WO2006063577A1

WO2006063577A1 - Method and projector for image projection

Info

Publication number: WO2006063577A1
Application number: PCT/DE2005/002265
Authority: WO
Inventors: Ulrich Hofmann; Georgios Fakas; Joachim Janes; Peter Blicharski; Bernd Wagner
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2004-12-16
Filing date: 2005-12-15
Publication date: 2006-06-22
Also published as: DE102004060576B4; DE102004060576A1

Abstract

The invention relates to a method and an image projector for image projection, whereby a projection beam (3) is intensity modulated and run over a projection surface (13) by deviation at a double-axis scanner to generate an image. During the image projection an instantaneous position value for an instantaneous position of the projection beam (3) on the projection surface (13) is determined, local image information corresponding to the instantaneous position is read from an image memory (10) and the intensity of the projection beam (3) adjusted according to the read local image information. According to the invention, the above method permits an image quality to be achieved which is less sensitive to external influences on the scanner micro-mirror.

Description

Method and projector for image projection

Technical application

The present invention relates to a method for image projection and to an image projector in which a projection beam is intensity-modulated and guided by deflection on a biaxial scanner to produce an image over a projection surface.

State of the art

In recent years, the display technology has steadily gained in importance. In particular, liquid crystal technology has made it possible to produce high-resolution flat displays which, due to their extremely compact dimensions, have been able to penetrate ever new areas of application. Despite this preference, there are applications in which even the compact LC displays are still too large and therefore reach their limits. A typical example is mobile phones. While the handset manufacturers are aiming to make the size smaller and more compact from generation to generation, the desire for ever higher image resolution and greater image reproduction is diametrically opposed to this trend. A possible way out could therefore be to provide image projection, provided that the projector can be integrated into the available construction volume. The image size is essentially determined by the projection distance and can therefore amount to many times the size of a mobile LC display. For this reason, the development of miniaturized laser scanners has been used for several years, with the help of which it should be possible to divert a correspondingly intinity-modulated laser beam over a projection surface so quickly that the impression of a stationary image is created for the human eye. The American company Microvision has already realized projection displays with SVGA resolution. A silicon microchip with a mirror plate (MEMS actuator) movably suspended in two axes deflects the incoming projection beam in the required manner. Analogous to the image structure in a television, the projection beam is deflected line by line very fast (up to 2OkHz), while a simultaneous superimposed vertical movement with the frequency of the image refresh rate (typically 30-60Hz) takes place. The horizontal movement and vertical movement must be synchronized with each other so that each image has an identical number of lines. As a rule, the fast line movement of the scanner is operated in a resonant manner, so that the required deflection angles are achieved by the resonance overshoot. Since such a resonant oscillating system can not be readjusted or corrected quickly enough in the case of a phase deviation (frequency shift), it is necessary to adapt the slow vertical motion to the movement of the fast axis. However, the slow axis does not have to be operated resonantly, so that the effect of the

Resonance overshoot in this axis can not be used. In addition, the modulation of the light source must be synchronized with the scanner motion so that each projected image begins at the same location and each line is made up of the same number of pixels. The synchronization required for such a raster image projection involves some not inconsiderable problems.

Micromechanically produced scanners, like any other mechanical product, have a certain amount of parameter dispersion. With reference to the resonant frequency, however, this means that every display constructed with such a scanner has to be calibrated. The control and synchronization electronics of scanner and light source must be tuned very accurately to the resonant frequency of the scanner. The required effort on the electronics side can be very high under certain circumstances. The same applies to the working hours required. Both stand in the way of cost-effective mass production.

High image resolution requires large optical scan angles given the diameter of the projection beam. In order to be able to achieve this, it is necessary, based on the resonantly operated line deflection, to achieve the highest possible resonance peaking or a high mechanical quality factor Q. The American manufacturer Microvision calls for its resonance scanner a quality factor of> 20,000. At the same time, however, such a high quality means that the resonance pattern in the frequency spectrum is extremely sharp. This means that the high resonance peak is only achieved if the drive frequency is extremely precise is matched to the sharp maximum of the resonance curve. A slight deviation of the drive frequency from the resonance frequency leads immediately to a significant decrease in the scan amplitude due to the large steepness. A high quality factor, however, also results in an enormous steepness of the phase frequency response. While the scanner is in phase with the excitation signal below the resonance frequency (phase angle = 0 °), the phase angle changes to 90 ° when the scanner is exactly in resonance, and finally oscillates in antiphase (phase angle = 180 °) when the excitation frequency is above the mechanical resonance frequency of the scanner is located. Due to the steep phase change when passing through the resonance even the smallest disturbances of the environmental conditions can lead to significant changes in the phase position and thus to considerable distortion of the image information within the line being projected. Again, at best, this can be compensated by considerable sensor and electronic effort.

The problems mentioned are particularly pronounced when the projection system is exposed to strong temperature fluctuations. If such a system to be used in the automobile, then in the

Usually a system capability in the temperature range of -40 ° to + 85 ° required. Temperature fluctuations lead to changes in the material properties and thus result in a shift of the mechanical resonance frequency, which adversely affects the image reproduction. The object of the present invention is to specify a method for image projection and an image projector in which the image quality is less sensitive to fluctuating environmental conditions.

Presentation of the invention

The object is achieved with the method and the image projector according to claims 1 and 9, respectively. Advantageous embodiments of the method and the image projector are the subject of the dependent claims or can be found in the following description and the embodiment.

In the present method of image projection, a projection beam is intensity modulated and guided by deflection on a biaxial scanner to produce an image over a projection surface. The method is characterized in that during the image projection in each case a momentary

Position value is determined, which is associated with a current position of the projection beam on the projection surface, a local image information associated with the current position is read from an image memory and the projection beam is adjusted in intensity according to the read-out local image information.

The associated image projector comprises a first light source for a projection beam, a

Modulation device for the modulation of the projection beam in intensity and a biaxial scanner on which the projection beam is vertical and is deflected horizontally. The image projector is characterized in that it comprises a position measuring device for determining an instantaneous position value which is assigned to a current position of the projection beam on a projection surface, and a control unit having an image memory which is connected to the position measuring device and the modulation device is configured such that it reads out a local image information assigned to the current position from the image memory and controls the modulation device in accordance with the read-out local image information.

The present method and the associated image projector are described below using the example of a

Micromirror scanner with a two-axis adjustable micromirror explained in detail, but without being limited to micromirror scanner. Instead of a micromirror scanner, other biaxial scanners can also be used. The scanner does not always have to be a reflective beam deflection system. A transmissive beam deflection system can also be used, for example based on two lenses which are suitably displaced relative to one another.

The present method is a scanning projection method in which the image projector can be operated in almost any manner with respect to the deflection parameters. Thus, the generation of the image on the projection surface can be done not only in the raster scan method but, for example, also in the Lissajous scan method. in the In contrast to the known standard methods of image projection, the light source in the present method is not modulated by a precalculated sequence of pixel data, ie intensity values within a line. Rather, a fast

Position measuring device used to determine at each moment of the image projection a, the exact position of the projection beam on the projection surface associated position value. A local image information associated with the current position, in particular a pixel value of the pixel which the projection beam currently traverses on the projection surface, is read from an image memory. The projection beam is set in intensity according to the read-out local image information. Thus, at each position of the projection beam on the projection surface, the correct pixel to be displayed at this point is always generated. The scanning geometry, d. H. the path on which the projection beam is guided over the projection surface is insignificant. The present method, as well as the associated image projector, thus always generate the image points at the correct location, irrespective of possible disturbances of the micromirror scanner or interfering influences on this scanner, so that no distortions caused by such disturbances occur.

The local image information corresponds to the image information that is to be displayed at a specific pixel within the image. This is usually gray scale information, in the simplest case a simple Dark information in the form of the bit values 1 or 0. However, the image information may additionally contain color information. In accordance with the image information read, the projection beam is set in intensity via the modulation device. The same is carried out for the next determined position of the projection beam, wherein the position determination preferably takes place continuously or at very short time intervals, in which the projection beam, depending on the resolution to be displayed, moves no more than one pixel on the projection surface. In this way, the projection beam is modulated as a function of the respectively determined position and the image information assigned to this position.

In the present method and the associated apparatus, instantaneous two-dimensional position values are thus determined during the image projection continuously or at very short time intervals in which the projection beam travels in dependence on a resolution to be displayed by one pixel on the projection surface each associated with a current position of the projection beam on the projection surface, each one of the current position of the projection beam associated local image information based on the determined position value read from an image memory and set the projection beam according to the read local image information in intensity. For determining the respective instantaneous position of the projection beam, the instantaneous position of the micromirror is preferably detected, from which this position on the projection surface, which corresponds to the image surface of the image to be displayed, can be derived. In a preferred embodiment of the present method as well as of the associated projector, the respective instantaneous position of the micromirror is detected via a measuring beam which impinges on the micromirror on an axis other than the projection beam and is directed onto a position-sensitive detector. By assigning the area of the position-sensitive detector swept by the measuring beam to the projection surface, the instantaneous position of the projection beam on the projection surface can be determined at any time via the position signal of the position-sensitive detector. These, for example, as x and y coordinates or row and column number with respect to the image matrix

(Rows and columns) of the image position information can be used directly to read the image information of an image memory constructed in the same way. Thus, based on the determined row and column number, the value written to this row and column number in the frame memory can be directly read out. In this way, a very fast adjustment of the correct instantaneous intensity of the projection beam can be carried out.

The enormous advantage of the present method is that the scanner is the projection beam basically in any way over the screen can lead and yet each pixel is always projected in the right place with the right brightness, if the scanner completely covers the screen. If, for example, fluctuating ambient temperatures occur, which result in changed deflections of the micromirror, then the unambiguous assignment of each pixel to a defined solid angle of the projection is not disturbed. When using a standard projection method with the video signals Vsync, Hsync and Pixelclock, on the other hand, there is a possibility of reaction in case of disturbances only from line to line. Within a line, it is therefore possible for image information to be distorted. This does not occur in the present process.

Another advantage of the present method and the associated image projector is that both axes of the micromirror scanner can be operated resonantly in order to exploit the resonance increase by the quality factor in both axes. In this embodiment, the scanner is always operated in its fundamental modes without having to force the projection beam onto a fixed path.

This mode of operation achieves the maximum excursion of the micromirror in both axes and thus a clear improvement in terms of resolution and image size. For use in harsh environments, such as in the mobile phone or in the automobile, this approach guarantees reliable operation even if the mechanical properties of the oscillator (scanner) should change, for example Temperature change. With this type of control, Lissajous scan figures are written which, depending on the frequency division ratio of the two axes, result in a high or low line density in the image. In order to increase the line density, the phase angle of the two axes can be specifically tuned to each other in order to complete a number of mutually offset partial images to a dense overall picture.

In the present image projector, a defective micromirror scanner can be replaced by a new micromirror scanner without further calibration measures, which generally does not have the identical characteristic shafts due to the unavoidable specimen scattering. This is made possible by the independence of the present image projection from the deflection properties of the micromirror scanner.

Another, already briefly indicated advantage of the proposed method and the associated

Image projector means that external vibration or shock to the scanner does not interfere with the image, as is the case with conventional synchronization techniques. The oscillation forced by the disturbance can be used just as well for image projection as the usually used electrostatic excitation of the micromirror.

Brief description of the drawings

The present method and the associated image projector will be described below with reference to an embodiment in conjunction with the drawings Without limiting the scope of the protection claims specified again briefly. Hereby show:

Fig. 1 is a schematic representation of

Position detection in the present method;

FIG. 2 is a schematic representation of individual units of an exemplary image projector according to the present invention; FIG. such as

3 shows an example of an application of the present image projector.

Ways to carry out the invention

1 shows a schematic representation of a 2D scanner chip 1, which forms the micro-mirror scanner (MEMS scanner) of the present image projector. In this chip, the micromirror 2 is formed, which is deflected by two perpendicular axes by electrostatic actuation. With a suitable drive frequency, this mirror, which is a mechanical oscillator with respect to each axis, can be operated in resonance in order to achieve maximum deflections. By means of these deflections, a projection beam 3, which is directed onto the micromirror 2 by a modulated light source 4, for example a laser diode, is guided over a projection surface (not shown). In addition to the light source 4 for the projection is a second light source 5, which may also be realized by a laser diode provided as part of the image projector. This second light source 5 emits a measuring beam 6, which is also directed at a different angle to the micromirror 2 and deflected by it. The deflection takes place on a two-dimensional, position-sensitive detector 7 arranged at the corresponding point. This position-sensitive detector 7 delivers a signal which depends on the point of impact of the measuring beam 6 on the detector surface. From this signal, the x and y position of the measuring beam 6 on the detector surface can be determined. By a fixed assignment between these landing positions and the position of the projection beam 3 on the

Projection surface, this position can also be derived from the signal of the position-sensitive detector 7. The two light sources 4, 5 are of course arranged rigidly and aligned.

Figure 2 shows an example of the individual units of an image projector, with which the present method is feasible. In this illustration, the light source for the image projection forms a laser unit 8 together with the modulation device. The modulated projection beam 3 generated by this laser unit 8 impinges on the micromirror 2 of the micromirror scanner and is deflected by the latter onto a projection surface (not shown).

At the same time, the measuring beam 6 is directed via the further light source 5 via the micromirror 2 onto the position-sensitive detector 7. From this detector 7, the four leakage currents are processed in an analog signal conditioning in block 14 to normalized x- or y-position signals (according to the scheme (X2-Xl) / (X2 + X1), if X2 and Xl represent the two partial currents in the x direction ). The two processed

Analog signals are digitized in an analog-to-digital converter 9 and directly respond to the image memory module 10 described with image information. At the output of the addressed memory cell is now the information "1" or "0". With this signal, the modulation device of the laser unit 8 is controlled via the control unit 11 in order to adjust the intensity of the light source accordingly. Preferably, in this case an image memory is used, which has two ports and can be simultaneously described and read out.

This data processing takes place in real time during operation of the image projector. While the projection beam crosses the position of a certain pixel on the projection surface, the electronics accesses the image memory with the aid of the determined x and y position and asks whether or not the corresponding image memory element is set in a one-bit representation , If the image memory element is set, then the laser is turned on. If the image memory element is not set, the laser remains off.

Of course, the coding can also be correspondingly inverted or transferred without restriction to a gray value representation. Accordingly, not a single bit is read from the image memory, but a data word consisting of a greater number of bits (e.g., 8, 12, 16, etc.) to adjust the brightness of the light source with the modulator. In the case of color information, such gray values, for example in the case of an RGB system, must be read out for each of the color channels involved in the color representation.

By the movement of the projection beam over the projection surface, such a position detection and adjustment of the intensity of the light source is made at any time, so that the projection beam is modulated on this basis to write the image on the projection surface.

Finally, FIG. 3 shows a possible use of the present image projector in a mobile telephone 12 with which the image can then be projected onto a currently available surface in a sufficient size (projection surface 13).

LIST OF REFERENCE NUMBERS

1 2D scanner chip

2 micromirror 3 projection beam

4 light source

5 light source

6 measuring beam

7 position-sensitive detector 8 laser unit

9 analog-to-digital converter

10 image memories

11 control unit

12 Mobile phone 13 Projection screen

14 Component for analog signal conditioning

Claims

claims

1. A method for image projection in which a projection beam (3) is modulated in intensity and guided by deflection on a biaxial scanner to produce an image over a projection surface (13), characterized in that during the image projection in each case a momentary position value is determined, which is assigned to a current position of the projection beam (3) on the projection surface (13), one of the current position associated local image information read from an image memory (10) and set the projection beam (3) in accordance with the read out local image information in intensity becomes.

2. The method according to claim 1, characterized in that the position value as a pair of values from a

Row and a column number determined with respect to an image matrix of the image and the local image information associated with the current position is read out of the image memory (10) by direct access via the determined row and column number.

3. The method according to claim 1 or 2, characterized in that in each case a current position of the scanner is detected to determine the instantaneous position of the projection beam (3) on the projection surface (13).

4. The method according to claim 3, characterized in that for determining the current position of the scanner, an additional measuring beam (6) via the scanner to a position-sensitive detector (7) is directed.

5. The method according to any one of claims 1 to 4, characterized in that the projection beam (3) line by line over the projection surface (13) is guided.

6. The method according to any one of claims 1 to 5, characterized in that as biaxial scanner, a micromirror scanner with a two-axis adjustable micromirror (2) is used.

7. The method according to claim 6, characterized in that the micromirror scanner is operated in resonance in both axes.

8. The method according to any one of claims 1 to 7, characterized in that a laser beam is used as the projection beam (3).

9. Image projector with a first light source (4) for a projection beam (3), a modulation device for modulation of the projection beam (3) in intensity and a biaxial scanner over which the projection beam (3) is deflected, characterized in that the image projector a position-measuring device (5, 7) for determining a respective current position value, which is assigned to a current position of the projection beam (3) on a projection surface (13), and one with the position-measuring device (5, 7) and Modulation device connected control unit (11) comprising an image memory (10) which is designed such that it reads out a local image information associated with the current position from the image memory (10) and controls the modulation device in accordance with the read-out local image information.

10. Image projector according to claim 9, characterized in that the position-measuring device (5, 7) and the control unit (11) are formed so that the position value determined as a pair of values from a row and a column number with respect to an image matrix of the image and the the local position information associated with the current position is read out of the image memory by direct access via the ascertained line and column number.

11. Image projector according to claim 9 or 10, characterized in that the position-measuring device (5, 7) comprises a position-sensitive detector (7) and a second light source (5) for a measuring beam (6) via the scanner on the position sensitive Detector (7) is deflected.

12. Image projector according to one of claims 9 to 11, characterized in that the first (4) and / or second light source (5) are lasers.

13. Image projector according to one of claims 9 to 12, characterized in that the scanner is a micromirror scanner with a two-axis adjustable micromirror (2).