[001] This application claims the benefit of United States (U.S.)

Provisional Application No. 62/243,707 entitled "Speckle Reduction Method for Laser Displays" filed on October 20, 2015. The complete disclosure of U.S. Provisional Application No. 62/243,707 is incorporated herein by reference.

FIELD

[002] The present application relates to methods and apparatuses for laser speckle reduction.

BACKGROUND

[003] Lasers are becoming promising light sources for object scanning and display (e.g., projection display) applications. Compared to traditional light sources, such as light emitting diodes (LED) and lamps, lasers can offer certain advantages, such as wider color gamut, higher brightness, narrow linewidth, longer life time and higher electrical-to-optical efficiency. However, laser speckle can cause image noise on displays and can contribute to detection error during scanning.

SUMMARY

[004] The various embodiments described herein generally relate to systems, apparatuses and methods for reducing speckle.

[005] In accordance with an embodiment, there is provided a system for reducing speckle. The system includes a set of light sources to produce light; a decoherence component to receive light from the set of light sources and to adjust the received light to provide an adjusted light having a reduced level of coherence than the received light; a homogenization component to receive the adjusted light from the decoherence component and to refine the adjusted light to provide a refined light; and an output component to receive the refined light from the homogenization component and to direct the refined light towards an object. [006] In some embodiments, the set of light sources includes two or more light sources.

[007] In some embodiments, the set of light sources includes one or more laser sources.

[008] In some embodiments, the one or more laser sources includes at least one of a single color laser, a three color laser and a laser array having two or more lasers.

[009] In some embodiments, the one or more laser sources includes at least one of a laser diode, a diode pumped solid state laser, and a vertical- cavity surface-emitting laser.

[0010] In some embodiments, the set of light sources can include a first light source generating light characterized by a first set of spectrum characteristics, and a second light source generating light characterized by a second set of spectrum characteristics different from the first set of spectrum characteristics. The spectrum characteristics can include a center wavelength and a linewidth.

[0011] In some embodiments, the output component includes: one or more intermediate optical elements for collecting the refined adjusted light from the homogenization component; a spatial light modulator for receiving the refined adjusted light from the one or more intermediate optical elements, and modulating the refined adjusted light to form one or more images; and a projection lens for projecting the one or more images onto the object.

[0012] In some embodiments, the decoherence component includes: a set of multimode fibers; and a fiber coupler for coupling the set of multimode fibers.

[0013] In some embodiments, the fiber coupler has at least one coil.

[0014] In some embodiments, the fiber coupler includes a tapered fiber end, the tapered fiber end.

[0015] In some embodiments, the system includes at least one lens to collimate the adjusted light from the tapered fiber end and to direct the adjusted light towards the object.

[0016] In some embodiments, the output component includes a balls lens coupled to an end of the fiber coupler for directing the adjusted light towards the object. [0017] In some embodiments, the output component includes a laser generator for receiving the refined adjusted light and directing the refined adjusted light towards the object. The laser generator can be a laser line generator or a laser spot generator

[0018] In some embodiments, the decoherence component includes an electroactive optical diffuser.

[0019] In some embodiments, the homogenization component includes a light pipe. The light pipe can have a rectangular structure.

[0020] In some embodiments, the homogenization component includes a pair of lens array.

[0021] In some embodiments, the decoherence component further includes a multimode fiber. The multimode fiber receives the light from the set of light sources and passes the light to the electroactive optical diffuser.

[0022] In some embodiments, the multimode fiber has at least one coil.

[0023] In some embodiments, the decoherence component further includes a mode scrambler for adjusting a modal distribution within the multimode fiber.

[0024] In some embodiments, the set of light sources includes two or more light sources; and the decoherence component further includes a fiber bundle composed of two or more multimode fibers, each multimode fiber receiving light from a light source of the set of light sources and the fiber bundle passes the light to the electroactive optical diffuser.

[0025] In some embodiments, the fiber bundle has at least one coil.

[0026] In accordance with an embodiment, there is provided an apparatus for reducing speckle. The apparatus includes the elements of the system described herein.

[0027] In accordance with an embodiment, there is provided a method for reducing speckle. The example method includes: identifying a set of light sources, the set of light sources comprises a first light source producing light at a first wavelength and a second light source producing light at a second wavelength different from the first wavelength; obtaining spectrum characteristics for each of the first light source and the second light source; determining a surface roughness of an object on which the speckle is formed; determining a power ratio for one of the first light source and the second light source using the operating parameters and the surface roughness to minimize a speckle contrast ratio (SCR) of the speckle; and setting the first light source and the second light source at a power level in accordance with the power ratio.

[0028] In some embodiments, the spectrum characteristics include a center wavelength and a linewidth for each of the first light source and the second light source.

[0029] In some embodiments, the speckle contrast ratio (SCR) is expressed as: SCR = Κ _β (Δν)μ(Αν) ² άΑν where v represents an illumination frequency, μ represents a complex correlation coefficient of two speckle light fields, and ¾(Δν) = f^ ^∞ g (ξ)§(ξ - Αν)άξ.

[0030] In some embodiments, the complex correlation coefficient of two speckle light fields can be expressed as:

μ(Δν) = M _h (Aq _z ) where M _h represents a first order characteristic function of surface height fluctuations and Aq _z = ^— (cos + cos# ₀ ) , such that 6 _t and θ ₀ are incident and observation angles, respectively. [0031] In some embodiments, the object comprises a display screen and the surface roughness of the display screen is approximately 100μιη.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Several embodiments will now be described in detail with reference to the drawings, in which:

[0033] FIG. 1 a is a block diagram of an example speckle reduction system in accordance with an example embodiment;

[0034] FIG. 1 b is a block diagram of another example speckle reduction system in accordance with an example embodiment; [0035] FIG. 2a is a schematic diagram of an example speckle reduction system in accordance with an example embodiment;

[0036] FIG. 2b is a schematic diagram of another example speckle reduction system in accordance with an example embodiment;

[0037] FIG. 3a is a schematic diagram of another example speckle reduction system in accordance with an example embodiment;

[0038] FIG. 3b is a schematic diagram of another example speckle reduction system in accordance with another example embodiment;

[0039] FIG. 4a is a schematic diagram of another example speckle reduction system in accordance with another example embodiment;

[0040] FIG. 4b is a schematic diagram of another example speckle reduction system in accordance with another example embodiment;

[0041] FIG. 4c is a schematic diagram of a partial example speckle reduction system in accordance with an example embodiment;

[0042] FIG. 5a is a schematic diagram of another example speckle reduction system in accordance with another example embodiment;

[0043] FIG. 5b is a schematic diagram of another example speckle reduction system in accordance with another example embodiment;

[0044] FIG. 6 is a flowchart of an example method of adjusting wavelength blending for reducing speckle in accordance with another example embodiment; and

[0045] FIG. 7 is a graph illustrating simulation and measured results from applying the method of FIG. 6.

[0046] The drawings, described below, are provided for purposes of illustration, and not of limitation, of the aspects and features of various examples of embodiments described herein. For simplicity and clarity of illustration, elements shown in the drawings have not necessarily been drawn to scale. The dimensions of some of the elements may be exaggerated relative to other elements for clarity. It will be appreciated that for simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the drawings to indicate corresponding or analogous elements or steps. DESCRIPTION OF EXAMPLE EMBODIMENTS

[0047] Speckle is a non-uniform pattern caused by random interference from coherent and/or partially coherent light sources, such as lasers. The random interference of coherent and/or partially coherent laser light can result in non-uniform distribution of the light intensity, resulting in the non-uniform pattern. As a result, speckle can cause errors.

[0048] For example, in display applications, the speckle can cause image noise. In laser scanning application, speckle can contribute to detection error during scanning. Laser scanning applications typically rely on the detection of the center of a laser line or spot. When scanning an object, speckle can cause errors during the detection of the center of the laser line or spot.

[0049] Reducing the effects of speckle, therefore, can improve the quality of applications that use lasers as light sources. Some prior systems rely on moving components, such as rotating lens array or vibrating mechanisms, for reducing the effects of speckle. These moving components, however, can cause instability to the system, which is critical since accuracy is essential for measurement systems, such as in laser imaging applications for example.

[0050] The level of speckle can be represented by a speckle contrast ratio (SCR). The speckle contrast ratio is defined as the standard deviation of intensity fluctuation (σ) over the average intensity (I) of the speckle, and can be expressed as follows:

SCR = - Eqn. (1 ) where SCR represents the speckle contrast ratio. Typically, a speckle contrast ratio of about 5% is the threshold at which the human eyes will see the speckle.

[0051] FIG. 1 a is a block diagram of an example speckle reduction system 100a. The speckle reduction system 100a includes a light source 1 10, a decoherence component 120, a homogenization component 130, and an output component 140.

[0052] The light source 1 10 can be a laser source in some embodiments, such as when the speckle reduction system 100a is used in a laser display application. Example laser sources can include one or more laser diodes, one or more diode pumped solid state lasers, and/or vertical-cavity surface- emitting lasers. The light source 1 10 can include a single color laser, a three color laser or a laser array with multiple lasers.

[0053] In laser scanning applications, the laser source can have scanning ranges up to 100 mm and a power of approximately 100 mW and greater, which will be suitable for medium-range and short-range laser scanning applications. For medium-range laser scanning applications, the laser source can generate light with a laser spot size of 200 to 1000 μηι and a field depth of 10 to 100 mm. For short-range laser scanning applications, the laser source can generate light with a laser spot size in a range of 10 to 200 μηι, and a field depth of 1 to 10 mm.

[0054] The decoherence component 120 can reduce the coherence of the light from the light source 1 10. Example implementations of the decoherence component 120 can include an electroactive optical diffuser (EOD), a multimode fiber (MMF), a fiber bundle formed of multimode fibers and/or a mode scrambler.

[0055] One example structure for the electroactive optical diffuser is a dielectric elastomer actuator. The dielectric elastomer actuator includes an electroactive polymer film with a rigid diffuser attached to it. Electrodes are connected to the electroactive polymer film so that when variable voltage is applied to the polymer film, the polymer film vibrates. The vibration at the polymer film causes the rigid diffuser to also vibrate, thereby homogenizes the light.

[0056] The diffuser surface can act as multiple uncorrelated structure cells. When laser light illuminates the dielectric elastomer actuator, the light passing through different uncorrelated structure cells overlap with each other and result in speckle suppression. When the diffuser is vibrating faster than the integration time of the detector (e.g. human eyes), the averaging effect can further decrease the appearance of speckles to the human observer.

[0057] Another example decoherence component 120 includes the multimode fiber.

[0058] A multimode fiber can have a core diameter on the order of hundreds of microns to support a large number of propagation modes. Different propagation modes refer to propagation of the light beams at different angles within the multimode fiber. As a result, the light travels along the multimode fiber in different optical paths. When a difference between the optical paths of two propagation modes are longer than a coherence length of the light source, the light travelling in these optical paths are out of phase and these two propagation modes are incoherent. When the light exits the multimode fiber, its coherence is reduced and therefore, the resulting speckle, if any, also reduces.

[0059] The multimode fiber, in some embodiments, can include at least one coil. The coil in the multimode fiber can result in higher propagation modes within the multimode fiber. The higher propagation modes involve light that travels longer optical paths and have stronger de-speckle effects. In some embodiments, the multimode fiber can include a coiled step index multimode fiber. The core diameter of the multimode fiber can vary with the design and/or operating parameters of the speckle reduction system 100a. For example, the core diameter can be 400 μιη.

[0060] A mode scrambler can, in some embodiments, be included with the multimode fiber. The mode scrambler can generate more modes and change the modal distribution inside the multimode fiber. The mode scrambler can increase the uniformity of the light field inside the multimode fiber.

[0061] The homogenization component 130 can refine the light received from the decoherence component 120. For example, the homogenization component 130 can improve the uniformity of the light to reduce speckle. The homogenization component 130 can, in some embodiments, adjust the light into a desired beam shape. Example implementations of the homogenization component 130 can include a light pipe and a lens system including one or more lens arrays. The light pipe may have a rectangular structure.

[0062] The output component 140 can collect light from the homogenization component 130 and direct the light to an object 150. The object 150, in some embodiments, can be a screen.

[0063] FIG. 1 b shows a block diagram of another example speckle reduction system 100b. As shown in FIG. 1 b, the output component 140' can include intermediate optical elements 142, a spatial light modulator 144 and a projection lens 146.

[0064] The intermediate optical elements 142 can collect and pass the light from the homogenization component 130 to the spatial light modulator 144. The spatial light modulator 144 receives the light from the intermediate optical elements 142 and modulates the light to form images. The projection lens 146 projects the image formed by the spatial light modulator 144 onto the object 150.

[0065] Example configurations of the speckle reduction system 100a will now be described with reference to FIGS. 2a to 5b.

[0066] FIG. 2a shows an example speckle reduction system 200a.

[0067] The speckle reduction system 200a includes a laser source 210 that directs light towards a decoherence component 220. In the speckle reduction system 200a, the decoherence component 220 includes an electroactive optical diffuser. The light passes the electroactive optical diffuser

220 and enters the homogenization component 230a.

[0068] The homogenization component 230a in FIG. 2a is a light pipe. The internal reflections within the light pipe create virtual images which overlap with each other at the end of the light pipe, creating a uniform light field with low speckle.

[0069] From the homogenization component 230a, the light is collected by the output component 240 and is then projected by the output component 240 towards the object. In the embodiment shown in FIG. 2a, like the example system shown in FIG. 1 b, the output component 240 can include intermediate optical elements 242, the spatial light modulator 244, and a projection lens 246 that projects the light to the object 150, which is a screen in this example.

[0070] FIG. 2b shows another example speckle reduction system 200b.

[0071] Similar to the speckle reduction system 200a of FIG. 2a, the speckle reduction system 200b includes the electroactive optical diffuser as the decoherence component 220. Unlike the speckle reduction system 200a, the speckle reduction system 200b includes a lens system as the homogenization component 230b. The lens system 230b includes two lenses 232, 234 and a pair of lens arrays 236 with a first lens array 236a and a second lens array 236b.

[0072] The first lens 232 is positioned to receive a diffused light from the electroactive optical diffuser 220 and to collimate the diffused light. The collimated light then passes through the pair of lens arrays 236. The second lens 234 is positioned to collect the light from the second lens array 236b and to form a low speckle uniform light field at its image plane. In some embodiments, the second lens array 236b is positioned at the focal plane of the first lens array 236a.

[0073] FIGS. 3a and 3b show example speckle reduction systems 300a and 300b, respectively.

[0074] The speckle reduction system 300a includes a laser source 310 that directs light towards a decoherence component 320 via a coupling element 312. The decoherence component 320 directs light towards the homogenization component 330a, which is a light pipe in this embodiment. The homogenization component 330a then directs light to the output component 340. Similar to the systems 200a and 200b, the output component 340 includes intermediate optical elements 342, the spatial light modulator 344 and the projection lens 346 for projecting the light onto the object 150.

[0075] Unlike the speckle reduction system 300a, the speckle reduction system 300b includes a lens system as the homogenization component 330b. Similar to the lens system 230b shown in FIG. 2b, the lens system 330b includes a first lens 332, a second lens 334 and a pair of lens 336 with a first lens array 336a and a second lens array 336b.

[0076] The decoherence component 320 in the speckle reduction systems 300a and 330b includes a multimode fiber 322 and an electroactive optical diffuser 324. In the speckle reduction systems 300a and 300b, the light source 310 directs light towards the coupling optics 312. The coupling optics 312 then couples the light for the multimode fiber 322. The multimode fiber 322 can reduce the coherence of the light beam generated by the laser source 310.

[0077] The multimode fiber 322 can include at least one coil. The number of coils in the multimode fiber 322 can vary with the operating parameters of the speckle reduction system 300a, 300b. In some embodiments, a mode scrambler can be included with the multimode fiber 322.

[0078] FIGS. 4a and 4b illustrate example speckle reduction systems 400a and 400b, respectively.

[0079] The speckle reduction systems 400a and 400b include a set of light sources 410 composed of one or more light sources 41 On _! to 41 On,. Each light source 41 On, directs light to a corresponding multimode fiber 422n, via a coupling element 412n,. As shown in FIGS. 4a and 4b, a set of multimode fibers 422^ to 422n, is provided and combined into a fiber bundle 423. The decoherence component 420 of the speckle reduction systems 400a and 400b includes the fiber bundle 423 and an electroactive optical diffuser 424 that receives light from the fiber bundle 423.

[0080] The number of multimode fibers 422n, within the fiber bundle 423 can vary with the design and/or operating parameters of the speckle reduction systems 400a, 400b, such as illumination requirements and laser performance. The multimode fibers 422^ to 422n, can be input multimode fibers, for example.

[0081] The multimode fibers 422n, in the speckle reduction systems 400a and 400b shown in FIGS. 4a and 4b are combined to form the fiber bundle 423. FIG. 4c illustrates another example fiber bundle 423' that includes at least one loop.

[0082] From the fiber bundle 423, 423, light is directed towards the homogenization component 130. The homogenization component 130 in FIG. 4a includes a light pipe 430a and the homogenization component 130 in FIG. 4b includes a lens system 430b. Similar to the lens systems 230b and 330b, the lens system 430b includes a first lens 432, a second lens 434 and a pair of lens 436 with a first lens array 436a and a second lens array 436b.

[0083] The homogenization component 430a, 430b then directs light to the output component 440 for directing the light onto the object 150.

[0084] FIGS. 5a and 5b illustrate example speckle reduction systems 500a and 500b. A camera 560 can collect data of the light being projected onto the object 150.

[0085] Similar to speckle reduction systems 400a and 400b, speckle reduction systems 500a and 500b include a set of light sources 510 composed of one or more light sources 510ni to 51 On,. Each light source 51 On, directs light to a corresponding multimode fiber 522n,. As shown in FIGS. 5a and 5b, a set of multimode fibers 522^ to 522n, is provided and coupled with a fiber coupler 523a. The fiber coupler 523a in the illustrated examples includes at least one coil.

[0086] In the speckle reduction system 500a, the homogenization component 130 and output component 140 includes a ball lens 530a coupled to the end of the fiber coupler 523a. The ball lens 530a directs a collimated beam towards the object 150. Depending on the design of the ball lens 530a, the collimated beam can have a diameter of less than 1 mm and a field of depth of 10 to 100 mm, which can be suitable for medium range laser scanning.

[0087] In the speckle reduction system 500b, the fiber coupler 523b has a tapered fiber end 528. The fiber coupler 523b directs the light towards the homogenization component 530b and output component 540, which can be provided as a pair of collimation lens 532a, 532b in some embodiments. The tapered fiber end 528 can reduce the spot size. For example, the laser spot projected onto the object 150 can have a diameter within a range of 10 to 200um and a depth of field of 1 to 10mm, which can be suitable for short range laser scanning.

[0088] In embodiments when laser line scanning is required, the fiber coupler 523a, 523b can direct the light towards a line generator (not shown). The fiber coupler 523a, 523b can direct the light towards the line generator using a Powell lens or a cylinder lens. The line generator can act as the output component 140 and throw the laser line or spot onto the object 150.

[0089] The speckle reduction systems described herein can be used for various different applications, such as, but not limited to, laser scanning, laser display and confocal microscopy. Example laser scanning applications can include three-dimensional laser scanning for constructing three-dimensional models.

[0090] In some embodiments, wavelength blending can be applied to reduce speckle effect. In speckle reduction systems in which an "n" number of lasers with different wavelengths are used, an "n" number of independent speckle patterns will be generated. As a result of wavelength blending, the speckle contrast ratio can be suppressed by l/Vn. For example, when light provided by two different laser sources have monochromic spectrums and equal power, the speckle contrast ratio will be reduced by V2. However, when the light sources 1 10 are characterized by different wavelengths and linewidths, a power ratio for the different light sources 1 10 need to be determine in order to minimize the speckle effect. [0091] For a speckle reduction system 100a with an "n" number of light sources 1 10, the normalized power spectrum density function for the speckle reduction system 100a can be represented as follows:

§ =∑ Ci9i Eqn. (2) where∑ ₌₁ C _t = 1 , g _n represents the normalized power spectrum density function of the n ^th wavelength, and C _n is the power ratio of the n ^th wavelength over the total power.

[0092] When the light from the light source 1 10 directly illuminates onto the object 150, the speckle intensity can be determined from the speckle captured by a detection lens of the camera 560. To obtain the expression for the speckle contrast ratio, the average speckle intensity 7 and standard deviation of the light field / are required.

[0093] The average speckle intensity 7 can be expressed as follows:

x. y) = Jo g i(x, y, v)dv Eqn. (3) where /(x,y,v) represents the speckle intensity at point (x,y) with an illumination frequency of v. From Eqn. (3), it can be seen that I ² can be expressed as follows:

I ² (x, y) = \ g (v ₁ )g(v ₂ ) I(x, y, v ₁ )I(x, y, v ₂ )dv ₁ dv ₂

Jo JQ

= K _§ (Av)T,(Av)dAv Eqn. (4) where: Κ _§ (Αν) = ί+ ^∞ § (ξ) ₉ (ξ - Αν)άξ Eqn. (5) and 1} is the statistical autocorrelation function of /(x,y,v). According to the circular complex Gaussian statistics, Γ, can be written as follows: ,(Av) = 7 ² (1 + μ(Δν) ² ) Eqn. (6)

[0094] By inserting Eqn. (6) into Eqn. (4) and combining it with Eqn. (3), the standard deviation of the light field / can be expressed as follows:

-2

σ ² = I ² - I ¾(Δν)Γ(1 + μ(Δν) ² )ίίΔν - ( [ g ldv

Jo = 7 ^ Κ _β (Αν)μ(Αν) ² άΑν Eqn. (7)

Thus, the speckle contrast ratio (SCR) of the speckle reduction system 100a can be expressed as follows:

where μ represents the complex correlation coefficient of two speckle light fields. By using the random height screen model, μ(Δν) can be expressed as follows: μ(Δν) = M _h (Aq _z ) Eqn. (9) where M _h represents the first order characteristic function of the surface height fluctuations and Aq _z = (cos + cos# ₀ ) , such that θ and θ ₀ are incident and observation angles, respectively. For an object with a rough surface with height fluctuation of Gaussian distribution:

\M _h (Aq _z ) \ ² = expC-σ^!) Eqn. (10) where a _h is the standard deviation of surface height fluctuation. For conventional display screens, a _h is approximately 100 μιη.

[0095] Based on the above derivation, the level at which the speckle effect is reduced as a result of wavelength blending can be determined. Using the normalized power spectrum density function g of the speckle reduction system 100a (Eqn. (2)) and the standard deviation of the surface roughness a _h of the object 150, the speckle contrast ratio can be determined.

[0096] FIG. 6 is a flowchart of an example method 600 for determining the power ratio for the light sources 1 10 to apply wavelength blending for reducing speckle.

[0097] At 610, identify the light sources 1 10 to be used in the speckle reduction system 100a, and at 620, for each light source 1 10, the spectrum characteristics, such as center wavelength and linewidth, of each light source 1 10 is obtained, which correspond to § _t in Eqn. (2).

[0098] For example, in laser scanning applications, red lasers with a center wavelength around 650 nm can be used. For laser display applications, red lasers with a center wavelength around 640 nm, green lasers with a center wavelength around 530 nm, and blue lasers with a center wavelength around 450 nm can be used.

[0099] At 630, the surface roughness of the object 150 is determined, which correspond to a _h in Eqn. (10).

[00100] At 640, the power ratio of each light source 1 10 is determined.

[00101] The power ratio of the light sources 1 10 can vary depending on the desired application. For example, the power ratio can be determined for minimizing the speckle contrast ratio, achieving a certain color hue, and/or reducing the cost of the light sources 1 10 while retaining a certain quality at the application. With the center wavelength and linewidth measured for each light source 1 10, and the surface roughness of the object 150, the various speckle contrast ratios can be determined using Eqn. (8) by changing the power ratio of c _t from 0 to 1 . An example will now be described with reference to FIG. 7.

[00102] FIG. 7 is a graph 700 illustrating simulated results 710 and measured results 720 resulting from wavelength blending in an example speckle reduction system described herein.

[00103] To conduct the measurements, a diode pumped solid state (DPSS) green laser is used as a first light source 1 10^ and a semiconductor green laser diode is used as a second light source 1 10n ₂ . The DPSS laser has a center frequency of 532 nm and a linewidth of about 0.15 nm, and the semiconductor laser diode has a center frequency of 520 nm and a linewidth of about 1 .5 nm. In this example, the light from the DPSS laser and the semiconductor laser diode are combined with a polarization beam splitter (PBS). The combined light was enlarged by a concave lens and projected onto the object 150, which is a screen in this example. The value of a _h for the screen was estimated as 100 μηι.

[00104] For the simulation, a Gaussian spectrum with a 1/e ² width of 0.15 nm and center wavelength of 532 nm was defined for the DPSS laser, and it was assumed that the semiconductor laser diode has several longitude modes around 520 nm, and the envelop of these longitude modes can be roughly considered as a Gaussian function with a 1/e ² width of 1 .5 nm. The value of a _h was estimated as 100 μηι.

[00105] To determine the speckle contrast ratio as a function of a power ratio (Ci) for the DPSS laser and a power ratio (C ₂ ) for the semiconductor laser diode, a normalized power spectrum density function (g^ for the DPSS laser, a normalized power spectrum density function ( g ₂ ) for the semiconductor laser diode, and a _h are entered into Eqn. (8). The power ratio (Ci) represents a ratio of a power of the DPSS laser and the total power of the two light sources (P-rotai), and the power ratio (C ₂ ) represents a ratio of a power of the semiconductor laser diode and the total power of the two light sources (P-rotai)- For this example, it is assumed that the incident and observation directions are perpendicular to the screen 150. By varying the power ratio Ci from 0 to 1 , the speckle contrast ratio can be determined as a function of the power ratio (Ci) of the DPSS laser. Similarly, by varying the power ratio C ₂ from 0 to 1 , the speckle contrast ratio can be determined as a function of the power ratio (Ci) of the semiconductor laser diode.

[00106] Referring to FIG. 7, the x-axis of the graph 700 represents the power ratio (Ci) of the DPSS laser, and the y-axis of the graph 700 represents the speckle contrast ratio. A power ratio of 0 indicates that the light source 1 10 is purely the DPSS laser, while a power ratio of 1 indicates that the light source 1 10 is purely the semiconductor laser diode. As can be seen from the measured results 720, the lowest speckle contrast ratio occurs when the power ratio of the DPSS laser is approximately 0.8.

[00107] At 650, the light sources 1 10 are set at the center wavelength and power in accordance with the power ratio determined at 640. In some embodiments, the speckle reduction system 100a may be simulated to determine whether the parameters of the light sources 1 10 comply with the design parameters.

[00108] At 660, if it is determined at 650 that the settings of the light sources 1 10 are not within the design parameters, steps 630 to 650 can be repeated. If it is determined at 650 that the settings of the light sources 1 10 are within the design parameters, the method 600 can terminate. [00109] Referring again to FIGS. 4a to 5b, the wavelength of the light from each of the light sources 41 On _! to 41 On, and 51 On _! to 51 On, can be blended with the method described with respect to FIG. 6. To minimize speckle, the power ratio of the light sources 41 Oni to 41 On, and 51 On _! to 51 On, can be adjusted in accordance with the method 600.

[00110] While the present application has been described with reference to examples, it is to be understood that the scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

[00111] All publications, patents and patent applications are herein incorporated by reference in their entirety. Where a term in the present application is found to be defined differently in a document incorporated herein by reference, the definition provided herein is to serve as the definition for the term.