WO2005060235A1

WO2005060235A1 - Illumination optics for scanning laser projector applications having a one-dimensional pixel array

Info

Publication number: WO2005060235A1
Application number: PCT/IB2004/052794
Authority: WO
Inventors: Marcellinus P. C. M. Krijn; Oscar Hendrikus Willemsen
Original assignee: Koninklijke Philips Electronics, N.V.; U.S. Philips Corporation
Priority date: 2003-12-15
Filing date: 2004-12-13
Publication date: 2005-06-30

Abstract

A laser scanning display device includes a linear array of light modulating devices (201), at least one optical emitter having an output that has a substantially non-uniform intensity distribution, and at least one refractive optical element (101, 102) that substantially homogenizes the intensity distribution of the output of the light from the laser.

Description

ILLUMINATION OPTICS FOR SCANNING LASER PROJECTOR APPLICATIONS HAVING A ONE- DIMENSIONAL PIXEL ARRAY

Laser scanning display devices are emerging as a viable option in high quality video with a large color gamut. The laser scanning device usefully includes red, green and blue (R, G and B) lasers, with light from these lasers being projected via front or rear projection. Often, three types of laser scanning display are used. A first type of device incorporates R, G, B lasers, scanned in both the line direction (usually the horizontal direction) and the frame direction (usually the vertical direction) using a scanning device such as scanning mirrors. A second type of device incorporates R, G, B lasers, are used in combination with optics to project a one-dimensional (ID) micro -display onto the screen. A complete frame is obtained by scanning the lasers in the frame direction. This is a hybrid between projecting and scanning. In a third type of device, the R, G, B lasers are used to project a 2D micro -display onto the screen. However, as will become clearer as the present description continues, in many desired laser scanner display applications, the desired output of the laser is substantially one- dimensional (i.e., a line) of substantially homogeneous intensity that is incident on a row of pixels. However, a laser produces a laser beam that is either circular or elliptical in cross- section and that has a Gaussian-shaped intensity distribution within the beam. To wit, the intensity at the beginning and at the end of the line will be much less than that at the center. Known methods that attempt to provide a homogeneous output include cropping the 'tails' of the laser output, and reducing the intensity of the central portion of the light with a filter or similar absorption device. Unfortunately, this is not very effective at providing a homogeneous output, and it is a very inefficient method as well. As such, this method may not provide the desired uniform illumination, but will provide a significant reduction in the intensity of the light, which may be deleterious to quality of the image at the display. In accordance with an example embodiment, a laser scanning display device includes a linear array of picture elements, at least one optical emitter having an output that has a substantially non-uniform intensity distribution, and at least one refractive optical element that substantially homogenizes the intensity distribution of the output of the light from the laser. In accordance with another example embodiment, a laser scanning display device includes at least one optical emitter having an output that has a substantially non-uniform intensity distribution. The device also includes a row of light modulating devices, which are adapted to operate in a first state and a second state and at least one refractive optical element that substantially homogenizes the intensity of the output of the light from the at least one optical emitter.

The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Fig. 1 is a schematic view from above of a scanning laser projector including a. ID pixel array in accordance with an example embodiment. Fig. 2 is a schematic view from the side of the scanning laser projector of the example embodiment of Fig.1. Figs. 3a and 3b are cross-sectional views of a ID pixel array, which may be incorporated into example embodiments, in an on state and an off state, respectively. Fig. 4 is a cross-sectional view of an optical system in accordance with an example embodiment. Fig. 5 is a cross-sectional view of an optical system in accordance with an example embodiment. Fig. 6 is an intensity distribution of redistributed light within a line-shaped beam via the lens system of Fig. 5 according to an example embodiment. Fig. 7 is a cross-sectional view of a wavefront of a laser at the laser output, after emerging from a known optical system, and after emerging from an optical system in accordance with an example embodiment. Fig. 8 is a perspective view showing the wavefront of the laser output from the optical system of an example incident on a row of pixel elements of an example embodiment. In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of the example embodiments. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments that depart from the specific details disclosed herein may be realized. It is noted that descriptions of well-known devices, methods and materials may be omitted so as to not obscure the description of the present invention. Finally, wherever practical, like reference numerals refer to like features. Briefly, in accordance with example embodiments, a scanning light projector device includes a light emitting device, which emits a light beam that is not homogeneous in intensity, a refractive optical system, a row of pixels, which modulate the light from the laser, and an oscillating reflective element, which selectively directs the light incident thereon to a display screen, where an image is formed. The refractive optical system is disposed before the pixels, and homogenizes the inhomogeneous light from the light emitting device incident on the refractive optical system. The light emerges refractive optical system having a wavefront of substantially homogeneous intensity. It is noted that in example embodiments described herein, the light emitting device is a laser, such as a semiconductor laser, or a super luminescent diode. As is well known, the output of these devices has a circular or elliptical wavefront, and the intensity versus angle of the far field patterns is substantially Gaussian in nature. As such, in order to form an image having a more uniform intensity this Gaussian intensity distribution must be made more uniform. The optical systems described in connection with the example embodiments include refractive optical elements, which re-distribute the light energy through non-unifonn refraction across the lens elements and convert the substantially circular or elliptical cross-section of the light beam of the laser into a light beam having a cross-section that is substantially wider than it is high. To wit, the light has the cross-section of a line. Thus, the optical system of an example embodiment provides a 'line' of light with a more uniform intensity distribution over the cross-section of the light beam. It is also noted that in accordance with example embodiments more than one row of pixels may be used. For example, three rows may be used in accordance with an example embodiment: one addressed by a red laser; one addressed by a green laser; and the third one addressed by a blue laser. In this manner the R, G, and B lines may be displayed in parallel instead of sequentially. Fig. 1 shows a scanning laser projection system 100 in accordance with an example embodiment. The system 100 includes a lens system comprised of a first lens element 101 and a second lens element 102, each of which are refractive lens elements. As will become clearer as the present description continues, the lens system of example embodiments may comprise more or fewer refractive lens elements than are shown in Fig. 1. Moreover, the details of these lens elements are described in further detail herein. The light 103 that is incident on the lens system emerges therefrom as a more homogeneous light beam having a substantial line cross-section. This light is incident on a row of pixels disposed in a light modulating device 104. The light modulating device 104 may include a grating light valve (GLN), described in further detail herein, or another type of light valve such as a liquid crystal display (LCD) device. For example, the light modulating device 104 may be a liquid crystal-on-silicon (LCOS) or similar device. Characteristically, through the function of the light modulating device, light incident on the light modulating device 104 ultimately may be projected onto a display screen 105, or may be prevented from reaching the screen 105. The former is known as 'bright' state light, while the latter is known as 'dark' state light. As the details of light valves devices is well- known to one of ordinary skill in the art of image projection display devices, further details are omitted so as to not obscure the description of the example embodiments. Upon selective reflection by the light modulator 104, the bright state light is incident on the display screen (surface) 105, having been focused by a lens element 106 that focuses the light through a diaphragm 107. The light is then scanned onto the screen 105 by a scanning device 108, which may be a rotating mirror, or rotating prism, or other suitable device. In this manner, an image is formed on the screen 105 by scanning each successive 'line' of light from the light modulator. Fig. 2 shows the scanning laser projection system 100 of Fig. 1 from another perspective. As can be seen, the light modulator 104 includes a plurality of pixels 201, which usefully direct the bright-state light 202 through the diaphragm 107, and prevent the dark-state light 203 from being transmitting through the aperture 107. As such, the bright-state light 202 is incident on the display screen 105, whereas the dark-state light is not. Figs. 3a and 3b show a light modulator 300, which includes a row of pixels 301. In an example embodiment, the modulator 300 may be used as the light modulator 104 with the pixels 301 serving as the pixels 201. The light modulator 300 is illustratively a GLN device. Such a device is described in detail in The Grating Light Naive: revolutionizing display technology, Proceedings of SPIE, vol. 3013, 165 (1997) by D.M. Bloom. The disclosure of this article is incorporated herein by reference. It is noted that in accordance with another example embodiment, the light modulator 300 may be a foil bar modulator (FBM). In operation, light of a laser beam is directed towards the row of pixels of the FBM. The FBM includes a scattering foil that is isolated from and sandwiched in between an upper and a lower glass plate. Electrodes are deposited onto each of the glass plates and onto the foil. The foil is flexible and can be attracted towards each of the glass plates by means of applying a voltage difference between the foil and, the appropriate glass plate. A prism can be used to couple the light into the upper plate. In the on-state, the foil is not in contact with the upper glass plate and the laser beam is reflected from the glass-to-air transition by means of total internal reflection (TIR). In the off- state, the foil is attracted towards the upper glass plate and TIR is frustrated. In this state, the light of the laser beam is scattered by the scattering foil and eventually blocked by an aperture (diaphragm). Moreover, in another example embodiment, the light modulator 300 may be a conformal grating electromechanical system (GEMS), which is rather similar to the grating light valve (GLN). Further details of this device may be found in an article by M.W. Kowarz, J.C. Brazas, J.G. Phalen, and entitled Conformal Grating Electromechanical System (GEMS) for High-Speed Digital Light Modulation, IEEE 15th International MEMS Conference, pages 568-573 (2002). The disclosure of this article is specifically incorporated herein by reference. In the example embodiment of Fig. 3a the modulator 300 is functioning in an 'on' state. Illustratively, the modulator is a GLN as referenced, and the incident light 302 is reflected at the surface 303 by total internal reflection as shown, and is the bright-state light 202, which traverses the diaphragm 107. Alternatively, the modulator 300 of Fig. 3b is in a state that some of the incident light 305 is refracted, and some light 304 is reflected as shown. The incident light 305 of the modulator 300, in the 'off state in Fig. 3b, experiences frustration of total internal reflection (FTIR), and as such none of the reflected light 304 will traverse the diaphragm 107. This light is the 'dark-state' light 203. Fig. 4a is a cross-sectional view of an optical system 400 in accordance with an example embodiment. Illustratively, the optical system 400 may be incorporated into the system 100 of the example embodiment of Figs. 1 and 2 as the lens elements 101 and 102. The optical system 400 includes a first lens 401 and a second lens 402, which is adapted to project a light beam 403 that has a substantially homogeneous intensity in the x-z plane onto a projection surface 404, when the light 405 incident on the optical system 400, is inhomogeneous. In the example embodiment, the light beam 405 is substantially homogeneous to illustrate effectively the function of the system 400. The incident light 405 is substantially homogeneous as can be appreciated by the flux lines in the figure. However, the lens element 401 disproportionately refracts the rays in the central portion 406 of the lens relative to the peripheral portions 407 of the lens. As such, the intensity of the light that emerges from the center of the lens (as seen by the reduction of the flux) is reduced relative to the light at the outer portions 407. Thereby light intensity can be redistributed by refraction of the first lens element 401. These and other details of this function will become clearer as the present description continues. ' Beneficially, in the present example embodiment, the second lens element 402 functions as a collimator to ensure clarity in the image. To wit, as viewed from the surface 404, the image appears to originate at a point. Moreover, but for this element 402, the rays from the first lens would cross and distort the image. The light beam 405 is relatively large in the x-dimension and relatively small in the y- dimension in the present embodiment. To wit the light beam is usefully a 'line' shape as desired in a scanned laser projection application. As such, if inhomogeneous in the x-direction at the input, the image at the surface 404 would be substantially homogeneous by virtue of the system 400. Finally, it is noted that first optical element 401 has an aspherical shape such that the inner rays are refracted outwards whereas the outer rays are less refracted. The second optical element also has an aspherical shape that is used to redirect the beams such that they appear to originate from the same position. The net effect is that the rays are redistributed within the beam such that some intensity from the centre of the beam is redistributed towards the periphery of the beam. Of course, the aspherical nature of the lens elements is merely illustrative of the example embodiment of Fig. 4a. Clearly other lenses that meet the characteristics of refracting the light in a center of an incident beam to a greater extent than the outer portions of the beam may be used and remains in keeping with the teachings of the example embodiments. Fig. 4b shows an incident beam 408 of non-uniform intensity (see the flux lines) incident on a lens system, which comprises a first refracting surface 410 and a second refracting surface 409. Mathematically, the position of a ray within a laser beam and the displacement of that ray that is required to ensure a uniform intensity distribution are outlined presently. There are two surfaces at which the rays are refracted. For simplicity, the surfaces are drawn as straight lines whereas in practice they will of course be curved. It is also assumed that the incoming and outgoing rays are parallel to the optical axis of the system. The intensity distribution of a laser beam in general obeys a normal (i.e. Gaussian) distribution: I(x) = I (0) exp (-x^σ²) (1)

The Gaussian- shaped intensity distribution at the first refracting surface 410 has to be redistributed in order to obtain a constant intensity distribution at the second refracting surface 409: Iι(xι) dxl=I2 (x₂) dx.2 , where I2(x) =c (i.e. a constant) (2) The constant c determines the length 1 of the line at the second refracting surface 409: C= (2%) I (0) σ/1 (3)

A ray starting at position xi at surface 410 is displaced to coordinate x₂= f(xι) at the second surface 409, where f(xι) is given by: f(xι)

order to obtain a constant intensity distribution at surface 409. The net displacement

The net refraction angle should therefore be: ΔΘ(xι) = arctan (Δ(xι)/ ΔL) (4) Fig. 5 shows a lens system 500 including an integrated lens element 501 in accordance with an example embodiment. This lens system may be implement as lens elements 101 and 102 of the example embodiments of Figs. 1 and 2. The integrated lens element 501 is illustratively an element formed of two aspherical lens elements each having a flat surface. For example, the integrated lens element may be the integration of the curved surfaces of lens elements 401 and 402 of Fig. 4a. The incident light 503 has a non-uniform intensity distribution. The intensity distribution may be a Gaussian distribution as from a laser. Moreover, the incident light has a dimension in the x-direction that is significantly greater than its dimension in the y-direction. A lens element 504 provides a collimation function. The incident light 503 emerges from the lens 504 collimated but have a non-uniform intensity that is shown by the flux of the rays being greater in the center 505 of the beam than in the peripheral region 506 of the beam. This is, of course, representative of a Gaussian intensity distribution. Upon emerging from the integrated lens element 501, the intensity distribution is substantially homogeneous, so that the substantially homogeneous beam is incident on the viewing surface 502. Fig. 6 shows an intensity distribution of a substantially uniform light beam 601, which has been redistributed from a non-uniform intensity beam, such as a Gaussian beam by refractive lens systems of example embodiments. The flux versus x-coordinate is shown at 602. As can be readily appreciated, the intensity is substantially uniform across the x- dimension, particularly compared to a Gaussian intensity distribution. Moreover, the flux versus y-coordinate 603 remains substantially Gaussian in nature. It is noted that the lens elements according to the example embodiments generally do not need to redistribute the beam in the 'height' dimension (e.g., the y-dimension in this embodiment), because this dimension is small compared to that of the x-dimension in scanned line applications. It is also noted that this is not essential, and the intensity in both the width and height (x, y) could also be effected in keeping with example embodiments. Fig. 7 shows a comparison of the redistribution of the energy of light from a laser using know optics, and using the refractive optical elements of example embodiments. The wavefront of a laser may be a circular or elliptical distribution 701 as shown. Upon emerging from the known optics (not shown) the beam 702 is expanded, but is no less inhomogeneous. Clearly, this will not enable the desired light output in scanned laser projection systems, and other applications addressed by the example embodiments. Contrastingly, the wavefront 703 of the homogenized beam in accordance with an example embodiment is shown. This wavefront is substantially homogeneous over the lateral (y) dimension as shown. When superposed over a row of pixels 801 as shown in Fig. 8, a more accurate reproduction of the intended image results. To this end, there are n- pixels (n=integer) 801, which have the substantially uniform intensity 'line' (in the y-direction) of light incident thereon. These pixels then selectively modulate the light so that the bright and dark pixels can be formed on the screen. As the reflective element described in connection with Fig. 1 scans line-by-line across the screen, the image is formed. It is noted from a review of the superposition of the light on the pixels 801, the light incident on the pixels in the center (e.g., at a point 802) is of substantially the same intensity as on the pixels at a peripheral x-coordinate (e.g., at points 803). In contrast, the superposition of the wavefront 702 would not yield the same result. The example embodiments having been described in detail, it is clear that modifications of the invention will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure. Such modifications and variations are included in the scope of the appended claims.

Claims

CLAIMS:

1. A scanning display device (100), comprising: a row of picture elements (301, 801); at least one optical emitter having an output that has a substantially non-uniform intensity distribution; and at least one refractive optical element (401, 402) that substantially homogenizes the intensity distribution of the output of the light from the at least one optical emitter.

2. A scanning display device as recited in claim 1, wherein the substantially non-uniform intensity distribution is a Gaussian distribution.

3. A scanning display device as recited in claim 1, wherein the at least one optical emitter is a laser.

4. A scanning display device as recited in claim 1 , wherein the at least one optical emitter is a super luminescent diode.

5. A scanning display device as recited in claim 1, further comprising two other rows of picture elements.

6. A scanning display device as recited in claim 5, wherein one row of the picture elements is addressed by a red laser, another row of the picture elements is addressed by a green laser and another row of the picture elements is addressed by a blue laser.

7. A scanning display device as recited in claim 5, wherein lines of an image are displayed in parallel.

8. A scanning display device as recited in claim 1, wherein lines of an image are displayed sequentially.

9. A scanning display device as recited in claim 1, wherein the light output (703) is substantially uniform in a lateral dimension, and not in a vertical dimension relative to the row of picture elements.

10. A scanning display device as recited in claim 1, wherein the at least one refractive optical element substantially refracts light of a center of an incident beam to a greater extent than light of outer portions of the incident beam.

11. A scanning display device as recited in claim 1, wherein the at least one refractive optical element is substantially aspherical.

12. A scanning display device (100), comprising: at least one optical emitter having an output that has a substantially non-uniform intensity distribution; a row of light modulating devices (300), wherein the light modulation devices each are adapted to operate in a first state and a second state; and at least one refractive optical element (401, 402) that substantially homogenizes the intensity of the output of the light from the at least one optical emitter.

13. A scanning display device as recited in claim 12, wherein in the first state substantially all of the light incident on the light modulating device is reflected to a display surface (105).

14. A scanning display device as recited in claim 12, wherein in the second state substantially none of the light incident on the light modulating device is reflected to a display surface.

15. A scanning display device as recited in claim 12, wherein the light modulating devices are grating light valves (GLVs).

16. A scanning display device as recited in claim 12, wherein the light modulating devices are foil bar modulators (FBMs).

17. A scanning display device as recited in claim 12, wherein the substantially non-uniform intensity distribution is a Gaussian distribution.

18. A scanning display device as recited in claim 12, wherein the at least one refractive optical element substantially refracts light of a center (503) of an incident beam to a greater extent than light of outer portions (506) of the incident beam.

19. A scanning display device as recited in claim 12, wherein the at least one refractive optical element is substantially aspherical.

20. A method of forming an image, the method comprising: providing light that has a substantially non-uniform intensity distribution; providing at least one refractive optical element; substantially homogenizing the intensity distribution of the output of the light; and reflecting the substantially homogenized light from a row of picture elements in a sequential manner.