Abstract:
In an image projector having at least one projection beam that is actuated in the raster mode and in the calligraphic mode for representing a raster component and a calligraphic component of a total image that is projected onto a display, to attain a higher image brightness and a sharper image contrast during projection in non-darkened rooms, the at least one projection beam is a laser beam ( 19 ) that is split into two linearly-polarized partial beams ( 21, 21 ′), with the two partial beams ( 21, 21 ′) being subjected to a separate modulation and deflection such that the one partial beam ( 21 ′) writes the raster component and the other partial beam writes the calligraphic component. The two partial beams ( 21, 21 ′) are projected simultaneously onto the display; the partial beams may be optically superposed prior to being projected.

Description:
FIELD OF THE INVENTION 
     The invention relates to an image projector of the generic type defined in the preamble to claim  1 . 
     BACKGROUND OF THE INVENTION 
     Known CRT image projectors, which operate in raster and calligraphic modes (U.S. Pat. No. 4,614,941), and are generally referred to as raster-calligraphic projectors, are used in, for example, flight simulators for displaying a computer-generated image of the aircraft environment. In raster mode, in which the image is written by horizontal and vertical deflections of the light beam, as in a television image, the actual environment scenario is represented with all of the details, such as the tower, landing strip, houses, roads, trees and the like; the calligraphic mode, in which the light or electron beam can be moved in any direction and at any speed, from non-movement to a high-speed pivot, permits the simultaneous display of the very bright runway lighting and colored regions within the environment scenario, resulting in extremely realistic displays of the airfield and its surroundings, as well as the surrounding landscape. 
     The light flux of raster-calligraphic CRT projectors is limited by the cathode-ray tube (CRT), and cannot increase significantly, so the image projection for representing a sufficiently bright simulated image is performed in darkened rooms. In raster-calligraphic CRT projectors, the raster component and the calligraphic component of the total image are written one after the other. This limits the image-repetition rate of the projector. If a large number of calligraphically-displayed lights (runway lighting) is displayed, or the image resolution is very high, the raster component must be displayed in an interlaced manner, in which case the lights are represented calligraphically between the two half-images. The use of half-images leads to flickering of the total image. 
     Laser projectors possessing a considerably higher light flux are used for projecting significantly brighter images that also have adequate contrast and brightness under daylight conditions. Currently, however, these laser projectors can only be operated strictly as raster projectors. 
     It is the object of the invention to render a raster-calligraphic image projector of the type mentioned at the 
     The light flux of raster-calligraphic CRT projectors is limited by the cathode-ray tube (CRT), and cannot increase significantly, so the image projection for representing a sufficiently bright simulated image is performed in darkened rooms. In raster-calligraphic CRT projectors, the raster component and the calligraphic component of the total image are written one after the other. This limits the image-repetition rate of the projector. If a large number of calligraphically-displayed lights (runway lighting) is displayed, or the image resolution is very high, the raster component must be displayed in an interlaced manner, in which case the lights are represented calligraphically between the two half-images. The use of half-images leads to flickering of the total image. 
     In a known image projector of the type mentioned at the outset (U.S. Pat. No. 5,582,518), the partial beam that writes the raster component is generated by a CRT, and the partial beam that writes the calligraphic component is generated by a laser. After the two partial beams have been modulated and deflected appropriately, they are guided by a semi-transparent mirror to a fish-eye lens that images the two separate images together on a spherical projection display. In a modified embodiment of this image projector, the CRT is replaced by a second laser, and the two image components of the projected image are written in the calligraphic mode. 
     In a known arrangement for generating polarized light (U.S. Pat. No. 5,073,830), a non-polarized light beam that is emitted by a light source, e.g., an HeNe laser, is split by a polarization beam splitter into two polarized partial beams having a half-brightness, and whose polarization planes are rotated by 90° relative to one another. The one partial light beam is guided directly to a lens, and the other partial beam is guided to the lens via a 90° deflection mirror and a λ/2 plate; the lens focuses them onto a common spot. The light source that is formed in this way and radiates polarized light is used for, for example, a video projector having a “liquid-crystal” display. 
     In a known image projector (WO 99/12358), after the laser beams emitted by the three lasers for the colors red, green and blue are modulated, they are guided with the image content by a light waveguide to a deflection system that images the laser beams on a display. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to render a raster-calligraphic image projector of the type mentioned at the outset laser-capable, so raster-calligraphically-written images can be projected with a much greater brightness. 
     The object is accomplished by the features of claim  1 . 
     An advantage of the raster-calligraphic image projector in accordance with the invention is that it uses a laser beam as the projection beam, and therefore has a much higher available light flux. Unlike in calligraphic CRT projectors, the splitting of the laser beam into two partial beams allows the raster component and the calligraphic component to be projected simultaneously, so the image-repetition in the raster component is not affected by the number of calligraphically-represented lights. Therefore, high-resolution images having numerous pixels and, simultaneously, a large number of light points, are also projected “non-interlaced.” The light points in the calligraphic component can be represented with a far sharper contrast. Their contrast to the raster component results from the longer sojourn of the partial laser beam at the light points relative to the sojourn of the other partial laser beam at a pixel of the raster line. In the use of a beam splitter that splits the two partial beams in a 1:1 ratio, with 1000 pixels per line in the raster component and five light points to be displayed in the calligraphic component, the partial laser beam for the light point remains 1000:5=200 times as long at one position. The maximum contrast for one light point is 200:1. In the display of numerous light points in the calligraphic component, a different splitting ratio can be selected for splitting the laser beam, so the high light flux compensates the shorter sojourn of the partial laser beam at the individual light points. Guiding together the separately-modulated partial beams of different polarities permits a virtually loss-free superposing of the two partial beams in the projection head. 
     Advantageous embodiments of the image projector in accordance with the invention, and advantageous modifications and embodiments of the invention, ensue from the further claims. 
     In accordance with an advantageous embodiment of the invention, the polarization directions of the polarized partial beams are rotated 90° relative to one another. The use of differently-polarized partial beams in accordance with the invention permits the virtually loss-free superposing of the two partial beams in the projection head. calligraphic component, the partial beam for the light point remains 1000:5=200 times as long at one position. The maximum contrast for one light point is 200:1. In the display of numerous light points in the calligraphic component, a different splitting ratio can be selected for splitting the laser beam, so the high light flux compensates the shorter sojourn of the partial laser beam at the individual light points. 
     Advantageous embodiments of the image projector in accordance with the invention, and advantageous modifications and embodiments of the invention, ensue from the further claims. 
     In accordance with an advantageous embodiment of the invention, the polarization directions of the polarized partial beams are rotated 90° relative to one another. The use of differently-polarized partial beams in accordance with the invention permits the virtually loss-free superposing of the two partial beams in the projection head. 
     In accordance with a preferred embodiment of the invention, a λ/2 plate is positioned in the beam path of one of the two partial beams for rotating the polarization directions. 
     In accordance with a preferred embodiment of the invention, a λ/2 plate is positioned in the beam path of one of the two partial beams for rotating the polarization directions. 
     In accordance with a preferred embodiment of the invention, the modulated partial beams are coupled into glass fibers that maintain the beam polarization, and are supplied to a projection head by being coupled back out of the glass fibers, and optically superposed for projection onto a display. This division of the image projector into a laser component and a modulation component, on the one hand, and a projection component, on the other hand, permits a spatial separation of the two components, which is advantageous in an application in a flight simulator, because only the lower-weight projection head must be disposed on the mobile part of the simulator; the weight of the mobile part can therefore be kept low. 
    
    
     The invention is described in detail below by way of an exemplary embodiment illustrated in the drawing. Shown are in: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a block diagram of a raster-calligraphic image projector having a laser and electronics component and a projection head that can be spatially separated therefrom; 
     FIG. 2 a schematic, detailed representation of the optical beam paths in the laser and electronics component in FIG. 1; 
     FIG. 3 a schematic, detailed representation of the optical beam paths in the projection head in accordance with FIG. 1; and 
     FIG. 4 a schematic, detailed representation of the optical beam paths in the laser and electronics component in a color projector. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The image projector shown schematically in FIGS. 1 through 3 has a laser and electronics component  10  and a projection head  11 , which can be spatially separated from this component; the two are connected by two glass fibers  12 ,  13 , and a signal line  14  and a current-supply line  15 . The laser and electronics component  10  has a power supply  16 , which has a 220-Volt current supply, electronics  17  and an optical component that will be described in detail below. 
     The image projector operates in raster mode and calligraphic mode, with a raster component and a calligraphic component of a total image being generated separately and projected onto a display. The optical component of the image projector is outlined in FIG. 2. A polarized laser beam  19  generated by a laser source  18  is split into two partial beams  21  and  21 ′ in a beam splitter  20 . The splitting ratio is preferably 1:1, but a different ratio can be selected for specific applications. Each partial beam  21  and  21 ′ passes through a modulation branch  28  and  28 ′, respectively. The raster component R of the total image is generated with the partial beam  21 , while the calligraphic component K is generated with the partial beam  21 ′. The polarized light of the partial beam  21  first passes through a polarizer  22 , whose polarization direction coincides with that of the polarized partial beam  21 , and is modulated according to the image content in a downstream electro-optical modulator  23 . The double-headed arrows and dots shown in the beam path symbolize the polarization direction or polarization plane of the light. The polarization plane is the plane in which the polarized light oscillates and propagates. In FIGS. 2 and 3, this is the drawing plane, and the plane extending perpendicular to this plane, respectively. The total image to be projected is generated in a so-called image generator  24 , which correspondingly controls the electronics  17 , which in turn actuates the electro-optical modulator  23  and the electro-optical modulator  23 ′. The modulators  23  and  23 ′ are configured such that, when the maximum modulation voltage is applied, the polarization plane or polarization direction of the partial beam  21  or  21 ′ rotates by 90°. The modulation is effected such that the maximum voltage is applied to the modulator  23  or  23 ′ for the maximum brightness of a pixel. Disposed behind the modulator  23  is a second polarizer  25 , whose polarization plane is oriented perpendicular to that of the first polarizer  22 . 
     The second partial beam  21 ′ of the laser  18 ′ is deflected after the beam splitter  20  with a deflecting prism  26 , and fed into the modulation branch  28 ′. Here, the partial beam  21 ′ passes through the same optical structural component, and in the same manner, as the partial beam  21 ; corresponding structural components of the modulation branch  28 ′ are therefore provided with the same reference characters and distinguished from the optical structural components in the modulation branch  28  by a prime symbol. 
     The partial beams  21 ,  21 ′ exiting the second polarizer  25  and  25 ′, respectively, are linearly polarized in the same polarization plane. To permit a later loss-free, optical superposing of the two partial beams  21 ,  21 ′ in the projection head  11 , the polarization plane of one of the two partial beams, here the partial beam  21 , is rotated by 90°, for which purpose a λ/2 plate  27  is disposed downstream of the polarizer  22 . The two modulation branches  28  and  28 ′ for the raster component R and the calligraphic component K of the total image are defined around this λ/2 plate  27 . Each partial image  21  or  21 ′ is coupled into one of the two glass fibers  12 ,  13  by way of an optical coupling optics  29  or  29 ′. 
     Each glass fiber  12 ,  13  is connected to an optical coupling optics  30  or  30 ′ in the projection head  11  (FIG.  3 ). Each partial beam  21  or  21 ′ coupled out of the glass fiber  12  or  13  passes through a polarizer  31  or  31 ′, which serves to suppress any rotations of the polarization planes of the partial beams  21 ,  21 ′ that may be experienced in the glass fibers  12 ,  13 , and to unambiguously define the polarization plane. The partial beam  21  is deflected horizontally in a deflection unit  32  or a scanner. This deflection corresponds to the line deflection of the partial beam  21 , and is performed with a correspondingly-high deflection frequency. 
     One possible embodiment of the deflection unit  32  is in the form of a rapidly-rotating polygonal mirror. Another possible embodiment of the deflection unit  32  is as a micro-optical mirror. The partial beam  21 ′, which is coupled out of the glass fiber  13  and is responsible for the calligraphic component K, passes through the same polarizer  31 ′ for the same purpose, and is deflected horizontally in a deflection unit  32 ′. In contrast to the line deflection of the raster component, the horizontal deflection of the calligraphic component can be effected slowly within a line for the light points. 
     One possible embodiment of the deflection unit  32 ′ is an electroplated mirror that is operated such that it approaches each light point within the line in quick succession. In the process, it writes the lines alternately from left to right and from right to left. This avoids a rapid return. Another possible embodiment of the deflection unit  32 ′ for horizontally deflecting the partial beam  21 ′ is, for example, a micro-optical mirror. The electronics  17  controls the two deflection units  32  and  32 ′ via the signal line  14 . 
     After the deflection units  32 ,  32 ′, the two partial beams  21  and  21 ′ are superposed in a polarization beam splitter  33 , for which purpose a deflection prism  34  has already deflected the partial beam  21 ′ to the polarization beam splitter  33 . The superposed partial beams  21 ,  21 ′ are deflected vertically in a further deflection unit  35 . An electroplated mirror is preferably used for this procedure. The mirror changes its angle by a small increment for each line. After each image, it returns to its initial position. Instead of an electroplated mirror, however, it is also possible to use a different deflection unit  35  that performs the same action, such as a micro-optical mirror. This deflection unit  35  is also controlled by the electronics  17  via the signal line  14 . The projection lens  36  projects the generated raster-calligraphic image onto the display or another projection surface. 
     According to the above-described principle of separate light modulation for the raster component and the calligraphic component, a monochromatic image having n gray stages is obtained in the color of the laser generated by the laser source  18 . The generation of color images requires three laser sources  18  having lasers of different wavelengths, as shown in FIG.  4 . Each laser source  18  emits light in the red, green and blue spectral range. The wavelengths can be, for example, 629 nm, 532 nm and 446 nm. Each laser beam  19  of the three laser sources  18  is split into the two partial beams  21  and  21 ′, as described in conjunction with FIG. 2, and passes through the modulation branch  28  or  28 ′. Prior to being coupled into the glass fibers  12 ,  13 , the partial beams  21  of all three laser beams  19  that write the raster component, and the partial beams  21 ′ of all three laser beams  19  that write the calligraphic component, are optically superposed with the aid of dichroic mirrors  37 ,  37 ′. The dichroic mirrors  37 ,  37 ′ have different transmissions and reflections in the three spectral ranges of red, green and blue. The dichroic mirrors  371  and  371 ′ have a high transmission for red and a high reflection for green, and the mirrors  372  and  372 ′ have a high transmission for red and green and a high reflection for blue. Only simple deflection mirrors  38  and  38 ′ are necessary for coupling in the partial beams  21  and  21 ′ in the red spectral range. 
     After the beams have been coupled into and out of the glass fibers  12 ,  13  in the projector head  11 , the course of the beam paths of the partial beams  21  and  21 ′ is as described in conjunction with FIG.  3 .