WO2009001274A1

WO2009001274A1 - Laser scanning projection device with splitting element

Info

Publication number: WO2009001274A1
Application number: PCT/IB2008/052465
Authority: WO
Inventors: Arnd Ritz
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2007-06-26
Filing date: 2008-06-23
Publication date: 2008-12-31

Abstract

The present invention relates to a projection device comprising at least one laser light source (2) and a scanning unit (7) arranged for scanning a scan area with a laser beam (10) emitted from said laser light source (2). The projection device comprises a beam splitting element (12) arranged in beam direction behind said scanning unit (7), said beam splitting element (12) splitting up said laser beam (10) to a range of directions. A projection unit (13) projects the image formed by scanning the scan area of the beam splitting element (12) to an external projection area (14). Due to the beam splitting element (12), an observer looking into the projection cone is prevented from focusing the eye to the laser light source (2) but sees the projected image on the beam splitting element (12) as the light source. Since this light source has a significantly larger extension than the focused laser light beam, the risk for damaging the eye when looking into the projection cone is significantly reduced.

Description

Laser scanning projection device with splitting element

FIELD OF THE INVENTION

The present invention relates to a projection device comprising at least one laser light source and a scanning unit arranged for scanning a scan area with a laser beam emitted from said laser source. Laser scanning projection devices of this kind are mainly used for image projection and can be designed, for example, as handheld devices.

BACKGROUND OF THE INVENTION

Laser scanning projection devices, also called laser projectors, which apply OD light valves, i.e. a modulator for one pixel at a time, in combination with 2D scanning of the laser beam are often referred to as flying spot systems. In these systems typically three laser sources, one for each of the RGB primaries (R: red; G: green; B: blue), are combined into a single beam which is then scanned over the area of an external projection screen. The laser beams of the three primary (RGB) lasers normally are first collimated and made to converge before they are combined into the single beam and directed to the scanning unit, which includes one or several scanning mirrors. Due to the convergence of the beams a small spot is formed on the projection screen allowing the desired image resolution.

According to the amount of light on the screen required for the application, laser beams of high power levels have to be applied. For every 25 lumen (RGB) on the screen, for example, a laser power (RGB) of about 100 mW is required. Such high levels of laser power may be harmful for the eyes of a person that gets into the scanning cone of the projection system and happens to look directly into the beam. While the brightness of the projected image is only dependent on the total laser power averaged over the screen, the risk for the eye depends in a complicated manner on the system and the laser parameters, such as laser power, duration of exposure and size of the laser source.

One of the main risks when operating such a scanning laser system is the possibility of the user or any third person to look into the beam and focus the eye to the very small source of light. This produces very high power densities on the retina, which can cause eye damage within fractions of a second.

US 2005/0035943 Al discloses a laser scanning projection device including detection means for detecting the presence or absence of an intruding object between the projection device and the projection screen. When such an intrusion is detected by the detection means, the output of the laser beam is lowered to a non harmful power level. This however requires in addition to the provision of such a detecting system also a repeated control of the proper work of this detection system.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a projection device which lowers the risk of damage of the eye when a person gets into the scanning cone of the projection device and happens to look directly into the beam, and which does not require any detection system or repeated control.

The object is achieved with the projection device according to claim 1. Advantageous embodiments of this projection device are subject matter of the dependent claims or are disclosed in the subsequent portion of the description.

The proposed projection device comprises at least one laser light source and a scanning unit arranged for scanning a scan area with a laser beam emitted from said laser light source. The projection device further comprises a beam splitting element arranged in beam direction behind said scanning unit and a projection unit projecting a scanned area of the beam splitting element, and with it an image formed by scanning the scanned area with the laser beam, to the external projection area. The beam splitting element is designed to split up the laser beam into a range of directions. The projection unit comprises at least one projection lens or curved projection mirror. With this construction of the projection device the laser source is effectively hidden from an observer. This reduces the hazard level considerably and results in an increased safety level of the projection device. For scanning systems in the pico beamer class of a few 100 mW of total laser power, which is a preferred power range of the proposed laser projection device, laser hazard is completely removed. The beam splitting element prevents an observer from directly looking into the beam with the eye accommodated to the position of the scanning mirror, which would form the laser source for the observer in absence of any splitting element. The beam splitting element forms the new light source of the system from the view of the observer. The scanning unit writes the image information (color and brightness of each pixel) to the surface of this stationary beam splitting element. The surface of the beam splitting element together with the image on it is projected via the projection unit to the external projection area, for example a projection screen, just as it is done in conventional DMD projectors (DMD: digital mirror device). A person looking into the cone of the projected light can no longer see the collimated laser beam itself but instead sees as the smallest light source the scanned area on the surface of the beam splitting element, which is much larger than the focused laser beam of a conventional laser scanning projection device. Due to this extended light source the risk of hazard of the eye is significantly reduced for the observer. There is no need for an additional detecting system or for any control of the operability of such a detection system. A further advantage of the proposed projection device is that it provides a two dimensional extended light source without a need for expensive 2D light valves such as DMDs or LCDs.

The beam splitting element of the proposed projection device is arranged in beam direction behind the scanning unit, i.e. between the scanning unit and the projection unit and projection screen. The beam splitting element is characterized in that it splits up the incoming laser beam to a range of directions, for example into a cone or fan having a cone or fan half angle of preferably between 5° and 30°. The beam splitting element and the projection unit are preferably designed and arranged such that a main portion of the laser energy impinging on the beam splitting element enters the projection unit to be used for projection of the image to the external projection area. The term "main portion" of the energy means more than 50% of the energy. This can be achieved by different measures, for example by adapting the distance between the beam splitting element and the projection unit, by designing the projection unit to have a sufficiently large aperture or by designing the beam splitting element to split up the laser beam in a limited range of directions. It is obvious for the skilled person that also any combinations of the above measures or even further measures or a combination with such further measures can be applied in order to make use of a main portion of the incoming laser energy for projection.

An additional measure preferably is to design the beam splitting element in such a manner that the mean or average direction of the range of directions to which an incoming beam is split up, is not the same for all positions or points on the scanned area of the splitting element. The splitting element is designed such in this embodiment that at each point of the scanned area the mean or average direction of the split up laser beam is directed to a central portion or centre of the curved projection mirror or projection lens of the projection unit. This can be achieved by a special geometrical form of the splitting element or by splitting properties varying over the scanned area of the splitting element.

In one embodiment of the proposed projection device the splitting element is a light scattering element which can be a light-transmissive or a light-reflective element. The scattering properties of this scattering element are preferably designed to scatter incoming light into a limited cone half angle of between 5° and 30°. Preferably this scattering element has a convex geometrical form to direct the scattering cones emerging from the scanned area towards the central portion of the projection lens or curved projection mirror.

In another embodiment the beam splitting element is a two- or three- dimensional phase grating that splits up the incoming laser beam to a two-dimensional array of sub-beams. The angular spread of the sub-beams and their energy content is determined by the structure of the grating, in particular by the lattice distance or lattice constant of the grating. In a preferred embodiment, this lattice distance or lattice constant is varying over the scanned area of the grating in such a manner that the mean of average direction of the sub-beams from each point or pixel of the scanned area is aligned to the central portion of the projection lens or curved projection mirror in order to make efficient use of the laser power.

The projection device may comprise several laser light sources, in particular laser light sources for red (R), green (G) and blue (B) laser light to form a RGB laser projection device. The laser beams of the different laser light sources are combined by means of a merging arrangement into a single laser beam which is directed to the scanning unit and, behind the scanning unit, split up with the beam splitting element according to the present invention.

In another embodiment, the laser beams of different wavelengths, for example of red, green and blue light, are not combined into a single beam but are scanned individually by the scanning unit to form a desired image on the beam splitting element and projection screen. The scanning unit in this case comprises different scanning mirrors for the different laser beams. Another possibility is to direct the different laser beams of the different colors at slightly different angles to a common scanning mirror. Using different primary laser beams which are not combined into a single beam but are scanned individually over the surface of the beam splitting element allows images with higher brightness and accordingly higher total laser power at lower power densities of the individual beams (compared to a device with a combined beam). This reduces both, the demands on the components and the laser hazard level. Generally, the scanning unit may be a ID or 2D scanning unit constructed in a known manner and may contain one or several scanning mirrors or other scanning elements. This scanning unit may comprise for example a rotating polygon mirror wheel for one scanning direction and a consecutive tiltable scanning mirror for the other scanning direction or a scanning mirror tiltable in both scanning directions. The laser light sources may comprise laser diodes emitting in the desired wavelength regions.

The proposed projection device may be formed as a handheld device and may also be included in other handheld devices like smart phones or PDAs (personal digital assistants). The invention, however, is not restricted to handheld devices.

In the proposed projection device the laser beam, for example the output of a laser diode, is focused to the surface of the beam splitting element, thereby producing the signal for one pixel. By scanning the beam over the surface of the splitting element, the complete image information is written on this surface. Every point of the scanned surface of the splitting element forms a stationary source for the corresponding pixel of the image on the projection screen. A person looking into the cone of the projected light then sees as the smallest light source the scanned area of the splitting element, which is significantly larger than the area of a focused laser beam. The scanned area of the splitting element is preferably about the size of a mirror array in a conventional DMD, i.e. around 1 to 3 cm². In that case projection lenses already used for DMD devices can also be used in the proposed projection device. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described herein after.

BRIEF DESCRIPTION OF THE DRAWINGS

The proposed projection device is described in the following by way of examples in connection with the accompanying figures without limiting the scope of protection as defined by the claims. The figures show:

Fig. 1 a schematic view of an example of the proposed projection device;

Fig. 2 a schematic view of the splitting up of the laser beam by a reflecting beam splitting element;

Fig. 3 a schematic view showing a transmissive beam splitting element having a convex shape; and Fig. 4 a schematic view of a further example of the proposed projection device.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 is a schematic view of an example of the projection device 1 according to the present invention. In this device 1 three laser light sources 2 emitting red (R), green (G) and blue (B) laser light beams are included. The laser light sources 2 may be for example laser diodes. The divergent laser beams emitted by these laser light sources 2 are collimated by a collimating optics 3 and combined by an arrangement of dielectric mirrors 4, 5, 6 to form one single laser beam 9. Dielectric mirror 4 is designed to reflect light in the red wavelength region. Dielectric mirror 5 reflects in the green wavelength region and is transparent in the red wavelength region, whereas dielectric mirror 6 reflects in the blue wavelength region and is transparent in the red and green wavelength regions. The combined single laser beam 9 is directed to a 2D scanning unit 7, which contains at least one scanning mirror 8. In Fig. 1 only for illustrative purposes one scanning mirror 8 is depicted, which is tiltable in the direction of the arrow. In order to prevent an observer from looking at the concentrated source of laser power on the scanning mirror 8, a beam splitting element 12 is arranged in beam direction behind the scanning unit 7. This beam splitting element 12, which is a transmissive element in the example of Fig. 1, splits up the scanning laser beam 10 into a range of directions indicated as a light cone 11 in the example of Fig. 1. The scanning mirror 8 scans a scan area on the beam splitting element 12 thereby writing the image information, i.e. color and brightness of each pixel, to the surface of this stationary beam splitting element 12. The scanning laser beam 10 may be focused to the beam splitting element 12 to this end. The beam splitting element 12 and with it the image on its surface is then projected via a projection lens 13 onto the external projection area, usually a projection screen 14.

With this construction the safety of the laser projector is increased reducing the risk of eye damage for an observer looking directly into the beam. A person looking into the beam between the device and the projection screen 14 and focusing to the splitting element 12 sees an extended source instead of the collimated laser beam of conventional scanning projectors. This extended source results in a considerable reduced radiation hazard of the projector. The cross section of the projection light cone is at all positions from the projector to the projection screen much larger than the aperture of the eye, such that under no circumstances the full laser power can enter the eye. This results in an inherently increased safety level of the projector. The beam splitting element 12 is a main component for achieving the object of the present invention. This splitting element can be a light-transmissive element, as shown in Fig. 1 or a reflective element as shown in Fig. 2. Fig. 2 illustrates the beam splitting of the scanning laser beam 10 coming from scanning unit 7 in a more detailed view in two scanning positions and for two embodiments. One scanning position is indicated with the solid lines, the other scanning position is indicated with the dashed lines. As can be seen from the figure, the scanning beam 10 when being reflected at the beam splitting element 12 is split up into a light cone 11 shown by two limiting light beams of the light cone 11. In order to achieve an efficient operation of the projection system, the beam splitting element 12 is designed to generate directed and limited light cones. Fig. 2 shows an example on the left hand side in which all of the light of the light cones enters the projection lens 13 to be projected to the external projection screen. The distance of the projection lens 13 to the beam splitting element 12 and the cone angle of the light cones 11 produced by beam splitting element 12 are adapted such that most of the laser energy entering the beam splitting element 12 during operation of the scanning unit passes through the projection lens 13.

In the case of a scattering element as the beam splitting element 12 of the left hand drawing of Fig. 2 all split up portions of the light have nearly the same wavelength. It is also possible to use a beam splitting element deflecting different wavelengths of the laser beam 10 in the different directions by diffraction, for example by using a two- dimensional or a three-dimensional phase grating. The angular spread of the sub-beams generated by diffraction, i.e. the light cone 11 shown in Fig. 2, and their energy content are determined by the structure of the grating, in particular by the lattice constant or lattice distance. Using a grating with a lattice distance appropriately varying from the center of the scanned area to the borders of the scanned area, the light cones 11 generated at the borders are also directed with their average direction to the central portion of the projection lens 13 as shown in the drawing of the right hand side of Fig. 2. This allows to direct all of the split up light cones 11 to the center of the projection lens 13, and therefore to make efficient use of the laser power. The mechanism used for splitting up the scanning light beam 10 may be based on different physical mechanisms. One mechanism may be a scattering on a reflective or transmissive element having a strongly directed scattering characteristics. An example for such a beam splitting element is in its simplest case a silica plate with adjusted surface roughness (forward scattering plate). Preferably a scattering element with a scattering surface having a narrow scattering behavior is used as the beam splitting element 12, that scatters most of the scattered light into a cone of a full cone angle of, for example, 22° (cone half angle of 11°) in order to fill the projection lens 13 similar to the situation in conventional DMD projectors. Fig. 3 shows a further example of a beam splitting element 12 which in this example has a convex shape. With such a convex shape the split light cones 11 can also be directed to the central portion of the projection lens 13 to achieve the above efficient use of the laser power. In addition, this convex shape fits better to the shape of the focal plane of the projection lens 13, which plane in reality also forms a convex surface, and therefore may improve the projection.

In the further example of the projection device shown in Fig. 4, the different colored laser beams of the RGB laser light sources 2 are not combined into one beam but are scanned individually over the surface of the splitting element 12. To this end, the scanning unit 7 includes several scanning mirrors 8 for individually scanning each of the colored laser beams to the scan area. These different colored laser beams may coincide at each instance in one point of the scan area of the beam splitting element 12 or may not coincide in one point of this scan area. The image is nevertheless correctly formed for the observer since the eye integrates over time if the scanning beams are moved with a sufficiently high velocity over the scan area. While the invention has been illustrated and described in detail in the drawings and forgoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention is not limited to the disclosed embodiments. The different embodiments described above and in the claims can also be combined. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. For example, although the proposed projection device has been described in these embodiments with three laser sources emitting red, green and blue light, the projection device may also include more than three or less than three laser sources. It may for example also be formed of only one laser source, which then produces a monochromatic image on the screen. Furthermore, this laser light source may emit a broad range of wavelengths which are then timely selected by an appropriate timely varying filter, for example a color filter wheel. The geometrical form of the beam splitting element may also vary depending on the intended effect. Furthermore, also other physical principles may be used for splitting up the incoming laser beam into a range of directions.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of these claims.

LIST OF REFERENCE SIGNS

1 projection device

2 laser light source

3 collimating optics

4 dielectric mirror

5 dielectric mirror

6 dielectric mirror

7 scanning unit

8 scanning mirror

9 incoming single laser beam

10 scanning laser beam

11 light cone

12 beam splitting element

13 projection lens

14 projection screen

Claims

CLAIMS:

1. A projection device comprising at least one laser light source (2) and a scanning unit (7) arranged for scanning a scan area with a laser beam (10) emitted from said laser light source (2), the projection device further comprising

- a beam splitting element (12) arranged in beam direction behind said scanning unit (7), said beam splitting element (12) splitting up the laser beam (10) to a range of directions, and

- a projection unit (13) projecting a scanned area of the beam splitting element (12) to an external projection area (14), said projection unit (13) comprising at least one projection lens or curved projection mirror.

2. The projection device according to claim 1, wherein the beam splitting element (12) and the projection unit (13) are designed and arranged such that a main portion of split light enters the projection unit (13).

3. The projection device according to claim 2, wherein the beam splitting element (12) is a transmissive or reflective scattering element.

4. The projection device according to claim 3, wherein the scattering element is designed to split up the laser beam (10) to a light cone (11) having a cone half angle of between 5° and 30°.

5. The projection device according to claim 3 or claim 4, wherein the scattering element has a convex shape adapted to direct a main portion of split light to a central portion of the projection lens or curved projection mirror.

6. The projection device according to claim 2, wherein the beam splitting element (12) is a transmissive or reflective two- or three- dimensional grating.

7. The projection device according to claim 6, wherein the grating is designed to have a varying lattice distance over its scanned area, said varying lattice distance being selected to direct a main portion of the split light to a central portion of the projection lens or curved projection mirror.

8. The projection device according to claim 1, wherein optical beam forming elements (3) are arranged between the laser light source (2) and the scanning unit (7), said optical beam forming elements (3) being designed and arranged to focus the laser beam (10) to said beam splitting element (12).

9. The projection device according to claim 1 comprising: at least three laser light sources (2) emitting laser beams in different wavelength regions and a merging arrangement (4-6) which combines the beams of the three laser light sources (2) to one single beam (9) which is directed to the scanning unit (7).

10. The projection device according to claim 1 comprising: at least three laser light sources (2) emitting laser beams in different wavelength regions, wherein the scanning unit (7) is designed to individually scan each of the laser beams over a surface of the beam splitting element (12).

11. The projection device according to claim 9 or claim 10, wherein the at least three laser light sources (2) emit in the red (R), green (G) and blue (B) wavelength region.