Illumination system to eliminate laser speckle and a single-panel projection system employing the same

[0001] This application claims priority from Korean Patent Application No. 2004-86543, filed on Oct. 28, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

[0002] 1. Field of the Invention

[0003] The present general inventive concept relates to an illumination system designed to eliminate speckle produced by laser light and a projection system employing the same, and more particularly, to an illumination system designed to efficiently reduce or eliminate laser speckle using a movable diffractive optical element to temporally and spatially average partial beams and a single-panel projection system employing the same.

[0004] 2. Description of the Related Art

[0005] A projection system that uses a laser light source provides a wide range of color rendition due to a wide color gamut while achieving high optical efficiency due to superior collimation. However, the projection system that uses the laser light source suffers from speckle caused by coherency of a laser beam. The speckle results from interference of beams having a random phase when coherent beams are reflected from a rough surface. The speckle degrades a resolution and a quality of an image produced by the projection system.

[0006] FIG. 1 illustrates a conventional illumination system disclosed in U.S. Pat. No. 6,606,173 B2 that attempts to eliminate laser speckle. Referring to FIG. 1, the conventional illumination system includes a laser light source 1, a beam expander 2, a holographic diffuser 3, a first field lens 4, and a second field lens 5. All elements of the conventional illumination system are coaxially aligned to convert a Gaussian laser beam emitted by the laser light source 1 into a plurality of uniform beams having rectangular cross-sections.

[0007] FIG. 2 illustrates beams having rectangular profiles created by the holographic diffuser 3 in the conventional illumination system of FIG. 1. Referring to FIGS. 1 and 2, a beam emitted by the laser light source 1 is converted to a parallel expanded beam having an increased diameter that is then partitioned into a plurality of rectangular beams by the holographic diffuser 3. Each rectangular beam diverges with horizontal and vertical angles θ_horiz and θ_ver, respectively, that can be controlled by a diffraction pattern of the holographic diffuser 3. The first field lens 4 superimposes the plurality of rectangular beams in order to convert the beam having a Gaussian distribution (i.e., the Gaussian laser beam) into a beam having a uniform intensity and to adjust a small rectangular profile of the uniform intensity beam to match an aspect ratio of a micro-display. The second field lens 5 converts the beam that passes through the first field lens 4 to be telecentric.

[0008] The holographic diffuser 3 produces rectangular beams with uniform intensity distributions that are then superimposed on one another, thereby eliminating speckle. However, using only the holographic diffuser 3 degrades efficiency of speckle removal and image quality.

[0009] The present general inventive concept provides a slim and compact illumination system designed to efficiently reduce or eliminate laser speckle by rotating or periodically moving a diffractive optical element partitioned according to wavelength and a projection system employing the same.

[0010] Additional aspects of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept.

[0011] The foregoing and/or other aspects of the present general inventive concept may be achieved by providing an illumination system including: a laser light source to sequentially emit a plurality of laser beams, and a first diffractive optical element that is partitioned into a plurality of segments having diffractive patterns that correspond to the plurality of laser beams to move synchronously with the sequential emission of the plurality of laser beams. Laser speckle is removed by moving the first diffractive optical element in such a manner as to temporally average the plurality of laser beams.

[0012] The first diffractive optical element may have a wheel-like shape that is rotatable. Alternatively, the first diffractive optical element may have a rectangular shape that is linearly reciprocatable. The first diffractive optical element may convert an incident beam emitted by the laser light source into a plurality of beamlets and superimpose and focus the plurality of beamlets, thereby spatially averaging the incident beam.

[0013] The illumination system may further include a second diffractive optical element to shape a beam that passes through the first diffractive optical element to match an aspect ratio of a display device that produces an image from the shaped beam. The second diffractive optical element may control a divergence angle of an incident beam in such a manner as to increase an F-number.

[0014] The foregoing and/or other aspects of the present general inventive concept may also be achieved by providing a projection system including: a laser light source to sequentially emit a plurality of laser beams, a first diffractive optical element that is partitioned into a plurality of segments having diffractive patterns that correspond to the plurality of laser beams to remove laser speckle by moving synchronously with the sequential emission of the plurality of laser beams in such a manner as to temporally average the laser beams, a second diffractive optical element to shape a beam that passes through the first diffractive optical element to match a geometry of a display device that produces an image from the shaped beam, a display device to produce the image using the shaped beam that passes through the second diffractive optical element, and a projection lens unit to enlarge and project the produced image onto a screen.

[0025] Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept while referring to the figures.

[0026] FIG. 3A illustrates a projection system designed to eliminate laser speckle according to an embodiment of the present general inventive concept. Referring to FIG. 3A, the projection system includes a laser light source 20, a first diffractive optical element 24 rotatable to remove the speckle of a laser beam emitted by the laser light source 20, and a display device 28 to produce an image using the beam that passes through the first diffractive optical element 24.

[0027] The projection system further includes a second diffractive optical element 26 that is disposed in an optical path between the first diffractive optical element 24 that removes the speckle and the display device 28 and to shape the beam that passes through the first diffractive optical element 24, and a projection lens unit 30 to enlarge and project the image produced by the display device 28 onto a screen 32. The laser light source 20 sequentially emits laser beams having a plurality of wavelengths.

[0028] FIG. 3B illustrates the first diffractive optical element 24 of the projection system of FIG. 3A. Referring to FIG. 3B, the first diffractive optical element 24 has a wheel-like shape and is divided into first, second, and third segments 24a, 24b, and 24c having wavelength-dependent diffractive patterns. For example, the first, second, and third segments 24a, 24b, and 24c may have patterns that correspond to red, green, and blue beams, respectively.

[0029] Since a diffraction efficiency of a typical diffractive optical element depends on a wavelength of an incident beam, the first diffractive optical element 24 may be designed to have a pattern that optimizes diffraction efficiency. Thus, the first diffractive optical element 24 may include segments having patterns that are optimally designed according to the wavelength(s) of the laser beam(s) used in the projection system.

[0030] Referring to FIGS. 3A and 3B, the first diffractive optical element 24 is rotated by a motor 22 synchronously with the sequential light emission of the laser light source 20 such that red, green, and blue beams emitted by the laser light source 20 respectively pass through the first, second, and third segments 24a, 24b, and 24c of the first diffractive optical element 24. The first diffractive optical element 24 rotates to temporally average the laser beams emitted by the laser light source 20, thereby efficiently removing laser speckle.

[0031] FIG. 4 illustrates profiles of partial beams produced by the first diffractive optical element 24 of the projection system of FIG. 3A. Referring to FIG. 4, the first diffractive optical element 24 includes a plurality of unit cells 24a. The laser beams emitted by the laser light source 20 are subdivided into a plurality of partial beams 27 having different phases and optical paths as the laser beams pass through the plurality of unit cells 24a. The plurality of partial beams 27 are then superimposed on one another and focused. Each of the plurality of partial beams 27 comprises multiple beamlets 27a. The black and white portions of the unit cells 24a represent the diffractive pattern of the corresponding segment. The first, second, and third segments 24a, 24b, and 24c may have diffractive patterns that are the same or different. In particular, the first, second, and third segments may have the diffractive pattern of the unit cells 24a illustrated in FIG. 4. The first diffractive optical element 24 rotates in such a manner as to average the laser beams both spatially and temporally, thereby eliminating laser speckle more efficiently.

[0032] FIG. 5A illustrates a laser beam in which laser speckle is observed, and FIG. 5B illustrates a laser beam from which the laser speckle is removed by rotating the first diffractive optical element 24 according to an embodiment of the present general inventive concept. When a contrast of the laser speckle is less than 4%, the laser speckle cannot be detected by the naked eye. The laser speckle contrast C is defined by Equation (1):

[0033] $\begin{array}{cc}C=\frac{\sqrt{\langle I_i^2\rangle -{\langle I_i\rangle }^2}}{\langle I_i\rangle }=\frac{\sigma }{\mu }\times 100\left(\%\right)& \left(1\right)\end{array}$
where I_i represents an average intensity of a beam that passes through an i-th unit cell of a diffractive optical element (e.g., the first diffractive optical element 24), σ represents a standard deviation of the intensity values, and μ represents a mean intensity value.

[0034] Referring to FIG. 3A, the first diffractive optical element 24 converts the laser beam emitted by the laser light source 20, which has the Gaussian distribution (i.e., the Gaussian laser beam) to have a uniform intensity distribution. That is, the laser beam having the Gaussian distribution is subdivided into a plurality of partial beams by the first diffractive optical element 24. The plurality of partial beams are then superimposed on one another, focused, and averaged both temporally and spatially by the first diffractive optical element 24 in order to produce the beam having the uniform intensity distribution, thereby improving the image quality.

[0035] The second diffractive optical element 26 shapes a cross-section of the uniform intensity beam that passes through the first diffractive optical element 24 to match the aspect ratio of the display device 28. The display device 28 may be a transmissive liquid crystal display (LCD), a Liquid Crystal on Silicon (LCoS) display, a Deformable Micromirror Device (DMD), or a Grating Light Valve (GLV). The display device 28 has a rectangular shape with an aspect ratio of 4:3 or 16:9. The display device 28 may alternatively have other aspect ratios. The beam emitted by the laser light source 20, which has a circular cross-section, is shaped to match the aspect ratio of the display device 28. Thus, as illustrated in FIG. 6, the circular beam is converted into a rectangular beam by the second diffractive optical element 26 to match the aspect ratio of the display device 28.

[0036] A diffractive pattern of the second diffractive optical element 26 controls a divergence angle of the incident beam. By controlling the divergence angle of the incident beam in such a manner as to increase an F-number of an illumination system, a slim and compact projection system can be provided. The rectangular beam shaped by the second diffractive optical element 26 is incident on the display device 28, and the image produced by the display device 28 is enlarged and projected onto the screen 32 by the projection lens unit 30.

[0037] The laser light source 20 includes a plurality of lasers to sequentially emit color beams having different wavelengths. For example, the laser light source 20 may include first, second, and third lasers to emit red, green, and blue beams, respectively. Images that correspond to the red, green, and blue beams are sequentially produced and projected onto the screen 32 in order to produce a color image. The display device 28 includes a single panel and sequentially modulates the red, green, and blue beams emitted by the laser light source 20 according to image signals that correspond to the respective wavelengths (i.e., of the color beams).

[0038] An illumination system according to an embodiment of the present general inventive concept includes a laser light source 20 having a plurality of lasers to emit beams having different wavelengths, a first diffractive optical element 24 to eliminate speckle of a beam emitted by the laser light source 20, and a second diffractive optical element 26 to shape an incident beam to match a geometry of a display device 28. The first diffractive optical element 24 may have a wheel-like shape that is divided into segments having diffractive patterns that correspond to wavelengths of the plurality of beams emitted by the laser light source 20.

[0039] The first diffractive optical element 24 may be designed to provide an optimal diffraction efficiency according to the wavelength of an incident beam. The diffraction efficiency D_eff of a diffractive optical element is defined by Equation (2):

[0040] $\begin{array}{cc}{D}_{\mathrm{eff}}={\left\{\frac{\mathrm{Sin}\left[\pi \times \left({\lambda }_D/\lambda -1\right)\right]}{\pi \times \left({\lambda }_D/\lambda -1\right)}\right\}}^2& \left(2\right)\end{array}$
where λ_D and λ represent a design wavelength of the diffractive optical element and an actual wavelength used in the diffractive optical element, respectively. By using a beam having a wavelength that is equal to the design wavelength, the diffractive optical element achieves 100% diffraction efficiency. However, using abeam having a wavelength that is different than the design wavelength degrades the diffraction efficiency. Therefore, the first diffractive optical element 24 can be designed to have a design wavelength that corresponds to the laser light source 20 that is used in the illumination system illustrated in FIG. 3A.

[0041] Although the first diffractive optical element 24 is described as removing speckle by rotating in such a manner as to temporally and spatially average partial beams, it should be understood that the first diffractive optical element 24 may have other configurations. In particular, FIG. 7A illustrates a projection system according to another embodiment of the present general inventive concept. The projection system includes a transport 25 to drive a first diffractive optical element 24′ having a rectangular shape to reciprocate linearly in order to average an incident beam both temporally and spatially.

[0042] Since elements of the projection system illustrated in FIG. 7A with the same reference numerals as the projection system illustrated in FIG. 3A perform similar functions and/or operations, another description of these elements will not be provided.

[0043] FIG. 7B is a front view illustrating the first diffractive optical element 24′. Referring to FIG. 7B, the first diffractive optical element 24′ has the rectangular shape and is divided into first, second, and third segments 24a′, 24b′, and 24c′ having diffractive patterns that correspond to red (R), green (G), and blue (B) beams, respectively. The first diffractive optical element 24′ reciprocates vertically such that the red, green, and blue beams sequentially emitted by the laser light source 20 pass through the first, second, and third segments 24a′, 24b′, and 24c′, respectively. Since the first diffractive optical element 24′ linearly reciprocates, blue, green, and red beams are sequentially emitted from the first diffractive optical element 24′. The display device 28 causes image signals to be synchronized with the color beams emitted by the laser light source 20.

[0044] As described above, the various embodiments of the present general inventive concept efficiently remove laser speckle by temporally and spatially averaging laser beams.

[0045] The various embodiments of the present general inventive concept provide an illumination system with a laser light source, which significantly reduces or removes speckle of a laser beam, and a single-panel projection system that provides improved image quality by employing the illumination system.

[0046] By using a diffractive optical element divided into segments having diffractive patterns that correspond to a plurality of wavelengths, the embodiments of the present general inventive concept achieve a maximum diffraction efficiency for each wavelength. By rotating or linearly reciprocating the diffractive optical element to temporally and spatially average a laser beam, laser speckle can be efficiently eliminated. By reducing a divergence angle using the diffractive optical element, the embodiments of the present general inventive concept also provide a slim and compact projection system.

[0047] Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.