Abstract:
A display device produces a virtual display image based on contents of display information. The device comprises a plurality of light emitting elements arranged in line, and an optical system made of transparent material so as to allow the light outputted from the light emitting elements to enter and exit the optical system. The optical system is rotated by a rotating unit, and a control unit controls lighting-on and light-off of the light emitting elements in synchronization with rotation movement of the optical system and according to the contents to be displayed. A color filter is putted on a surface of the optical system and selectively passes the light in a predetermined wavelength band through the color filter.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a display device which is available on motor vehicles, boats and ships, aircrafts, and others and can provide a virtual display image based on image information, such as drive information.  
         [0003]     2. Description of the Related Art  
         [0004]     A display device of this kind is disclosed in Japanese patent laid-open No. (Tokkaihei) 7-20794. This display device includes plural light emitting elements disposed in line, a polygonal column-like mirror with plural reflective surfaces, a lens to diverge light outputted from the light emitting elements, a drive unit to rotate the mirror, a screen to display two-dimensional virtual display image, and a controller to control light-on timing of the light emitting elements according to a rotational position of the mirror.  
         [0005]     In the conventional display device, light outputted from the light emitting elements passes through the lens or is reflected on the mirror to produce the virtual display image on the screen.  
         [0006]     The screen generally is made of transparent material and provided at its front and back sides with a satin finished surface, or made of an opalescent plate. The screen causes incident rays to diffuse in every possible direction, such as a transmitted side direction and/or a reflected side direction. Therefore, some of the rays do reach an eye position of a driver because of light reflection and diffusion, causing light-energy loss to decrease brightness of the image.  
         [0007]     In order to obtain a sufficiently bright display image on the screen, a large scaled and powerful light-emitting resource is required, which causes the display device to grow in size and increase running cost due to high electric power consumption. In addition, it also increases generation of heat, decreasing its life.  
         [0008]     On the other hand, in the conventional display device, many light sources for emitting different colors are needed in order to produce a multicolor virtual display device. This increases manufacturing cost. In addition, a multangular-prism mirror of the conventional display device also increases the manufacturing cost.  
         [0009]     It is, therefore, an object of the present invention to provide a display device which overcomes the foregoing drawbacks and can decrease light energy loss so as to obtain a sufficiently display brightness by a smaller and lower-cost light source and produce multicolor virtual display at lower manufacturing cost.  
       SUMMARY OF THE INVENTION  
       [0010]     According to a first aspect of the present invention there is provided a display device for producing a virtual display image based on contents of display information. The display device comprises a plurality of light emitting elements which are arranged in line and capable of emitting light, and an optical system made of transparent material so that the optical system allows the light outputted from the light emitting elements to enter and exit the optical system. The display device also comprises a rotating unit for rotating the optical system and a control unit for controlling lighting-on and light-off of the light emitting elements in synchronization with rotation movement of the optical system and according to the contents of the image information to be displayed. A color filter is putted on a surface of the optical system and selectively passes the light in a predetermined wavelength band through the color filter.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The objects, features and advantages of the present invention will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which:  
         [0012]      FIG. 1  is a schematic side view showing an optical system used in a display device of a first embodiment according to the present invention;  
         [0013]      FIG. 2  is a schematic plan view showing the optical system and an electronic system, which are used in the display device of the first embodiment shown in  FIG. 1 ;  
         [0014]      FIG. 3  is an enlarged perspective view of a light emitting diode (LED) array and a slit plate having a slit for passing light from the LED array therethrough, both being used in the display device of the first embodiment shown in  FIGS. 1 and 2 ;  
         [0015]      FIG. 4  is a schematic diagram illustrating an example of how a virtual display image is produced by the display device of the first embodiment when the optical system is tilted at 40° in a counterclockwise direction relative to an optical axis;  
         [0016]      FIG. 5  is a schematic diagram showing virtual display images when the optical system is tilted at angles −40°, −20°, 0°, +20° and +40° relative to the optical axis in the first embodiment;  
         [0017]      FIG. 6  is a diagram showing a relationship between wavelengths and transmittance characteristics of light when it passes through yellow and/or blue filters;  
         [0018]      FIG. 7A  is a schematic diagram showing an example of a two-dimensional virtual display image obtained by using five light emitting elements of the LED array of the display device in the first embodiment, and  FIG. 7B  is a schematic diagram showing an example of two-dimensional virtual display image, representing letters “ABC”, obtained by using twenty-four light emitting elements of the LED array of the display device in the first embodiment;  
         [0019]      FIG. 8  is a time chart of rotation control of the optical system and light-on control of the light emitting elements of the display device of the first embodiment;  
         [0020]      FIG. 9  is a schematic side view showing an optical system used in a display device of a second embodiment according to the present invention;  
         [0021]      FIG. 10A  is a schematic diagram showing an example of a two-dimensional virtual display image obtained by using five light emitting elements of the display device of the second embodiment, and  FIG. 10B  is a schematic diagram showing an example of two-dimensional virtual display image, representing letters “ABC”, obtained by using twenty-four light emitting elements of the display device of the second embodiment;  
         [0022]      FIG. 11  is a schematic side view showing an optical system used in a display device of a third embodiment according to the present invention;  
         [0023]      FIG. 12  is a schematic plan view showing the optical system and an electronic system, which are used in the display device of the third embodiment shown in  FIG. 11 ;  
         [0024]      FIG. 13  is a perspective view showing the optical system of the display device of the third embodiment;  
         [0025]      FIG. 14  is a schematic diagram showing an example of a two-dimensional virtual display image obtained by using five light emitting elements of the display device of the third embodiment;  
         [0026]      FIG. 15  is a schematic side view showing an optical system used in a display device of a fourth embodiment according to the present invention;  
         [0027]      FIG. 16  is a schematic plan view showing the optical system and an electronic system, which are used in the display device of the fourth embodiment shown in  FIG. 15 ;  
         [0028]      FIG. 17  is a schematic diagram showing an example of a two-dimensional virtual display image obtained by using five light emitting elements of the display device of the fourth embodiment;  
         [0029]      FIG. 18  is a time chart of rotation control of the optical system and light-on control of the light emitting elements of the display device of the fourth embodiment;  
         [0030]      FIG. 19  is a schematic diagram showing an optical system of a display device of a fifth embodiment according to the present invention;  
         [0031]      FIG. 20  is a schematic plan view showing the optical system and an electronic system, which are used in the display device of the fifth embodiment shown in  FIG. 19 ;  
         [0032]      FIG. 21  is an enlarged perspective view showing light emitting elements and a slit plate having a slit for passing light from the light emitting elements therethrough, both being used in the display device of the fifth embodiment shown in  FIGS. 19 and 20 ;  
         [0033]      FIG. 22  is a schematic diagram showing an example of a two-dimensional virtual display image obtained by using five light emitting elements of the display device of the fifth embodiment; and  
         [0034]      FIG. 23  is a time chart of rotation control of the optical system and light-on control of the light emitting elements of the display device of the fifth embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     Throughout the following detailed description, similar reference characters and numbers refer to similar elements in all figures of the drawings, and their descriptions are omitted for eliminating duplication.  
         [0036]     Referring to FIGS.  1  to  8  of the drawings, there is shown a first preferred embodiment of a display device according to the present invention.  
         [0037]     A display device of the first embodiment is used in motor vehicles to produce a virtual display image for providing, for example, a driver with information he or she wants. In this case, the display device is installed in an instrument panel of the motor vehicle, for example.  
         [0038]     The display device includes a prism  2 , a light emitting diode (LED) array  1 , a slit plate  3 , a control part  4  and a drive part  21  as shown in  FIGS. 1 and 2 . The LED array  1  and the drive part  21  are electrically connected to the control part  4 , all of them constitute an electric system of the display device. The prism  2  acts as an optical system of the present invention.  
         [0039]     The LED array  1  has five light emitting elements composed of a first LED  1   a,  a second LED  1   b,  a third LED  1   c,  a fourth LED  1   d  and a fifth LED  1   e  in this embodiment. The first to fifth LEDs  1   a  to  1   e  are arranged in line with one another in a horizontal direction, indicated by an arrow AD in  FIG. 2 , and equally spaced from the adjacent one. These five LEDs  1   a  to  1   e  are also arranged in parallel with a rotation axis O 1  of the prism  2 , where the third LED  1   c  is on an optical axis D which is perpendicular to the rotation axis O 1  and passes a center  20  of the prism  2  and an eye position  5  of the driver.  
         [0040]     The LEDs  1   a  to  1   e  are constructed to emit white light toward the eye position  5  through the prism  2  in the first embodiment, and light-on and light-off timing of them are controlled by the control part  4 .  
         [0041]     The slit plate  3  is disposed between the LED array  1  and the prism  2  to block off the light emitted from the LED array  1  except through a slit  3   a  formed thereon.  
         [0042]     The slit  3   a  is formed to be parallel with an LED arrangement direction indicated by the arrow AD and also to the rotation axis O 1  of the prism  2  so that the optical axis D can pass through the slit  3   a.  The slit  3   a  is set to have an opening height H and an opening width so that diffusion angles of the light outputted from the first to fifth LEDs  1   a  to  1   e  through the slit  3   a  are restricted and all of the light passing through the slit  3   a  can travel to the prism  2 .  
         [0043]     The prism  2  is shaped in a quadrangular prism, which is made of transparent acrylic by injection molding. The prism  2  has four surfaces for receiving the light from the LEDs  1   a  to  1   e  through the slit  3   a  and outputting the light therefrom toward the eye position  5 . They are a first surface  2   a,  a second surface  2   b  perpendicular to the first surface  2   a,  a third surface  2   c  perpendicular to the second surface  2   b  and parallel with the first surface  2   a,  and a fourth surface  2   d  perpendicular to the first and third surfaces  2   a  and  2   c  and parallel with the second surface  2   b.    
         [0044]     Two opposite surfaces of the prism  2  act as a set of paired surfaces of the present invention. Therefore, the prism  2  of the first embodiment has two sets of the pared surfaces consisting of a set of the first surface  2   a  and the third surface  2   c,  and a set of the second surface  2   b  and the fourth surface  2   d.  One of them acting as first pared surfaces in first color of the present invention and the other of them acting as second pared surfaces in second color of the present invention, respectively. The pared surfaces are designed to have a distance of 20 mm between the paired surfaces in this embodiment.  
         [0045]     The prism  2  is rotatable at the center of the rotation axis O 1  to change its surfaces so that one of the first to fourth surfaces  2   a  to  2   d  can be subjected to incident light passing through the slit  3   a.    
         [0046]     A first yellow filter Y 1  and a second yellow filter Y 2  are putted on the first and third surfaces  2   a  and  2   c,  respectively. A first blue filter B 1  and a second blue filter B 2  are putted on the second and fourth surfaces  2   b  and  2 d, respectively. The filters Y 1 , Y 2 , B 1  and B 2  act as a color filter of the present invention, and are made of colored translucent material, such as  15  translucent color film, translucent color printing, or the like. The filters Y 1 , Y 2 , B 1  and B 2  adhere to the surfaces  2   a,    2   c,    2   b  and  2   d  of the prism  2 , respectively, but they are illustrated to be apart from each other for facilitating visualization of their presence in the drawings.  
         [0047]     The control part  4  is electrically connected to the LED array  1  and the drive part  21 . It comprises a not-shown microcomputer for controlling timing of light-on and light-off of the first to fifth LEDs  1   a  to  1   e  and a not-shown display electric circuit for supplying electric pulse current to them according to a first control signal outputted from the microcomputer. The first control signal is determined based on a rotational angle of the prism  2  and display information. The control part  4  acts as a control unit of the present invention.  
         [0048]     The drive part  21  is electrically connected to the control part  4  and mechanically connected with the rotation axis O 1 . It includes a not-shown electric motor and a not-shown electric power supply circuit for supplying electric power to the motor according to a second control signal outputted from the control part  4 . The second control signal is determined based on the number of the surfaces  2   a  to  2   d  of the prism  2  and time for obtaining afterimage effect. The drive part  21  acts as a drive unit of the present invention.  
         [0049]     The display device of the first embodiment can produce a two-dimensional virtual display image without a screen, although it is needed in the conventional display device.  
         [0050]     The operation of the display device of the first embodiment will be described.  
         [0051]     As shown in  FIGS. 1 and 2 , the first to fifth  1   a  to  1   e  of the LED array  1  output the white light toward the prism  2  through the slit plate  3 . The prism  2  is positioned at a rotation angle 0° relative to the optical axis in D  FIG. 1 . At this rotation angle, the first surface  2   a  of the prism  2  is in a state perpendicular to the optical axis D to face the slit  3   a  of the slit plate  3  and receive the incident light. Therefore, the ray travels outputted from the third surface  2   c,  which is also perpendicular to the optical axis D, straight through the prism  2  along the optical axis D to the eye position  5 .  
         [0052]     The ray outputted from the prism  2  becomes deflected according to the rotation angle of the prism  2 , when the rotation angle of the prism  2  is not equal to 0°, ±90°×n where n is a natural number.  FIG. 4  shows an example when the rotation angle is 40° relative to the optical axis D in a counterclockwise direction in  FIG. 4 .  
         [0053]     In this case, the ray inputted to the first surface  2   a  is reflected and deflected by the prism  2 , a deflection ray L 4  outputting from the third surface  2   c  at a deflection angle α of 0.62° relative to the optical axis D in the counterclockwise direction. Therefore, the driver can see its virtual display image S(+40°), which is produced on an LED array side of an extended line of the deflection ray L 4  and on an image plane PS.  
         [0054]     Thus, the ray outputted from the prism  2  is deflected by the prism  2  according to the rotation angle of the prism  2 .  
         [0055]      FIG. 5  shows a relationship between the virtual display images and the rotation angles of the prism  2  when the prism  2  is rotated in a rotating direction AR to have the rotation angles −40°, −20°, 0°, +20° and +40°. Incidentally, “+” indicates the counterclockwise direction from the optical axis D, and “−” indicates a clockwise direction from the optical axis D. According to these rotation angles, five virtual display images S(−40°), S(−20°), S(0°), S(+20°) and S(+40°) can be obtained on the image plane PS due to their deflection rays L 1  to L 4  deflected by the prism  2 . A deflection angle difference between the ±40° deflection rays L 1  and L 4  becomes an angle γ of 1.243°.  
         [0056]     This means that deflection angles of the rays outputted from the prism  2  can be varied according to the rotation angle of the prism  2  to produce the virtual display images S(−40°), S(−20°), S(0°), S(+20°) and S(+40°) on the image plane PS from downward toward upward as the prism  2  moves from −40° to +40° in the counterclockwise rotating direction AR. Therefore, the rays outputted from the prism  2  are deflected at different deflection angles according to the rotation angle of the prism  2  and travel to the eye position  5 . The driver can see these two-dimensional virtual display images produced by the deflection rays on the image plane PS. Not that light-on of the LEDs  1   a  to  1   e  is required in order to produce the virtual display images in addition to rotation control of the prism  2  described above.  
         [0057]     The LEDs  1   a  to  1   e  are controlled to emit white light for a predetermined short time by the control part  4  when the rotation angle becomes a predetermined angle, for example −40°, −20°, 0°, +20° and +40°. The light-on time and the rotation speed of the prism  2  are determined based on the afterimage effect and display information.  
         [0058]      FIG. 7A  shows the two-dimensional virtual display image S in three colors by the rotation control of the prism  2  and the light-on control of the LEDs  1   a  to  1   e.  The display image S consists of five-column and eleven-line dot images as shown in  FIG. 7A , including two-yellow-column dot images S 1   a  and S 1   b,  two-blue-column dot images S 1   c  and S 1   d  and one-white-column dot images S 1   e.  The positions of the five column dot images S 1   a  to S 1   e  are produced corresponding to five LEDs  1   a  to  1   e  and these positions. The eleven lines are produced by driving the LED array I to emit the white light eleven times. The colors of the dots of the display image S are produced from the white light of the LEDs  1   a  to  1   e  by passing through the yellow and blue filters Y 1 , Y 2 , B 1  and B 2 .  
         [0059]     How to obtain yellow, blue and white rays from the white light will be described with reference to  FIG. 6 .  
         [0060]      FIG. 6  shows a relationship between wavelengths and transmittance characteristics of light when it passes through the yellow and/or blue filters Y 1 , Y 2 , B 1 , and/or B 2 . The transmittance characteristic when the white light passes through the yellow filters Y 1  and Y 2  is indicated by a line Ta, the transmittance characteristic when the white light passes through the blue filters B 1  and B 2  is indicated by a line Tb, and the transmittance characteristic when the white light passes the yellow filter Y 1  or Y 2  and the blue filter B 1  or B 2  is indicated by a line Tc.  
         [0061]     Therefore, the yellow ray outputted from the prism  2  is produced from the white light emitted from the LED array I by passing the white light through the yellow filters Y 1  and Y 2  on the first and third surfaces  2   a  and  2   c  of the prism  2 . Similarly, the blue ray outputted from the prism  2  is produced from the white light by passing it through the blue filters B 1  and B 2  on the second and fourth surfaces  2   b  and  2   e  of the prism  2 .  
         [0062]     The white ray, outputted from the prism  2  with the color filters Y 1 , Y 2 , B 1 , and B 2 , can be obtained by passing the white light outputted from the LED array  1  through the yellow filter Y 1  or Y 2  and the blue filter B 1  or B 2 , because yellow light and blue light are complementary colors. But, in the first embodiment, the white ray is obtained not by passing the white light through one of the yellow and blue filters and then through the other of them. It is obtained by individually producing the yellow ray and the blue ray by respectively using the yellow filters Y 1  and Y 2  and the blue filters B 1  and B 2  at the same positions so as to be mixed up into the white ray due to the afterimage effect. Note that the LEDs  1   a  to  1   e  are controlled to stop emitting the white light so that it does not pass through the yellow filter Y 1  or Y 2  and the blue filter B 1  or B 2  at the same time  
         [0063]     As described above, the color of the deflection ray can be changed according to the rotation angle of the prism  2  and the light-on timing of the LED array  1 .  
         [0064]     The control part  4  controls the LED array  1  to emit the white light predetermined times when the prism  2  are rotated at high speed and reaches predetermined rotation angles of the prism  2 . The high speed rotation control of the prism  2  and light on/off control of the five LEDs  1   a  to  1   e  can produce the two-dimensional virtual display image due to afterimage effect.  
         [0065]     In this embodiment, the five LEDs  1   a  to  1   e  are used and their light-on control is performed eleven times according to the rotation angles of the prism  2 . This provides the virtual display image S represented by five-column and eleven-line dot images. Each dot image produced by the LED  1   a  to  1   e  becomes vertically long, since the prism  2  rotates at constant angular speed and light-on time of the LEDs  1   a  to  1   e  has limited length. The vertical length of the dot image can be adjusted by changing the opening height H of the slit  3   a  of the slit plate  3  shown in  FIG. 3  and/or by controlling a length of the light-on time of the LEDs. The horizontal length of the dot image is determined by a distance between the first and fifth LEDs  1   a  and  1   e.    
         [0066]     The rotation control of the prism  2  and light-on control of the LED will be described with reference to a time chart shown in  FIG. 8 .  
         [0067]     The most upper part of a time chart in  FIG. 8  indicates a rotation clock signal CR, which appears at every one rotation of the prism  2 . Accordingly, an interval between adjacent rotation clock signals is equal to a time for one rotation of the prism  2 .  
         [0068]     The second part of the time chart indicates a surface clock signal CS, which appears when every one surface  2   a,    2   b,    2   c,    2   d  passes through a predetermined position. In this first embodiment, four surface clock signals SF 1  to SF 4  appear in one rotation cycle time t 1  between the adjacent rotation clock signals, because the prism  2  has the four surfaces  2   a  to  2   d.  An interval of adjacent surface clock signals is a time for producing one display image, corresponding to a display period TD indicated at a bottom part in the time chart. In this time chart, WLED indicates a waveform of the LEDs and consists of waveforms W 1   a  to W 1   e  of electric power pulses to be supplied to the first to fifth LEDs  1   a  to  1   e,  respectively.  
         [0069]     In one surface time of an interval between the adjacent surface clock signals, some of the five LEDs  1   a  to  1   e  are supplied with eleven electric power pulses r 1  to r 11  according to contents of the display information a predetermined rest period IP after the surface clock appears. Note that fifth to ninth pulses of the eleven electric power pulses r 1  to r 11  are omitted in  FIG. 8  for simplicity. The eleventh pulse r 11  disappears the rest period IP before the next surface clock signal appears.  
         [0070]     After next one surface time, the same LEDs are supplied with eleven electric power pulses r 1  to r 11  again similarly, which is repeated till the contents are changed, producing the virtual display image by using the rotation control of the prism  2 , the light-on control of the LED array  1  and the afterimage effect.  
         [0071]     In general, the virtual display image using the afterimage effect can be obtained without image-flickering by displaying the image at the rate of at least not less than 50 Hz, preferably not less than 100 Hz.  
         [0072]     In this first embodiment, a display cycle is set to be 100 Hz, and therefore a rotation clock signal interval (the one rotation cycle time) t 1  becomes 0.04 second and a surface clock signal interval (the one surface time: TD/2 in the first embodiment) becomes 0.01 second. This means that the drive part  21  rotates the prism  2  at 0.04 sec/rev., corresponding to 1500 r.p.m., a relatively low speed for current electric motors. Accordingly, a low cost motor can be employed and its control becomes easier in the first embodiment, thereby increasing its life.  
         [0073]     The color of the image S is obtained as follows.  
         [0074]     As shown in  FIG. 8 , in this first embodiment, one display period TD consists of one yellow display period YD and one blue display period BD, each of which corresponds to one surface clock signal interval. Each of the yellow display period YD and the blue display period BD consists of the two rest periods IP and one light-on control period DP between the rest periods IP, respectively. In the light-on control period DP, the eleven pulses r 1  to r 11  are supplied to some of the LEDs  1   a  to  1   e  according to the contents of the display information.  
         [0075]     In the yellow display period YD, the first, the second and fifth LEDs  1   a,    1   b  and  1   e  are supplied with the eleven pulses r 1  to r 11  at the same time in its light-on control period DP to each emit the white light. These white lights pass through the yellow filters Y 1  and Y 2  on the first and third surfaces  2   a  and  2   c  of the prism  2  to be changed into three yellow deflection rays. These three yellow rays appear eleven times at first and second columns (two left columns in  FIG. 7A ) and a fifth column (a right column in  FIG. 7A ) of the display image S in the yellow display period YD.  
         [0076]     In the next blue display period BD, the third, fourth and fifth LEDs  1   c,    1   d  and  1   e  are supplied with the eleven pulses r 1  to r 11  at the same time and also at the same timing of the yellow display period YD in a light-on control period TD to each emit the white light. These white lights pass through the blue filters B 1  and B 2  on the second and fourth surfaces  2   b  and  2   d  of the prism  2  to be changed into three blue deflection rays. These three blue rays appear eleven times at third and forth columns and the fifth column of the display image S as shown in  FIG. 7A  in the blue display period YD.  
         [0077]     In the result, the display image S is produced by five-column and eleven-line dot images, which are arranged in the first and second columns in yellow, the third and fourth columns in blue, and the fifth column in white. Note that the fifth column dot images S 1   e  become white, because the driver see the yellow dot images in the yellow display period YD and then the blue dot images in the blue display period BD at the fifth column before the afterimage of the yellow dot images disappear. Accordingly the driver mixes up the yellow and blue dot images, which are produced at the same positions because of their light-on timing, into the white dot images due to the afterimage effect, since they are complementary colors. Incidentally, the white dot images are aligned on the line with the yellow and blue dot images, because the fifth LED  1   e  is controlled to emit its white light simultaneously with the light-on of the other LEDs.  
         [0078]     The five LEDs  1   a  to  1   e  are used in the first embodiment, but the number of the LEDs may be changed according to need for contents of display information by varying a horizontal length of the display image. For example, twenty four LEDs may be used as shown in  FIG. 7B , which represents a virtual display image S′ of letters “ABC”. In addition, the LEDs may be supplied with more or less electric power pulses to change the number of lines of the virtual display image.  
         [0079]     The display device of the first embodiment has the following advantages.  
         [0080]     The display device of the first embodiment can produce the two-dimensional display image without a screen, so that it can decrease light energy loss to obtain a sufficiently display brightness. This enables the display device to have a smaller and lower-cost light source, and its dimension, manufacturing and running cost can be reduced.  
         [0081]     The display device does not need a high-cost multangular-prism mirror, and the prism  2  can be formed by injection molding of transparent resin, enabling it to decrease its manufacturing cost.  
         [0082]     The display device can produce the virtual display image in different colors at lower manufacturing cost by using the color filters Y 1 , Y 2 , B 1  and B 2  in different colors on the paired surfaces  2   a  and  2   c,  and  2   b  and  2   d  of the prism  2  under its rotation control and the LEDs  1   a  to  1   e  in the same colors under the light-on control. This can provide the virtual display image in excellent visibility and representable in copious contents of display information.  
         [0083]     The color filters change the color of the light emitted from the LEDs  1   a  to  1   e  by selecting the color filters, which enables the virtual display image to be produced in different colors even when using the same type LEDs emitting the same color light. This reduces its manufacturing cost.  
         [0084]     Using the two different-color filters in complementary colors, such as yellow and blue, the display device can provide the virtual display device in three colors, such as yellow, blue and white.  
         [0085]     Next, a display device of a second embodiment according to the present invention will be described with reference of the accompanying drawings of  FIGS. 9 and 10 B.  
         [0086]     As shown in  FIG. 9 , the display device of the second embodiment includes a prism  2  shaped in a quadrangular prism and made of transparent acrylic by injection molding. The prism  2  has a first surface  2   a,  a second surface  2   b  perpendicular to the first surface  2   a,  a third surface  2   c  perpendicular to the second surface  2   b  and parallel with the first surface  2   a,  and a fourth surface  2   d  perpendicular to the first and third surfaces  2   a  and  2   c  and parallel with the second surface  2   b.  The first and third surfaces  2   a  and  2   c,  and the second and fourth surfaces  2   b  and  2   d  constitute first pared surfaces and second pared surfaces of the present invention, respectively. Yellow filters Y 1  and Y 2  are putted on the second and fourth surfaces  2   b  and  2   d,  respectively, but no color filter is putted on the first and third surfaces  2   a  and  2   c  in the second embodiment.  
         [0087]     The other parts of the display device of the second embodiment are similar to those of the first embodiment.  
         [0088]     In this second embodiment, the display device can produce virtual display image S consisting of two-column yellow dot images S 1   a  and S 1   b  and three-column white dot images S 1   c  to S 1   e  as shown in  FIG. 10A . The two-column yellow dot images S 1   a  and S 1   b  are produced by emitting white light from first and second LEDs of an LED array  1  to pass through the second and fourth surfaces  2   b  and  2   c  of the prism  2  with the yellow filters Y 1  and Y 2 . The third- to fifth-column white dot images S 1   c  to S 1   e  are produced by emitting the white light from third to fifth LEDs to pass the first and fourth surfaces  2   a  and  2   c  of the prism  2  with no color filters. The LEDs are controlled to emit the white light eleven times during one surface clock signal interval.  
         [0089]      FIG. 10B  shows a two-dimensional virtual display image S′ consisting of eight-column and eleven-line yellow dot images and sixteen-column and eleven-line white dot images and representing letters “ABC” (“A” in yellow and “BC” in white) by using twenty four LEDs instead of five LEDs.  
         [0090]     White display image can provide good visibility at night and/or in a case where its background is black.  
         [0091]     In this second embodiment, LEDs emit the white light, but they may emit light in other color, providing the virtual display image in the same color as the light of LEDs by passing the light through the surfaces of the prism  2  without color filters.  
         [0092]     The display device of the second embodiment has the following advantages in addition to those of the first embodiment. The display device can provide a brighter display image, because the light energy is not attenuated by the color filter when the light passes through the surfaces of the prism  2  without the color filters.  
         [0093]     Next, a display device of a third embodiment according to the present invention will be described with reference of the accompanying drawings of  FIGS. 11 and 14 .  
         [0094]     Referring to  FIGS. 11 and 12 , there is shown the display device of the third embodiment. The display device includes a prism  2  shaped in a quadrangular prism and made of transparent acrylic by injection molding. The prism  2  has a first surface  2   a,  a second surface  2   b  perpendicular to the first surface  2   a,  a third surface  2   c  perpendicular to the second surface  2   b  and parallel with the first surface  2   a,  and a fourth surface  2   d  perpendicular to the first and third surfaces  2   a  and  2   c  and parallel with the second surface  2   b.  The first and third surfaces  2   a  and  2   c,  and the second and fourth surfaces  2   b  and  2   d  act as pared surfaces of the present invention, respectively. One of them corresponds to first pared surfaces in first color of the present invention, and the other of them corresponds to second pared surfaces in second color of the present invention.  
         [0095]     As shown in  FIGS. 12 and 13 , yellow filters Y 1 ′ and Y 2 ′ are putted on half portions of the second and fourth surfaces  2   b  and  2   d  of the prism  2 , respectively, and no color filter is putted on the first and third surfaces  2   a  and  2   c.  A first LED  1   a  and a second LED  1   b  are arranged at one side of an optical axis D and a third LED  1   c,  a fourth LED  1   d  and a fifth LED  1   e  are arranged at the other side of the optical axis D so that white lights emitted from only the first and second LEDs  1   a  and  1   b  can pass through the yellow filters Y 1 ′ and Y 2 ′. A space between the second LED  1   b  and the third LED  1   c  is set to be larger than those between the first and second LEDs  1   a  and  1   b,  between the third and fourth LEDs  1   c  and  1   d,  and between the fourth and fifth LEDs  1   d  and  1   e  so that a driver can recognize color difference of a virtual display image more clearly.  
         [0096]     The other parts of the display device of the second embodiment are similar to those of the first embodiment.  
         [0097]     In this third embodiment, the display device can provide the virtual display image consisting of yellow dot images S 1   a  and S 1   b  and white dot images S 1   c  to S 1   e  similarly to that of the first embodiment as shown in  FIG. 14 .  
         [0098]     In addition, the three- and fourth- column white dot images S 1   c  and S 1   d  can be displayed at a low level and the fifth-column white dot images S 1   e  can be displayed at a high level. The three- and fourth-column white dot images S 1   c  and S 1   d  at the low level are obtained by emitting the white light from the third and fourth LEDs  1   c  and  1   d  to pass through only the first and third surfaces  2   a  and  2   c  of the prism  2  with no filter. The fifth-column white dot images S 1   e  at the high level are obtained by individually emitting the white light from the third and fourth LEDs  1   c  and  1   d  to pass through the first and third surfaces  2   a  and  2   c  of the prism  2  with no filter and to pass through no-filter portions of the second and fourth surfaces  2   b  and  2   d  of the prism  2 , adding them to each other. Therefore, a brightness level of the fifth-column white dot images S 1   e  becomes higher than those of the yellow dot images S 1   a  and S 1   b  and the white dot images S 1   c  and S 1   d.    
         [0099]     The display device of the third embodiment has the following advantages in addition to those of the first and second embodiments. The display device can provide the virtual display images at different brightness levels without changing electric power level for the LEDs and using different types of the LEDs.  
         [0100]     Next, a display device of a fourth embodiment according to the present invention will be described with reference of the accompanying drawing of FIGS.  15  to  18 .  
         [0101]     Referring to  FIGS. 15 and 16 , there is shown the display device of the fourth embodiment, which includes an LED array  1  for emitting white light and a prism  6  rotatable for deflecting the white light outputted from the LED array  1 .  
         [0102]     The prism  6  is shaped in a hexangular prism and made of transparent acrylic by injection molding to act as an optical system of the present invention. The prism  6  has a first surface  6   a,  a second surface  6   b,  a third surface  6   c,  a fourth surface  6   d,  a fifth surface  6   e  and a sixth surface  6   f  as shown in  FIG. 15 .  
         [0103]     The first surface  6   a  and the fourth surface  6   d  are parallel with each other, and blue filters B 1  and B 2  adhere to them, respectively. The second surface  6   b  and the fifth surface  6   e  are parallel with each other, and green filters G 1  and G 2  adhere to them, respectively. The third surface  6   c  and the sixth surface  6   f  are parallel with each other, and red filters R 1  and R 2  adhere to them, respectively.  
         [0104]     The first surface  6   a  and the fourth surface  6   d,  the second surface  6   b  and the fifth surface  6   e,  and the third surface  6   c  and the sixth surface  6   f  act as pared surfaces of the present invention. Therefore, the prism of the fourth embodiment has three sets of the pared surfaces. One of them corresponds to first pared surfaces in first color of the present invention, another of them corresponds to second surfaces in second color of the present invention, and the other of them corresponds to third surfaces in third color of the present invention. Note that the first to third colors are light&#39;s primary colors, specifically blue, green and red.  
         [0105]     The other parts of the fourth embodiment are similar to those of the first embodiment except light-on control.  
         [0106]     The prism  6  and first to fifth LEDs  1   a  to  1   e  of the LED array  1  are controlled so as to produce a virtual display image S shown in  FIG. 17  under rotation control of the prism  2  and light-on control of the LEDs  1   a  to  1   e  performed by a control part  4 .  
         [0107]      FIG. 18  shows a time chart of the rotation control and the light-on control.  
         [0108]     In this embodiment, six surface clock signals SF 1  to SF 6  appears in one rotation of the prism  2 .  
         [0109]     As shown in  FIG. 18 , one rotation clock signal interval t 1  consists of two display periods TD, and the one display period TD consists of one blue display period BD, one green display period GD and one red display period RD.  
         [0110]     Each blue, green and red display period BD, GD and RD corresponds to one surface clock signal interval (TD/3 or t 1 / 6  in the fourth embodiment) and has two rest periods IP and one light-on control period DP between the rest periods IP. In the light-on control period DP, the control part  4  supplies first to sixth electric power pulses r 1  to r 6  to some of the LEDs  1   a  to  1   e.    
         [0111]     In the blue display period BD, the first LED  1   a,  the fourth LED  1   d  and the fifth LED  1   e  are supplied with the six pulses r 1  to r 6  to emit white light to pass through the first surface  6   a  of the prism  6  with the blue filter B 1  and the fourth surface  6   d  of the prism  6  with the blue filter B 2 , being changed into blue rays.  
         [0112]     In the next green display period GD, the second LED  1   b,  the fourth LED  1   d  and the fifth LED  1   e  are supplied with the six pulses r 1  to r 6  to emit the white light to pass through the second surface  6   b  of the prism  6  with the green filter G 1  and the fifth surface  6   e  of the prism  6  with the green filter G 2 , being changed into green rays.  
         [0113]     In the following red display period RD, the third LED  1   c,  the fourth LED  1   d  and the fifth LED  1   e  are supplied with the six pulses r 1  to r 6  to emit the white light to pass through the third surface  6   c  of the prism  6  with the red filter R 1  and the sixth surface  6   f  of the prism  6  with the red filter B 2 , being changed into red rays.  
         [0114]     Therefore, these blue, green and red rays produce the virtual display image S consisting of five-column and six-line dot images S 1   a  to S 1   e.  The first-column dot images S 1   a  is blue, the second-column dot images S 1   b  is green, the third column dot images S 1   c  is red, and the fourth- and fifth-column dot images S 1   d  and S 1   e  are white. This white color is produced by mixing up the blue, green and red rays, which are light&#39;s three primary colors, at the same positions by using the afterimage effect.  
         [0115]     The display device of the fourth embodiment has the following advantages in addition to those of the first and second embodiments. The display device can provide a full-color virtual display image, thereby improving its display quality.  
         [0116]     A display device of a fifth embodiment will be described with reference to the accompanying drawings of FIGS.  19  to  23 .  
         [0117]     Referring to  FIGS. 19 and 20 , there is shown the display device of the fifth embodiment. The display device is provided with an LED array  11  having an upper LED array  11  and a lower LED array  12 , which are spaced by a height Hc from each other as shown in  FIG. 21 . The upper LED array  11  consists of a first upper LED  11   a,  a second upper LED  11   b,  a third upper LED  11   c,  a fourth upper LED  11   d,  a fifth upper LED  11   e,  and the lower LED array  12  consists of a first lower LED  12   a,  a second lower LED  12   b,  a third lower LED  12   c,  a fourth lower LED  12   d,  and a fifth lower LED  12   e.    
         [0118]     The first to fifth upper LEDs  11   a  to lie are aligned in line with one another in a horizontal direction to emit green light, and under them, the first to fifth lower LEDs  12   a  to  12   e  are aligned in line with one another in the horizontal direction to emit white light.  
         [0119]     A slit plate  3  is disposed between the LED arrays  11  and  12  and a prism  2  and is formed with an upper slit  3   a  and a lower slit  3   b  parallel with the upper slit  3   a.  The first slit  3   a  allows the green light emitted from the first to fifth upper LEDs  11   a  to  11   e  to pass therethrough, and the second slit  3   b  allows the white light emitted from the first to fifth lower LEDs  12   a  to  12   e  to pass therethrough.  
         [0120]     The other parts of the fifth embodiment are similar to those of the first embodiment except light-on control of the LEDs  11   a  to  11   e  and  12   a  to  12   e.    
         [0121]      FIG. 23  shows a time chart of rotation control of the prism  2  and the light-on control of the LEDs  11   a  to  11   e  and  12   a  to  12   e.    
         [0122]     In this time chart, waveforms of green lights emitted from the first to fifth upper LEDs  11   a  to  11   e  are indicated by W 1   a  to W 1   e,  respectively. Waveforms of white light emitted from the first to fifth lower LEDs  12   a  to  12   e  are indicated by W 2   a  to W 2   e,  respectively.  
         [0123]     One display period TD consists of one yellow display period YD and one blue display period BD.  
         [0124]     Light-on timings of the upper and lower LED arrays  11  and  12  are controlled in connection with each other so that the upper LED array  11  starts to emit the green light a delay time ta after the lower LED array  12  stats to emit the white light. This can a virtual display image produced by the upper LED array  11  and a virtual display image produced by the lower LED array  12  at the same positions not so as to be dislocated from each other due to the height Hc between the upper and lower LED arrays  11  and  12 . Therefore, the display time ta is set based on the height Hc and rotation speed of the prism  2 .  
         [0125]     Incidentally, a horizontal length of the virtual display image can be adjusted by changing opening widths Ha and Hb of the slit plate  3 .  
         [0126]     Each yellow and blue display period YD and BD corresponds to one surface clock signal interval (TD/2 in the fifth embodiment) and has two rest periods and one light-on control period between the rest periods similarly to that of the first embodiment. In the light-on control period, a control part  4  supplies first to eleventh electric power pulses r 1  to r 11  to some of the LEDs  11   a  to  11   e  and  12   a  to  12   e.    
         [0127]     In the yellow display period YD, the second lower LED  12   b  and the third lower LED  12   c  are supplied with the six pulses r 1  to r 11  to emit the white light. The delay time ta after the first pulses r 1  for the second and third lower LEDs  12   b  and  12   c  appear, the first pulses r 1  for the first and fifth upper LEDs  11   a  and lie start to appear, and the six pulses r 1  to r 11  are supplied to them to emit the green light.  
         [0128]     In the next blue display period BD, the second, fourth and fifth lower LEDs  12   b,    12   d  and  12   e  are supplied with the six pulses r 1  to r 11  to emit the white light. The first to fifth LEDs  11   a  to  11   e  are at rest.  
         [0129]     Therefore, green virtual display dot images S 1   a,  shown in  FIG. 22 , are obtained by using the green light from the first upper LED  11   a  and the yellow filters Y 1  and Y 2  on a first surface  2   a  and a third surface  2   c  of prism. White virtual display dot images S 1   b  are obtained by using the white light emitted from the second lower LED  12   b  and yellow and blue filters Y 1 , Y 2 , B 1  and B 2  on first to fourth surfaces  2   a  to  2   d  of the prism  2 . Yellow virtual display dot images S 1   c  are obtained by using the white light emitted from the third lower LED  12   c  and the yellow filters Y 1  and Y 2 . Blue virtual display dot images S 1   d  are obtained by using the white light emitted from the fourth lower Led  12   d  and the blue filters B 1  and B 2 . Cyan virtual display dot images S 1   e  are obtained by using the green light emitted from the fifth upper LED  1   e  and the yellow filters Y 1  and Y 2 , and also by using the white light emitted from the fifth lower LED  12   e  and the blue filters B 1  and B 2 . Note that these images S 1   a  to S 1   e  are produced by using the afterimage effect.  
         [0130]     The display device of the fifth embodiment has the following advantages in addition to those of the first and second embodiments.  
         [0131]     In the display device of the fifth embodiment, the lights in different colors (green and white in the fifth embodiment) are emitted from the upper and lower LED arrays  11  and  12  can provide the virtual display image in colors produced from a combination of the light-colors and the color filters, improving display quality of the display device.  
         [0132]     While there have been particularly shown and described with reference to preferred embodiments thereof, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.  
         [0133]     For example, the optical system may have other configuration instead of the quadrangular prism and the hexangular prism used in the embodiments.  
         [0134]     The color filter may be constructed to have color different from those in the embodiment, and the light emitting elements may be constructed to emit light different from those of the embodiments.  
         [0135]     The entire contents of Japanese Patent Application No. 2005-090296 filed Mar. 28, 2005 is incorporated herein by reference.