WO2003013150A1

WO2003013150A1 - Raster projection of an image with light beam guidance thereto and therefrom

Info

Publication number: WO2003013150A1
Application number: PCT/EP2002/007751
Authority: WO
Inventors: Christhard Deter; Klaus Hiller
Original assignee: Schneider Laser Technologies Ag
Priority date: 2001-07-20
Filing date: 2002-07-11
Publication date: 2003-02-13
Also published as: DE10135418A1; DE10135418B4

Abstract

The invention relates to a raster projection device for an image, using an intensity-modulated light beam (26), comprising a dual-axis deflector device (20) provided with a moveable line mirror (21) deflecting the light beam (26) in lines on top of said image (103) and a moveable image mirror (22) deflecting the light beam (26) perpendicular thereto. The line mirror (21) and the image mirror (22) are respectively embodied as tilting mirrors. The line mirror (21) can be excited in order to perform a rotating oscillation. The image mirror (22) can be moved into various angle positions (α) and remains in a fixed angle position (α) or carries out a specific angle function when the line mirror (21) deflects the light beam (26) along a line (104), each line (104) being associated with a specific angle position (α) or angle function.

Description

Raster projection of an image with back and forth light beam guidance

The invention relates to a device for raster projection of an image by means of an intensity-modulated light beam, with a biaxial deflection device which has a movable line mirror deflecting the light beam in lines across the image and a movable image mirror deflecting the light beam perpendicular thereto, each as a tilting mirror are formed, wherein the line mirror can be excited into a rotational movement vibration. The invention further relates to a method for raster projection with reciprocating light beam guidance.

Video projection systems that write an image with a direct writing light beam using a biaxial deflector are known. The basic principles of the beam deflection systems required for this are described, for example, in Stan Reich, "The use of electromechanical mirror scanning devices", SPIE, Vol. 84, Laser Scanning Components and Techniques (1976). The structure of the DE 198 60 017 A1, in which the biaxial deflection device has a rotating polygon mirror and a galvanometrically operated tilting mirror. The light beam is deflected horizontally by the polygon mirror and vertically by the tilting mirror. The polygon mirror is therefore referred to as a line mirror and the tilting mirror as an image mirror.

The correspondingly modulated light beam is deflected by a horizontal angle in the line direction from the line mirror. The difference between the minimum and maximum horizontal angle, which is influenced by the number of mirror facets in the case of the polygon mirror, is referred to as the line deflection angle and is an influencing variable for the ultimately achievable image width. The image mirror deflects the light beam by a vertical angle. The difference between the minimum and maximum vertical angles is the image deflection angle, which determines the image height.

In a deflection device with a polygon mirror, the light beam is guided on the projection surface in a manner similar to the electron beam of a television tube, ie each line is always written in the same direction, for example from left to right, and the lines are built up in succession from top to bottom. In order to achieve the required speed when building an image, rotating polygon mirrors are required very quickly; the rotational frequencies are of the order of 2.5 kHz, which requires considerable mechanical demands on the polygon mirror as well as sophisticated mirror bearings. Alternatively, the line mirror which effects the horizontal deflection can be designed as a tilting mirror which carries out a rotary motion oscillation with the required frequency. The light beam is then moved back and forth across the lines by the line mirror, while the image mirror causes the vertical deflection perpendicular to it. However, the projected lines are no longer parallel, but have the shape of a sawtooth, which results in poor image quality. In the prior art, this is either tolerated if it is possible, as is the case with barcode scanners, for example, or a wide image area around the reversal points, at which adjacent lines are particularly close to one another, is hidden by blanking the light beam in order to avoid the To fix the image deterioration at least partially. In the latter solution, however, a reduced line deflection angle has to be accepted, since the length of the effectively usable line section decreases.

The invention is therefore based on the object of developing a device for raster projection of the type described at the outset such that the image quality is increased without reducing the line deflection angle.

This object is achieved in a device mentioned at the outset in that the image mirror carries out a non-uniform rotary movement, while the line mirror carries out a partial period of the rotary movement vibration which runs in one direction of rotation.

The line mirror deflects the light beam horizontally and guides it over the lines of the image. These each consist of an effectively usable line section in which the light beam is modulated with usable image information, and remaining line sections in which the light beam is blanked.

The line mirror performs a periodic rotary motion oscillation and directs the light beam over an effectively usable line section during a partial period. The image mirror effects the vertical position perpendicular to it in a movement which is suitable for the partial period of the line mirror oscillation. This movement of the image mirror runs differently for the effectively usable line segments than for the remaining line segments.

During the blanking of the light beam, the image mirror moves into a new angular position, which it has to assume at the beginning of its effective line section in order to write the next line. The movement of the image mirror during the effective line section depends on the projection geometry, in particular on a possible oblique projection correction.

Due to the overall nonuniform movement of the image while writing an image

Image mirrors, which can, for example, fulfill an approximately staircase-shaped angular function, each stair step being assigned to a corresponding line, ensure that the lines are exactly parallel to one another during the reciprocating deflection by the line mirror, even with any Oblique projection. The sawtooth-shaped line image of the prior art is thus avoided by the non-uniform movement of the image mirror.

Due to the now parallel position of the effective line sections on the projection surface, the image quality is significantly improved, in particular the image quality, which decreases significantly with conventional tilting mirrors, is eliminated; the image quality is equally good over the entire image.

It is essential that the movement of the image mirror, during the period in which a line is written along its effectively usable line section with image information, fulfills a certain angular function and changes to the appropriate angular position for the next line after the effectively usable line section.

For example, the image mirror deflects the light beam by a fixed vertical angle (α), while the line mirror deflects the light beam by a horizontal angle along the effective line section of a line. The angle function then corresponds to the previously mentioned staircase function, with each stair step being assigned to a corresponding line. The angular function is consequently designed such that a certain angular position of the image mirror is assigned to each effective line section.

The change from a first fixed angular position which is assigned to a first line and which the image mirror has at the end of the effective line section of this line, into a second fixed angular position which is assigned to a following line and which the image mirror occupies at the beginning of its effectively usable line section would lead to the projection of non-parallel line segments during this change. In a further development it is therefore provided that the image mirror carries out the change, while the intensity-modulated light beam is blanked out in an area of the line lying outside the effective line section. The image mirror thus assumes a certain angular position while the line is written with image information along its effectively usable line section. Only after writing the effectively usable line section is the image mirror transferred during the line dead time to the fixed angular position which is assigned to the next line.

The resulting line dead time is determined by the positioning speed of the image mirror and can therefore be much shorter than the line dead time in the prior art determined by still tolerable image errors. The possible line deflection angle and thus the achievable image size is considerably increased for a given distance between the deflection device and the projection surface due to the significantly reduced line dead time.

This concept can help to improve the image quality, especially when projecting obliquely from above or below. For such cases it is preferable that the angular positions assigned to the beginning of the line are not equidistant from one another. In the majority of applications for a device for raster projection, an alignment is possible in which the main projection axis is perpendicular to the projection surface. Sometimes, however, spatial conditions force an oblique projection. In the case of an oblique projection which brings about a horizontal angle between the main projection axis from the projection optics onto the projection surface and the normal of the projection surface, the lines on the projection surface normally diverge in a star shape. To compensate for this image distortion, it is expedient to provide a rotational movement of the image mirror while writing the effectively usable line section, so that the lines are nevertheless parallel on the projection surface. For this purpose, it is provided that the line mirror deflects the light beam along a line in each partial period, which line has an effective line section in which the light beam is intensity-modulated for image display, and the image mirror, while each effective line section executes a certain vertical angular movement with a rotational movement speed other than zero, whereby a certain vertical angular position (α) is assigned to each line.

For this purpose, for example, a control device can be provided which controls the movement of the image mirror in accordance with a staircase function, a step being assigned to each line section. The steps of this staircase function each run at a certain angle, i.e. the image mirror executes a rotational movement with a certain angular velocity depending on the position of the line in the image during each effective line segment. The angle function mentioned at the outset therefore provides for a specific linear angle adjustment during each stage.

Due to the reciprocating movement of the line mirror, it is expedient to compensate for a lateral oblique projection that adjacent steps have an incline with opposite signs. With this configuration, any lateral inclined position can be corrected, so that parallel lines are projected and therefore no optimal image quality is achieved.

In special applications, an oblique projection from above or below, e.g. B. from a ceiling of a room on a room wall, desired; this is, for example, regular in the lecture halls of the

Case. The device for raster projection is then aligned so that its main projection axis is no longer perpendicular to the projection surface. This projection geometry also causes an essentially trapezoidal image distortion with a line length that varies over the projection surface.

The invention enables a particularly simple one for those cases which have hitherto usually been treated in the prior art with a corresponding electronic conversion of the image

Solution in that the correction of the line length in the case of an oblique projection of the image at a projection angle between the main projection axis and the projection surface is not equal to 90 °

Vibration amplitude of the line mirror is designed depending on the line. Because of that

Projection geometry different line lengths can be compensated for by a function for the line deflection angle which is dependent on the position of the lines in the image. It is therefore unnecessary to convert the image information, which is not only computationally intensive, but often also involves loss of information. Instead, the control of the rotary motion oscillation of the line mirror can be adapted to the projection conditions in a simple manner. In addition, this procedure does not change the resolution in the projected image, which is particularly advantageous in applications that require high image fidelity (for example CAD systems). Finally, this simple image equalization enables rapid adaptation to different projection geometries, without the need for expensive electronic image equalization.

In particular, this concept makes it possible in a simple manner to correct the pillow distortions which are particularly disruptive at large deflection angles and which likewise bring about different line lengths.

The lines now running parallel have the same angular spacing in relation to the location of the deflection device. In the projected image, however, the line spacing is usually not the same across the entire image; even if the main projection axis is perpendicular to the screen, the lines are closer to each other in the center of the screen than at the top and bottom of the screen. In order to avoid a streaky image representation caused thereby, it is particularly expedient to carry out the aforementioned control of the image mirror in accordance with an angular function (e.g. stair function). Then the vertical angles of the beam deflection assigned to the individual lines are not equidistant, the differences being chosen such that the line spacings in the projected image are the same.

The type of angular function depends on the projection geometry. If there is a projection in which only the line spacings in the image vary, it is preferred that the image mirror remain in a fixed vertical angular position, while the line mirror deflects the light beam along the effective line section of a line, each line being assigned a specific vertical angular position ,

A corresponding design of the angular functions can compensate for a variation in the line spacing due to the oblique projection, so that the effective line segments on the projection surface are not only parallel but also equidistant. The stair function already mentioned then has different step heights. This further improves the picture quality.

Line and image mirrors are expediently controlled by a control device, which not only influences the movement sequence of these mirrors, but also the modulation of the light beam. In this way, a synchronization between the blanking of the light beam and the control of the vertical and horizontal angles of the beam deflection can be carried out in a simple manner in a single control device. Any suitable drivable tilting mirror can be used as a line mirror. A tilting mirror is particularly preferred, which is driven at a fixed working frequency close to its resonance frequency to produce a rotary motion oscillation about the axis of the line deflection. Then the desired line opening angle, ie the desired vibration amplitude, can be set by simply regulating the drive voltage or the drive current of the exciting vibration.

Alternatively, such a line mirror can be subjected to an exciting oscillation with a fixed drive voltage or a fixed drive current, and the oscillation amplitude variation can be achieved by a slight change in frequency. Even such a small change in frequency, with constant drive current or constant drive voltage, causes a noticeable change in amplitude of the mirror oscillation, which can be regulated well. Such a frequency modulation has the advantage over the aforementioned current or voltage modulation that it is generally less interference, requires less effort and shows better control behavior.

When the line mirror is operated with a rotary motion oscillation somewhat apart from the natural frequency, that is to say in the supercritical or subcritical range, a particularly exact control of the oscillation amplitude is ensured and at the same time the desired amplitude increase is realized. When operation is possible in principle on the natural frequency, instabilities are difficult to control.

The object on which the invention is based is further achieved in a method of the type mentioned at the outset in that the light beam is guided over a line in a forward direction, the deflection about the image axis not being changed or being changed at a first angular velocity, then intensity modulation blanking the light beam and at the same time there is a change in the deflection around the image axis at a second angular velocity and then the light beam is again guided along a further line in a backward direction, the deflection of the image axis again optionally not being changed or at a third angular velocity.

This method according to the invention likewise ensures a parallel line course in the projected image and avoids the severe deflection angle restrictions which arise in the case of the described conventional sawtooth-shaped course of the light beam over the projection surface. The advantages mentioned above with regard to better image quality are also achieved with this method.

To improve the image quality, it is preferable to change the deflection around the image axis depending on the position of the line in the image. This can achieve on the projection, an equidistant Zeilenverte ^'tlung, which enhances the image quality again. In the case of oblique projection, the position of the deflection device in relation to the projection surface can also be taken into account. To correct the distortion, it is preferable to select the deflection around the image axis also as a function of the position of the lines on the projection surface, in order to compensate for a line length inhomogeneity caused by the projection geometry.

Of course, the corrections mentioned do not all have to be implemented together; Depending on the projection conditions and image quality requirements, some of the corrective measures mentioned or suitable combinations thereof can be taken.

The invention is explained in more detail below by way of example with reference to the drawing. The drawing shows:

1 shows a device for raster projection of an image,

2 shows the angle control of a light beam deflected with two tilting mirrors according to the prior art via a projection surface,

3 a control of the vertical angle of the beam deflection according to a staircase function with the same angle steps,

4 shows the angle control of a light beam deflected using the staircase function shown in FIG. 3,

5 to 8 geometry-related image distortions in straight and oblique projection,

9 shows diagrams for illustrating the amplitude control in a tilting mirror which executes a rotary motion oscillation and which is used as a line mirror,

10 shows the angular control of a light beam which is deflected with an image mirror controlled according to the staircase function of FIG. 3 and with an amplitude-modulated line mirror.

11 shows a staircase function for angular control of an image mirror with differently large angular steps,

FIG. 12 shows the angular control of a light beam which is controlled with an image mirror controlled according to FIG. 11 and a line mirror controlled with amplitude modulation to compensate for oblique projection distortions according to FIG. 6

13 shows the line-dependent course of a horizontal angle of the beam deflection for different projection conditions, 14 shows the line-dependent course of a vertical angle of the beam deflection for different projection conditions,

15 shows a staircase function similar to FIG. 12, but to compensate for a lateral oblique projection according to FIG. 7, and

FIG. 16 shows the angle control of a light beam which is deflected with a line mirror controlled according to FIG. 9 and with an image mirror controlled according to FIG. 10 in order to compensate for an oblique projection according to FIG. 8.

A device for raster projection is shown in FIG. 1 in the form of a video projection system which has an electromechanical radiation deflection system. This video projection system has two locally separated assemblies: an RGB light source 10 and a projection head 20. However, this division into assemblies is not mandatory. Housing the RGB light source 10 in the projection head 20 is expedient, for example, if it is designed to be correspondingly small and light.

The RGB light source 10 consists of several laser light sources 1, each of which is followed by a modulator 2. The laser light sources 1 and the modulators 2 generate three light beams in the primary colors red, green and blue, which are then coupled into an optical fiber 15 via a fiber plug-in connection 16 in a beam combination 3 and a coupling optic 4 as an intensity and color-modulated light bundle.

The RGB light source 10 also contains suitable (not shown) driver electronics units for operating the laser light sources 1. The RGB light source 10 is supplied with a signal VIDEO, which is converted by an input module 6 into a device-specific digital signal R-G-B. This signal is received by an image calculation unit 7, which controls the modulators 2. The image calculation unit also contains a synchronization signal RGBsync, which is used for synchronization with the projection head to be described. The modulators 2 are controlled taking this synchronization signal RGBsync into account. Furthermore, the RGB light source 10 also has driver circuits 8 and 9 for controlling the projection head 20, which are fed with corresponding signals Hsync and Vsync.

The color-modulated light bundle is transmitted via the optical fiber 15 to the projection head 20 and is in turn fed in there at a fiber plug connection 16. From there, it arrives via a decoupling optic 25 as an essentially parallel light beam on a mirror surface of a line mirror 21. The light beam has a diameter which is chosen accordingly for the pixel size of the image to be represented. For example, the diameter can be 2-5 mm. However, only the position of the center of the light bundle is essential for the further description, which is why a light beam is referred to below for simplification. The line mirror reflects the light beam 26 onto a mirror surface of an image mirror 22 and causes a horizontal deflection about a line axis; the horizontal angle of the deflection occurring as the deflection angle is entered in FIG. 1 with ß. The image mirror 22 causes a vertical deflection about an image axis lying perpendicular thereto; the vertical angle of the deflection occurring as the deflection angle is designated by α.

The illustration in FIG. 1 uses a coordination system, the origin of which lies at the location of the maximum deflection in the beam direction to the left and upward, since this only results in positive horizontal angles β and vertical angles α. This choice is advantageous for the sake of simple presentation, but is not otherwise mandatory.

Both the line mirror 21 and the image mirror 22 are tilting mirrors. The line mirror 21 preferably has an electromechanically driven mirror surface which is suspended from torsion straps within a frame shown schematically in FIG. 1.

The line mirror 21 and the image mirror 22 act as a deflection device for a two-axis deflection of the light beam 26 over the projection surface, such that an image is projected onto the projection surface 101 in the direction of a main projection axis OA.

Depending on the projection geometry, the image is thereby of the desired rectangular image 103, the aspect ratio of which, for. B. is specified by a TV standard, distorted to a distorted image 102. In the example in FIG. 1, an oblique projection takes place from above at an angle ω to the horizontal H. The oblique projection parameters (angles ω and ε) can be entered via a control panel 5.

The line mirror 21 is driven near its resonance frequency to rotary motion vibrations. It is controlled in the supercritical or subcritical range with an operating frequency f _A close to the resonance frequency f _{R in} order to achieve sufficient amplitude control and at the same time to implement a desired increase in amplitude. The control of the vibration amplitude of the rotary motion vibrations, which the line mirror 21 executes, is carried out by a current / voltage modulation. The increase in resonance enables a relatively large mechanical deflection angle in the range of typically +/- 12 °.

The image mirror 22 is an anti-resonance tilt mirror with an adjustable angular position of the mirror surface; preferably it is of the mirror galvanometer type.

It is essential for the functionality of the video projection system shown in FIG. 1 that the timing of the modulators 2 and the instantaneous angular position of the line mirror 21 are synchronous. For this purpose, a detector system is provided to detect the angular position of the line mirror 22. It has a laser diode 23 and a photodetector 24. The laser diode 23 illuminates the rear of the line mirror 21 and the photodetector 24 designed as an array makes it possible to resolve the angular position of the line mirror 21. The so won Synchronization signal RGBsync is fed via a line 11 to the image calculation unit 7 and, as mentioned, is fed in there. The image calculation unit 7 uses this to generate the aforementioned signal Hsync and thus acts on the driver circuit 8, which generates an AC voltage, the frequency of which is the working frequency f _{A of} the line mirror 21. The signal Hsync supplies information about the line number i of the line to be written in each case in order to design the control of the vibration amplitude of the line mirror 21 in a manner to be described. Furthermore, a signal Vsync is generated by the driver circuit 8 or by the image calculation unit 7, with which the driver circuit 9 is fed, which drives the image mirror 22 via a line 13 in a manner to be explained.

2 shows, by way of example, the sawtooth-like course that takes place without further measures and with which the light beam would be deflected from the projection head 20 over the projection surface. The angular deflection is plotted as a vertical angle α over the horizontal angle β in the aforementioned coordination system.

In a forward scan 40, which corresponds, for example, to a partial period with left-hand movement of the line mirror 21 during the rotational movement oscillation executed by it, the light beam is guided with increasing horizontal angle β from maximum left-hand deflection to maximum right-hand deflection. Since at the same time the vertical angle α increases due to the continuous movement of the image mirror 22, the light beam 26 also moves in the image direction, i. H. in the representation of Fig. 2 down.

This takes place up to the deflection point 41, at which the line mirror 21 has reached its maximum deflection in one direction and therefore the horizontal angle β has reached its maximum value. The rotary motion oscillation then runs in the opposite direction of rotation, for example clockwise. A backward scan 42 takes place with a decreasing horizontal angle β, the vertical angle α also increasing due to the continuous movement of the image mirror. At the deflection point 43, the movement of the line mirror 21 is reversed again, since the latter has again reached its maximum deflection of its rotary motion oscillation at a now minimal horizontal angle.

This process continues until the image mirror has reached the maximum value of the vertical angle α and the last line (i = z) has been written. The image mirror then jumps back to the minimum value of the vertical angle α in a dead time of the image.

Since the lines are written alternately from left to right and from right to left and the image mirror 22 carries out a continuous movement, there is no uniform line spacing. At the edges of the lines near the deflection points 41 and 43, two lines are even partially written into one another owing to the beam diameter. This can be reduced by choosing a corresponding line blind spot range ßt1 and ßt2 for the vertical angle near the deflection points 43 and 41, in which the light beam 26 in each case blanked, that is switched off. This measure, however, drastically reduces the mechanically possible line deflection angle βm, which is given by the difference between the minimum and maximum value of the horizontal angle β, to an effective line deflection angle β.

Despite the corresponding blanking of the light beam 26 in the angular ranges ßt1 and ßt2, a loss of resolution and disturbing brightness differences remain in the image between the center of the image and the edges of the image.

In order to remedy these image defects, the image mirror 22 is now controlled according to the staircase function 30 shown in FIG. 3. The image mirror 22 remains at a constant value 32 for the vertical angle α during an effective line segment of each line. This corresponds to a step of the staircase function 30. Between the steps, the image mirror 22 is adjusted by a change 34 to the next value 32 of the vertical angle α for the next line with the next line number i in a short line dead time tz. During this line area corresponding to a line dead time tz, the laser beam 26 is blanked out. Each line therefore consists of an effective line section on which the light beam for image display is intensity-modulated with image information, and two line areas that are blanked out.

The values 32 for the vertical angle α are distributed equidistantly in Fig. 3, i.e. Amendment 34 has exactly the same amount for each line number. If all line numbers of i = 1 of i = z have been processed in this way, the image mirror 22 is moved back into the starting position. This is shown by a swivel flank 33 in the staircase function 30 of FIG. 3. While the image mirror 22 is being driven with this return flank 33, the laser beam 26 is again blanked so that an image dead time tb is present.

The deflection of the light beam shown in FIG. 4 results from the control of the image mirror 22, which is effected by the driver circuit 9. The type of representation corresponds to FIG. 2; Identical elements are identified with the same reference symbols. As can be seen, the effective line sections of all lines in the forward scan 40 and backward scan 42 are now parallel since the image mirror 22 is set to a constant value 32 of the vertical angle α during each effective line section. In the line blind spot range ßt1 and ßt2, which is now greatly reduced compared to FIG. 2, the image mirror 22 is set to the next value 32 of the vertical angle α, whereby the light beam is deflected along triangular tips 44 and 45 or 46 and 47, the tip of which at the deflection point 41 and 43 lies.

By controlling the image mirror 22 according to the staircase function 30 of FIG. 3 it is achieved that the effective line deflection angle β, ie the length of the effective line section, comes very close to the maximum possible mechanical line deflection angle βm, that is to say the entire line length; the image is thereby significantly enlarged compared to the principle shown in FIG. 2. The image quality is also noticeably improved, since lines are no longer interleaved. 5 shows how a rectangular image appears on a projection surface 101 during a straight projection without further measures. The projection head is located centrally above the projection surface 101 with respect to both the vertical V and the horizontal H. However, parallel lines 104 are not equally spaced on the projection surface 101, but their mutual distance increases with increasing vertical distance from the center of the image. In addition, the line length also increases, so that instead of a desired rectangular image 103, a pillow-shaped image 102 results.

A similar distortion also causes the projection geometry shown in FIG. 6, in which the projection head 20 lies on the vertical V of the projection surface 101, but is arranged above the projection surface 101 with respect to the horizontal H. There is therefore an angle ω between the horizontal H and the main projection axis OA. It is consequently an oblique projection from above in the case shown. The upper image edge, which is closer to the projection head 20, is shown shorter than the lower image edge. The side edges run essentially obliquely, so that in addition to the cushion distortion according to FIG. 5, a trapezoidal distortion occurs. As a result, the length of lines 104 of the image varies across the image area.

FIG. 7 shows the distortion which arises in the case of an oblique projection from the side when the projection head 20 is located horizontally in the center of the projection surface 101, but is laterally displaced with respect to the vertical V. Then an angle ε between OA and V is not equal to zero. As a result, the image 102 is distorted in a pillow-like and trapezoidal manner, lines 104 aligned in parallel for straight projection then diverging in a star shape.

8 shows the conditions in the case of an oblique projection both from the side and from above, that is to say for w and ε not equal to zero. The projection head 20 thus lies next to the center of the projection surface 101 with respect to the horizontal H and also with respect to the vertical V a combination of the distortions described with reference to FIGS. 4 to 7 results. Lines 104 are also fanned out in a star shape.

Such distortions caused by geometry can be eliminated by various measures as follows:

In the case of an image 102 which is recorded in a pillow shape and additionally trapezoidally in the case of an oblique projection, as is shown as a dashed line in FIG. 5 or 6, the lines 104 have line spacings which depend on the number of lines i. This image error is caused by the biaxial beam deflection. Of course, if, as shown in FIG. 6, the projection head 20 is centered in the horizontal direction with respect to the projection surface 101 and at an angle ω above the center of the image in the vertical direction, an undistorted image 103 is desired. In order to achieve this, the control of the line mirror 21 or the control of the image mirror 22 are designed accordingly. In the oblique projection shown in FIG. 6, the horizontal lines 104 of the image have different lengths due to the geometric arrangement. Furthermore, they lie at different distances on the projection surface 101, as shown in broken lines in the image 102. The same occurs with straight projection due to the pillow distortion shown in FIG. 5.

The image calculation unit 7 and the driver circuit 8 therefore effect a corresponding correction of the actuation of the image mirror 22 to achieve a uniform line spacing and of the line mirror 21 to achieve a uniform line length. For this purpose, the value of the oblique projection angle ω, ε, in addition to other parameters for image display, is entered into the image calculation unit 7 via the control panel 5. This then changes a stored recoding table so that the desired line profile is achieved on the projection surface 101. For this purpose, the image calculation unit 7 calculates values of the horizontal angle β from the angle ω and effects a corresponding amplitude modulation of the rotary motion oscillation carried out by the line mirror 21 by suitably designing the drive of the line mirror 21. The time course of the amplitude modulation is dependent on the line number.

For this purpose, as shown in FIGS. 9 and 10, the mechanical line deflection angle βm defined as the difference between the minimum and maximum horizontal angle varies depending on the line. In the example shown in FIG. 9 there is an adjustment between 21.1 ° for the last line in the image with the line number i = z and 26.3 ° for the first line with the line number i = 1 for an oblique projection according to FIG ω = 15 ° and ε = 0 ° through an amplitude modulation of the torsional vibration of the line mirror.

The amplitude modulation is achieved in that the line mirror is driven with a drive frequency f _A, which is spaced by a frequency deviation Δf from the resonance frequency f _R , which has the line mirror 21 which can be excited for the rotary motion oscillation. For amplitude modulation, the drive voltage or the drive current can be changed either at a fixed operating frequency f _A. Alternatively, it is possible to record the drive current or the drive voltage and to adjust the drive frequency f _A somewhat, as shown in dotted lines in FIG. 9. The working frequency f _A is therefore constant in the case of a current / voltage modulation and is close to the natural frequency f _{R of} the vibratory mechanical system of the line mirror 21.

For the control behavior of the line mirror 21, it is favorable if the working frequency f _A does not exactly match the natural frequency f _R. With a high damping of the oscillating mirror 21, the working frequency f _{A is} closer to the natural frequency f _R than with a low damping. The frequency deviation .DELTA.f is therefore selected in accordance with the existing equipment. For frequency modulation, however, the working frequency f _{A is} controlled within a range. The variability of the mechanical line angle βm obtained in this way is used to correct the line length differences caused by the image distortion, for example by B. in oblique projection from above with increasing line number i the mechanical line angle ßm of the line opening is reduced by exactly the amount by which the respective line 104 is extended by the geometric conditions. This results in a constant line length on the projection surface even with oblique projection.

10 shows the deflection of a light beam with respect to the vertical angle α as a function of the horizontal angle β in the amplitude modulation of the line mirror 21 explained and in the control of the image mirror 22 according to the staircase function shown in FIG. 3 with constant changes 34 in the vertical angle α. The type of representation corresponds to FIG. 2; corresponding elements are provided with the same reference numerals.

As can be seen in FIG. 10, the mechanical line deflection angle βm decreases with increasing line number i due to the amplitude control. Hence ßm = ßm (i). At constant line dead angle ranges ßt1 and ßt2, which are usually specified by the speed of movement of the line mirror and the line dead time of the video standard of the image to be displayed, the effective line deflection angle ß is varied as a function of line number i, so esse = esse (i). The total line length is reduced in such a way that lines with the same length are written onto a projection surface 101 when the projection is oblique. This avoids keystone distortion. This concept can also be used to compensate for the pillow distortion in a straight projection according to FIG. 5, which results from the different distances of the projection surface 101 from the deflection point of the projection head 20.

By combining the staircase function control of the image mirror 22 (according to FIG. 3) and the amplitude modulation of the line mirror 21 (according to FIG. 10), the lines lie parallel to one another in the range of the effective line deflection angle β. Only in the line blind spot areas ßt1, ßt2 is the image mirror 22 adjusted by the change 34 and rotated into the respective vertical angular position for the next line. The amplitude modulation of the rotational movement oscillation of the line mirror 21 results in a decreasing effective line deflection angle ß with increasing line number i, by means of which an extension of the lines 104 caused by the projection geometry is compensated for.

Depending on the projection conditions, there are also different line spacings. In the case of a straight projection, as shown in FIG. 5, this is due to the fact that the lines 104 which are further away from the center of the image are at a greater distance from one another than the lines in the center of the image. The same occurs with an oblique projection from above, as shown in FIG. 6, in which the spacing of the lines increases across the image.

For correction, control of the image mirror 22 is necessary in accordance with a staircase function shown in FIG. 11. The staircase function 31 shown in FIG. 11 essentially corresponds to that in FIG Fig. 3 shown. The crucial difference is that the values 32 of the vertical angle α in Fig. 11 are now not evenly spaced. Instead, the change 34 between two successive values 32 of the vertical angle α depends on the line number i. The change 34 of the vertical angle α is greater for the transition from the first (i = 1) to the second (i = 2) line than for all subsequent lines. The smallest change 35 occurs for the transition to the last (i = z) line.

Fig. 11 shows a staircase function 31, in which the change 34, 35 decreases with increasing line number, in order, for. B. in an oblique projection according to FIG. 6 from above to compensate for the increasing line spacing. The same applies to a straight projection or an oblique projection from below. Then of course other changes 34, 35 are to be selected. In the case of a straight projection, the changes 34, 35 will generally be symmetrical to the center of the image.

A combination of the staircase function 31 of FIG. 11 for controlling the image mirror 22 with the explained amplitude modulation of the line mirror 21 provides the angular deflection of the light beam 26 shown in FIG. 12. As can be seen, both the mechanical line deflection angle βm and the line spacing between adjacent ones increase Lines 104 with increasing line number i. This compensates for image distortions arising from an oblique projection from above, so that the desired undistorted image 103 is then projected onto the projection surface 101.

How the vertical angle α and the changes in the values 32 for the vertical angle α depend on the line number i is influenced by the projection conditions and in particular the degree of the oblique projection. 13 shows in two curves 77 and 78 the line number-dependent course of the vertical angle α for different projection geometries. Curve 77 shows the dependency of the vertical angle α on the line number i in the case of a straight projection for correcting the cushion distortion which arises on a flat projection surface. Curve 78 shows the course of an oblique projection with a projection angle ω = 15 ° to the horizontal H.

14 shows the corresponding course of the mechanical line deflection angle βm as a function of the line number i for adapting the line length changes which are dependent on the projection geometry. Curve 98 shows the mechanical line deflection angle βm for correcting the cushion distortion in the case of a straight projection. Curve 98 shows the value pairs for an oblique projection with a projection angle ω of 15 ° to the horizontal.

In order to correct image distortions caused by lateral oblique projection, such as occur in FIGS. 7 and 8 due to the non-zero angle ε and lead to diverging lines 104, the staircase function of FIG. 3 and FIG. 11 is corrected in such a way that the individual steps 32 run obliquely. This is shown in FIG. 15. FIG. 15 shows the time course of the vertical angle α. As can be seen, the vertical angle α is also changed while writing a line; This corresponds to the inclined stair steps 32 of the stair function 31. The change made while writing a single line is dependent on the line number. The change 35.2 carried out, for example, during the second line (i = 2) differs both in direction and in amount from the change 35.i which is carried out during the writing of the last (i = z) line. The same applies to the changes in the vertical angle α which are carried out during the line dead time tz. The change 34.1 in the transition from the first (i = 1) to the second (i = 2) line differs from the change 34.i which is carried out in the transition to the last (i = z) line.

The alternating changes 35 of the vertical angle α during the writing of a line compensate for a diverging line course. Because of the geometry-related distortion of the lines, which would otherwise diverge radially, the changes in the vertical angle α which occur during the line dead time tz at the image edge which is further away from the projection head 20 are markedly greater than the changes in the vertical angle α those in one Line dead time can be made, which is assigned to the vertical image edge closer to the projection head 20.

The image mirror thus executes a certain swiveling movement while writing each effective line section, the angular speed of this swiveling movement being dependent on the position of the line, and consequently on the line number i. This achieves the angle control of the light beam shown in FIG. 16, which in the end results in parallel and equally spaced lines on the projection surface 101 when the projection is inclined sideways.

Claims

Expectations

1. Device for raster projection of an image by means of an intensity-modulated light beam (26) with a biaxial deflection device (20) which has a movable line mirror (21) deflecting the light beam (26) in lines (104) and a movable line mirror (21) has a deflecting image mirror (22) perpendicular to it, the line mirror (21) and the image mirror (22) each being designed as a tilting mirror and the line mirror being able to be excited into a rotational movement oscillation, characterized in that the image mirror (22) runs in a direction of rotation during one Partial period of the rotational oscillation of the line mirror (21) performs a rotational movement with varying rotational speed.

2. Device according to claim 1, characterized in that the line mirror (21) deflects the light beam (26) along a line (104) in each partial period, which has an effective line section in which the light beam is intensity-modulated for image display, and that Image mirror (22), while each effective line section with zero rotational speed remains in a fixed vertical angular position (α), each line (104) being assigned a specific vertical angular position (α).

3. Apparatus according to claim 1, characterized in that the line mirror (21) deflects the light beam (26) along a line (104) in each sub-period of the rotary motion oscillation, which line has an effective line section in which the light beam is intensity-modulated for image display, and the image mirror (22) executes a certain vertical angular movement during each effective line section with non-zero rotational speed, each line (104) being assigned a specific vertical angular movement.

4. Apparatus according to claim 2 or 3, characterized in that the image mirror (22) changes from a first angular position (α) at the end of a line (104) to a second angular position (α) at the beginning of the following line, while the intensity-modulated light beam (26) is blanked in a cell area outside the effective line section.

5. Device according to one of claims 2, 3 or 4, characterized in that the angular positions (α) assigned to the beginning of the line are not equidistant from one another.

6. The device according to claim 3, characterized in that the rotational movement of the image mirror during adjacent sub-periods of the rotational movement vibration of the line mirror (21) each have sections with opposite directions of rotation.

7. Device according to one of the above claims, characterized in that the line mirror (21) is a tilting mirror with controllable oscillation amplitude which can be excited to rotational vibrations near its resonance frequency.

8. The device according to claim 7, characterized in that the vibration amplitude of the line mirror (21) is line-dependent.

9. A method for raster projection of an image in which a light beam (26) is modulated in intensity and guided by means of a deflection device (20) by deflection about a line and an image axis in lines (104) over a projection surface (101), characterized in that a) the light beam (26) is guided over a line (104) in a forward direction, the deflection around the image axis not being changed or being changed at a first angular velocity, b) then an intensity modulation blanking the light beam (26) and at the same time a change in Deflection around the image axis takes place at a second angular velocity and c) the light beam (26) is then guided along a further line in a reverse direction.

10. The method according to claim 9, characterized in that the deflection around the image axis depending on the position of the line (104) in the image and the position of the deflection device (20) to the projection surface (101) is designed.

11. The method according to any one of claims 9 or 10, characterized in that the length of an effective line section, in which the light beam is intensity-modulated for image display, for distortion correction depending on the position of the line (104) in the image and the position of the deflection device (20th ) is selected for the projection surface (101).

12. The method according to any one of claims 9 to 11, characterized in that in step c) the deflection around the image axis is not changed or at a third angular velocity.