Abstract:
The invention relates to a display module, a portable device comprising a display module, and a method of displaying graphical information. A virtual image is generated using a micro-display, imaging optics and a diffractive beam expander. The virtual image is observable through the exit aperture of the diffractive beam expander. The aspect ratio of the displayed virtual image is substantially different from the aspect ratio of the exit aperture. A visible portion of the displayed image may be selected by tilting the display module. Thus, e.g. an entire displayed page of text may be examined through a wide but low display module, viewing two or three lines of text at a time.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority under 35 USC §119 to Finnish Patent Application No. 20045513 filed on Dec. 31, 2004.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a display module to form a virtual image, said display module comprising at least a micro display, imaging optics, and a diffractive beam expander, said virtual image being adapted to be observable through an exit aperture of said diffractive beam expander. The present invention relates also to a device comprising a display module, and to a method to display an image.  
       BACKGROUND OF THE INVENTION  
       [0003]     Display modules are used in portable devices to display information in graphical form. Small size is an important aspect in portable devices. The small size of a portable device also sets a limitation to the size of a display module incorporated in said device. The drawback of small display modules is that, when viewing at a distance, an observer can examine only a small portion of a large displayed image at a glance, while preserving adequate resolution.  
         [0004]     A visible portion of the image is typically selected using scroll buttons or a scroll knob. For example, when a page of text is examined, the portion of the text to be viewed may be selected by turning a scroll knob of a computer mouse.  
         [0005]     Another approach to display a large detailed image using a small display module is to use a near-eye-display. An exit aperture of a small display module is brought close to the eye of an observer. However, some people find it slightly unpleasant to bring objects close to their eyes. A near eye display based on a diffractive beam expander is disclosed in patent application EP 0535402.  
         [0006]     The aspect ratio of the exit aperture is typically selected to correspond to the aspect ratio of the displayed image. For example, when a display module is designed to display images with an aspect ratio 4:3, the aspect ratio of the exit aperture is typically substantially close to 4:3. The aspect ratio refers to the ratio of the width to the height.  
       SUMMARY OF THE INVENTION  
       [0007]     It is an object of the present invention to provide a small display module, which allows viewing a virtual image with high resolution. A further object of the present invention is to provide a portable device comprising said a display module. It is yet an object of the present invention to provide a method to display a virtual image.  
         [0008]     According to a first aspect of the invention, there is a method to display a virtual image, said method comprising at least forming a virtual image by a micro display, imaging optics, and a diffractive beam expander, said virtual image being an image of the active area of said micro display, and said virtual image being observable through an exit aperture of said diffractive beam expander, wherein the aspect ratio of said virtual image is substantially different from the aspect ratio of said exit aperture.  
         [0009]     According to a second aspect of the invention, there is a display module to form a virtual image, said display module comprising at least a micro display, imaging optics and a diffractive beam expander, said virtual image being an image of the active area of said micro display, and said virtual image being observable through an exit aperture of said diffractive beam expander, wherein the aspect ratio of said virtual image is substantially different from the aspect ratio of said exit aperture.  
         [0010]     According to a third aspect of the invention, there is a portable device comprising a display module to form a virtual image, said display module in turn comprising at least a micro display, imaging optics and a diffractive beam expander, said virtual image being an image of the active area of said micro display, and said virtual image being observable through an exit aperture of said diffractive beam expander, wherein the aspect ratio of said virtual image is substantially different from the aspect ratio of said exit aperture.  
         [0011]     A display module according to the present invention comprises a micro-display, imaging optics and a diffractive beam expander. The display module converts a real image formed by the micro-display to a virtual image, which is observable through an exit aperture of said diffractive beam expander. The dimensions of the exit aperture may be, for example 1×10 cm. The displayed image may be a page of text. The display module is advantageously positioned at 0.2-0.6 meters from the eyes of an observer, and from that distance two or three lines of text are visible at a glance. The examined text is selected by tilting the display module. In other words, the visible portion of the virtual image is selected by changing the angular orientation of a combination of the micro-display and the imaging optics with respect to an observer.  
         [0012]     The image can be observed by keeping the display module far from the eye of an observer, i.e. at a distance greater than 0.1 meters. The display module or the portable device comprising such a display does not block the vision of an observer. The observer can see the environment while viewing the displayed image. Thus, the observer can even walk while viewing the displayed image.  
         [0013]     The virtual image is formed at a distance, which is in the range from one meter to infinity. Thus, the virtual image is further away from the observer than the display module. Thus, the eyes of an observer do not need to accommodate to the physical distance of the display module.  
         [0014]     Some people with hyperopia (farsightedness) do not need spectacles to examine the displayed image.  
         [0015]     In general, people wearing spectacles appreciate that the display unit can be viewed at a distance.  
         [0016]     The method of scrolling the image by tilting the entire device is user-friendly, and the observer learns the method in a short time.  
         [0017]     The display and the device according to the present invention may be slim, and consequently easy to carry and to handle. The power consumption of the display module is small. The visual appearance of the displayed virtual image is exciting when compared with images displayed by conventional planar displays.  
         [0018]     The embodiments of the invention and their benefits will become more apparent to a person skilled in the art through the description and examples given herein below, and also through the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     In the following examples, the embodiments of the invention will be described in more detail with reference to the appended drawings, in which  
         [0020]      FIG. 1  shows schematically a display module according to the present invention comprising a diffractive beam expander and an optical engine,  
         [0021]      FIG. 2  shows schematically optical rays transmitted from a micro-display through the diffractive beam expander, as seen from a direction parallel to the dimension W 1  of an exit aperture of said diffractive beam expander,  
         [0022]      FIG. 3   a  shows schematically the tilting of the exit aperture with respect to a horizontal axis,  
         [0023]      FIG. 3   b  shows schematically the tilting of the exit aperture with respect to a vertical axis,  
         [0024]      FIG. 4   a  shows schematically optical rays transmitted from a first point, as seen from a direction parallel to the dimension W 2  of the exit aperture,  
         [0025]      FIG. 4   b  shows schematically optical rays transmitted from the first point and from a second point, as seen from a direction parallel to the dimension W 2  of the exit aperture,  
         [0026]      FIG. 5   a  shows schematically how the projected dimension W 1 ′ of the exit aperture defines the portion of the virtual image visible to an observer.  
         [0027]      FIG. 5   b  shows schematically the angular dimensions of the virtual image and the exit aperture, and  
         [0028]      FIG. 6  shows schematically a portable device comprising a display module according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     Referring to  FIG. 1 , a display module  40  according to the present invention comprises at least an optical engine  20  and a diffractive beam expander  10 . The optical engine  20  transmits a plurality of light beams corresponding to the virtual image to be displayed. The aperture of the optical engine  20  is typically round or rectangular. The aperture is small, and therefore the light beam emitted from the optical engine  20  is expanded in at least one dimension using the diffractive beam expander  10 . The expanded beam is further transmitted to the eye of the observer  100 .  
         [0030]     According to the present invention, the dimension W 1  of the exit aperture  16  of the diffractive beam expander  10  may be small, and therefore it may be enough to expand the beam transmitted from the optical engine  20  only in one dimension, i.e. in the direction parallel to the dimension W 2 .  
         [0031]     The beam expander comprises at least two diffractive elements  12 ,  14  arranged on a substantially planar transparent substrate. The perimeter of the second diffractive element  14  substantially defines the height W 1  and the width W 2  of the exit aperture  16 , providing that the output aperture of the optical engine  20  is large enough. The exit aperture  16 , in turn, defines the maximum height and width of the expanded light beam transmitted towards the observer  100 .  
         [0032]     Referring to  FIG. 2 , the optical engine  20  comprises a micro-display  22  and imaging optics  24 . The imaging optics  24  may comprise one or more optical elements, such as lenses, mirrors, prisms or diffractive elements. Light rays transmitted by a point P 1  of the micro-display  22  are collimated by the imaging optics  24  to form a parallel or a slightly diverting beam of light. The micro-display  22  is positioned such that the active pixels of the micro-display  22  are at the focal distance of the imaging optics  24 . The active area of the micro-display  22  is defined by the area consisting of controllable pixels. The active area of the micro-display  22  has a dimension d 2  substantially parallel to the dimension W 2  of the exit pupil  16 .  
         [0033]     The beam transmitted from the optical engine  20  impinges on the first diffractive element  12  of the beam expander  10 . The first diffractive element  12  diffracts light towards the second diffractive element  14 . The light propagates inside the transparent beam expander  10  by a plurality of total internal reflections. The second diffractive element  14  diffracts an expanded beam of light BE 1  towards the eye of the observer  100 .  
         [0034]     A real image formed by the pixels of the micro-display  22  is converted to a virtual image by the imaging optics  24 . Each point of the micro-display  22  corresponds to a parallel or a slightly diverting beam of light transmitted from the exit aperture  16 . Thus, the eye of the observer  100  sees a virtual image at a distance.  
         [0035]     When using planar diffractive elements  12 , 14 , the virtual image is formed at infinite distance. However, the distance between the virtual image and the observer  100  may also be shorter than infinity. Said distance may be, for example, in the range 1 to 2 meters. Distances shorter than infinity may be implemented using a curved diffractive beam expander disclosed in a patent application PCT/IB2004/004094. Said diffractive beam expander comprises at least one non-planar diffractive element having a finite curvature radius.  
         [0036]     A patent application PCT/FI2003/000948 discloses a split diffractive grating element to balance diffraction efficiency with respect to variations in the angle of incidence. It is advantageous to use such an element in the diffractive beam expander  10 .  
         [0037]     Light escapes from the second diffractive element  14 . Advantageously, the local diffraction efficiency of the second diffractive element  14  is adjusted to provide uniform intensity. Otherwise the intensity would be substantially higher at positions near the first diffractive element  12  than at positions far from the element  12 .  
         [0038]     The beam emitted from the optical engine  20  may also be expanded in the direction parallel to the dimension W 1 . In that case a third diffractive element may be used. A beam expander based on three diffractive elements is disclosed in a patent application PCT/IL99/00183.  
         [0039]     A patent application US2004/0062502 discloses a beam expander with one or more surface interfaces to improve color uniformity in the beam expander, e.g. when expanding red, green and blue light beams simultaneously.  
         [0040]     The diffractive beam expander may be implemented using periodic surface relief patterns. The diffractive beam expander may also be a holographic diffractive beam expander, comprising periodic absorbing and non-absorbing features, implemented by holographic manufacturing techniques.  
         [0041]     The micro-display  22  may be a reflective, emissive or transmissive two-dimensional light-modulating array. The micro-display  22  may be an array of light emitting diodes (LED, Organic Light Emitting Diode), an array of micromechanical mirrors (MEMS display), or an array of liquid crystal cells (liquid crystal on silicon). The micro display  22  may also be implemented using opto-mechanically scanned light beams, e.g. using a modulated light beam, which is deflected and/or shifted by rotating mirrors.  
         [0042]     Micro-displays  22  that are not self-illuminating, such as the MEMS display, require an additional light source. The light source may be integrated into the system. Alternatively, light may be introduced using a waveguide.  
         [0043]     Advantageously, the micro-display  22  provides a resolution of 640×400 pixels (VGA display), or a higher resolution. The micro-display  22  is controlled by a control unit (not shown) to display a page of text and/or figures.  
         [0044]     The imaging optics  24  comprises advantageously an achromat. Also a lens system comprising several lenses, e.g. a Hastings triplet may be used. The imaging optics  24  may also be implemented using a diffractive element. Said collimating diffractive element may also act as a part of the diffractive beam expander  10 .  
         [0045]     Referring to  FIG. 3   a , the exit aperture  16  may be tilted with respect to a horizontal axis  8 . The center of the exit aperture  16  and the eye of the observer  100  define a viewing line VL and a plane NP perpendicular to said viewing line VL. The dimension W 1  of the exit aperture and the plane NP define an angle γ.  
         [0046]     Referring to  FIG. 3   b , the exit aperture  16  may be tilted with respect to a vertical axis  9 . The dimension W 2  of the exit aperture and the plane NP define an angle φ.  
         [0047]      FIG. 4   a  shows the path of optical rays transmitted from the micro-display  22 , as seen from a direction parallel to the dimension W 2  of the exit aperture  16 . The point P 1  is in the center of the active area of the micro-display  22 . The point P 1  transmits light, which is collimated by the imaging optics  24  to form a collimated beam B 1 . The collimated beam is transmitted through the beam expander  10  to form an expanded beam BE 1 . The expanded beam BE 1  impinges on the eye of the observer  100 , and the observer  100  is able to see a point of a virtual image corresponding to the point P 1 .  
         [0048]     f denotes the focal distance of the imaging optics  24 . d 1  denotes the dimension of the active area of the micro-display  22  in the direction parallel to the dimension W 1  of the diffractive beam expander  10 . L 1  denotes the distance between the exit aperture  16  and the eye of the observer  100 .  
         [0049]      FIG. 4   b  illustrates how the portion of the virtual image visible to the observer  100  is selected by tilting the display module. In  FIG. 4   b , the angular orientation of the display module  40  with respect to the observer  100  has been changed by the angle γ when compared with the orientation shown in  FIG. 4   a . Now, the light beam BE 1  originating from the point P 1  shown in  FIG. 4   a  does not impinge on the observer&#39;s eye  100 , whereas a second light beam BE 2  originating from a second point P 2  impinges on the observer&#39;s eye  100 . Thus the observer is able to see a point of the virtual image corresponding to the point P 2 , but not a point of the virtual image corresponding to the point P 1 .  
         [0050]     The projection of the exit aperture  16  on a plane normal to the viewing line VL has height W 1 ′ and width W 2 ′ (W 2 ′ is not shown in  FIG. 4   b ). Said dimensions of the projection define the portion of the virtual image visible to the observer  100 . W 1 ′ is equal to W 1  multiplied by cos(γ). W 2 ′ is equal to W 2  multiplied by cos(φ). (See  FIG. 3   b )  
         [0051]     Referring to  FIG. 5   a , the observer  100  sees the displayed virtual image V 1  through the exit aperture  16 .  
         [0052]     When the virtual image V 1  is formed at infinite distance, it does not have dimensions that could be expressed using units of length, i.e. in meters. However, the observer  100  perceives the features of the virtual image V 1  in terms of angular dimensions, i.e. in radians or degrees.  
         [0053]     The angular dimensions of the virtual image V 1  are defined by the dimensions of the active area of the micro-display  22 . The height and the width of the active area of the micro-display  22  are d 1  and d 2  (see  FIGS. 2 and 4   a ). The focal length of the imaging optics  24  is f ( FIG. 4   a ). The angular height α 1  of the virtual image is substantially equal to the height d 1  of the active area of the micro-display  22  divided by the focal distance f of the imaging optics  24 .  
         [0054]     Referring to  FIG. 5   b , the angular width α 2  of the virtual image is substantially equal to the width d 2  of the active area of the micro-display  22  divided by the focal distance f of the imaging optics  24 .  
         [0055]     The aspect ratio of the virtual image V 1  is defined to be the ratio of the angular width α 2  to the angular height α 1 . Thus, the angular height α 1  of the displayed virtual image V 1  is equal to d 1 /f and the angular width α 2  of the displayed virtual image V 1  is equal to d 2 /f. Thus, the aspect ratio of the virtual image V 1  is equal to d 2 /d 1 .  
         [0056]     The projected dimension W 1 ′ of the exit aperture  16  defines a portion V 2  of the virtual image V 2  visible to the observer  100 . The maximum angular height β 1  of said visible portion V 2  is defined by the ratio W 1 ′/L 1 . The maximum angular width β 2  of said visible portion V 2  is defined by the ratio W 2 ′/L 1 , respectively.  
         [0057]     The portion V 2  can be selected by changing the angular orientation of the display module  40  with respect to the eye of the observer  100 . β 1  and/or β 2  becomes zero, when the tilting angle γ or φ of the display module  40  is too large.  
         [0058]     Advantageously, the observer  100  sees the virtual image V 1  at a distance, which is in the range from 1 meter to infinity. Thus, a change of the distance between the exit aperture  16  and the observer  100  does not require significant accommodation of the observer&#39;s eye. However, in general, people prefer to keep objects at a distance from their eyes. Advantageously, the distance L 1  between the exit aperture  16  and the observer  100  is in the range 0.2 to 0.6 meters.  
         [0059]     It is advantageous to select the dimensions W 1 , W 2  of the exit aperture such that the size of the exit aperture  16  corresponds to two or three lines of text having a font size of 12 points. The dimension W 1  is advantageously 10 mm. The dimension W 2  is advantageously an order of magnitude greater than the dimension W 1 , allowing two or three entire lines of text to be visible at a glance, when the distance L 1  is in the range 0.2 to 0.6 meters.  
         [0060]     According to the present invention, the aspect ratio of said virtual image V 1  is substantially different from the aspect ratio of said exit aperture  16 . In other words, the ratio of the first angular dimension α 1  of said virtual image V 1  to the second angular dimension α 2  of said virtual image V 1  is substantially different from the ratio of the first dimension w 1  of said exit aperture  16  to the second dimension w 2  of said exit aperture  16 . The expression “substantially different” means that the ratio of the aspect ratio of the virtual image V 1  to the aspect ratio of the exit aperture is outside the range 0.5 to 2. Thus, a virtual widescreen image V 1  with 16:9 aspect ratio viewed through a 10:1 exit aperture  16  belongs into the category “substantially different”, but a virtual widescreen image V 1  with 16:9 aspect ratio viewed through a 1:1 exit aperture  16  does not fall into said category.  
         [0061]     When determining the aspect ratio, the substantially uniform light-transmitting area should be considered. For example, when two 5 cm×1 cm exit apertures  16  are positioned next to each other, the substantially uniform exit aperture has dimensions 10 cm×1 cm and a respective aspect ratio of 10:1.  
         [0062]     Advantageously, the aspect ratio of the exit aperture  16  is greater than or equal to 10:1, or when oriented vertically, the aspect ratio is smaller than or equal to 1:10.  
         [0063]     Referring to  FIG. 6 , a portable device  200  may comprise a display module  40  according to the present invention. The observer  100  may select the visible portion V 2  of the virtual image V 1  by tilting the device  200 , for example around the axis  8 . The diffractive beam expander  10  has small dimensions, and therefore the device  200  may be very compact. Advantageously, the device  200  is a slim pen-type device, which is easy to carry in a pocket and easy to handle.  
         [0064]     The device  200  may be, for example, selected from the following list: a display module connectable to a further device, portable device, device with wireless telecommunicating capabilities, imaging device, image scanner, digital camera, mobile phone, gaming device, music recording/playing device (based on e.g. MP3-format), remote control transmitter or receiver, wrist watch, compass, heartbeat monitoring device, medical instrument, measuring instrument, industrial measuring instrument, process control device, target finding device, aiming device, navigation device, personal digital assistant (PDA), communicator, portable internet appliance, hand-held computer, accessory to a mobile phone  
         [0065]     The device  200  may comprise two or more display modules  40  to enlarge the effective exit aperture  16 . The device  200  may comprise a display module  40  for the left eye and a second display module  40  for the right eye of an observer  100 . The device  200  may also comprise two separately controlled display units  40  to display three-dimensional images.  
         [0066]     The micro-display  22  is operatively connected to a controlling unit (not shown), which provides controlling signals to the micro-display  22 . The displayed image may be a still image or a moving image. The device  200  may also comprise battery, telecommunicating unit, control buttons, keyboard, audio devices, data storage units etc.  
         [0067]     Advantageously, the micro-display  22  is able to display e.g. Latin, Arabic or Chinese alphabets. When viewing vertically oriented text, e.g. typical Chinese writing, the device  200  may be held vertically. The device may have at least two different modes of operation, one for horizontal text and one for vertical text.  
         [0068]     A mask with an aperture may be positioned over the diffractive beam expander  10  to protect the surface and to enhance the visual appearance of the device  200 . The mask may slightly reduce the dimensions of the exit aperture  16 .  
         [0069]     The display module  40  may comprise elements to affect the angular height α 1  of the virtual image V 1  irrespective of the angular width α 2 , and vice versa. The display module  40  may comprise further reflecting or image rotating elements to optimize the use of available space in the device  200   
         [0070]     Advantageously, the entire display module  40  or the entire portable device  200  is tilted. However, for the selection of the visible portion, it is sufficient to tilt only the combination of the micro-display  22  and the imaging optics  24 .  
         [0071]     The exit aperture  16  may also have another form than a rectangular form. For example, it may be ellipsoidal, it may have rounded corners or it may even have a rhombic form to create a fancy impression. In those cases the dimension W 1  of the exit aperture refers to the greatest dimension of the exit aperture  16  and the dimension W 2  refers to the dimension perpendicular to the dimension W 1 , allowing the aspect ratio W 2 /W 1  to be defined.  
         [0072]     It is also possible to hold the display module  40  near the eye of the observer, i.e. within a distance less than 3 cm. In that case the display module  40  acts as a near-eye-display.  
         [0073]     For the person skilled in the art, it will be clear that modifications and variations of the devices and method according to the present invention are perceivable. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.