directions are lateral movements in the plane of the screen and movement in the z direction are axial movements perpendicular to the screen. Such mechanical vibration methods must be carefully tuned to avoid standing wave patterns and regions of uneven or zero vibration. None of the speckle reduction methods cited report quantitative performance results, or discuss their potential impact on image sharpness. There is still a need, therefore, for additional methods of laser speckle reduction in projection systems.