TECHNICAL FIELD

The present invention relates to an optical system for combining at least a first and a second beam onto a spatial light modulator, beam combiner for combining at least a first and a second beam, and a method for combining at least a first and a second beam onto a spatial light modulator.

In particular the invention relates to a pico projection system, especially for the increasing of brightness of projectors.

BACKGROUND

There are several technologies which are used in projectors for creating and projecting images on a screen. Usually the projector creates images with different colors consecutively. A projector uses a spatial light modulator, color filters, and a color wheel with a lamp and a beam combiner with three light sources of different wavelengths.

In recent years solid-state light sources and other narrow-wavelength-band and/or polarized light sources capable of producing visible light have drawn significant attention as alternative light sources for use in imaging systems, such as projection systems. These light sources exhibit a compact size, greater durability, longer operating life, and lower power consumption. Often white lamps are used as a light source within these projectors.

Beam combiners often use a plurality of optical components which have to be adjusted relative to each other. There are many different types of beam combiners, such as beam combiners using dichroic mirrors or different combs with prismatic arrays which use total internal reflection or refraction, diffraction gratings and others.

Document US 4,084,180 describes a color splitting prism assembly for color mixing with LED sources.

Document US 5,098,183 describes dichroic optical elements for use in a projection type display apparatus.

Document US 6,676,260 describes a projection apparatus using a spatial light modulator with relay lens and dichroic combiner.

Document US 5,504,544 describes a linear prismatic film for beam combining. Document US 5,231,433 describes a refractive comb for beam combining. SUMMARY

It is the object of the invention to provide an improved technique for combining multiple beams.

This object is achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

The invention is based on the finding that multiple light sources with different wavelengths and a beam combiner are an effective and simple solution for an optical projector, e.g. a pico projector. The light sources can be light emitting diodes or lasers. These light sources can be used for a switching of different colours. A beam combiner combines beams of different colours and redirects light into the same area and direction. There are different types beam combiners that are used in projection systems, such as dichroic mirrors or X-prisms. Often beam combiners inefficient and their manufacturing is difficult and expensive, since several special optical coatings are used. In general light transmission should be increased and size and construction of the beam combiner should be reduced.

According to a first aspect, the invention relates to an optical system for combining at least a first and a second beam onto a spatial light modulator comprising a first light source for outputting a first beam of a first wavelength; a second light source for outputting a second beam of a second wavelength; and a beam combiner having a plurality of first triangular prisms having first and second sides, the first sides of the first prisms facing the first light source for inputting the first beam and for internally reflecting the second beam, the second sides of the first prisms facing the second light source for inputting the second beam and for internally reflecting the first beam; and a plurality of second triangular prisms with first and second sides, the first sides of the second prisms for outputting and refracting the first and second beam parallel onto the spatial light modulator or a screen.

In a first possible implementation form of the optical system according to the first aspect, the optical system comprises a third light source for outputting a third beam of a third wavelength.

In a second possible implementation form of the optical system according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the beam combiner comprises a flat side face for inputting the third beam.

In a third possible implementation form of the optical system according to the second implementation form of the first aspect, the third light source is arranged for perpendicularly inputting the third beam into the flat side face.

In a fourth possible implementation form of the optical system according to the third implementation form of the first aspect, the second sides of the second prisms are arranged for refracting the third beam parallel to the first and second beam refracted by the first sides of the second prisms.

In a fifth possible implementation form of the optical system according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the first sides of the first prisms and the second sides of the first prisms are arranged in an acute angle with respect to each other.

In a sixth possible implementation form of the optical system according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the first light source is arranged for perpendicularly inputting the first beam into the first sides of the first prisms.

In a seventh possible implementation form of the optical system according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the second light source is arranged for perpendicularly inputting the second beam into the second sides of the first prisms.

In an eighth possible implementation form of the optical system according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the optical system comprises a homogenizer for creating a beam with uniform shape from the first, the second and the third beam output by the beam combiner.

According to a second aspect, the invention relates to a beam combiner for combining at least a first and a second beam comprising a plurality of first triangular prisms having first and second sides, for inputting the first beam and internally reflecting the second beam, the second sides of the first prisms for inputting the second beam and internally reflecting the first beam; and a plurality of second triangular prisms with first and second sides, the first sides of the second prisms for outputting and refracting the first and second beam onto a spatial light modulator or a screen. In a first possible implementation form of the beam combiner according to the second aspect, the beam combiner comprises a flat side face for inputting a third beam of a third wavelength.

In a second possible implementation form of the beam combiner according to the second aspect as such or according to the first implementation form of the second aspect the second sides of the second prisms are arranged perpendicularly to an output side of the beam combiner.

According to a third aspect, the invention relates to a method for combining at least a first and a second beam onto a spatial light modulator, comprising the steps of outputting a first beam of a first wavelength; outputting a second beam of a second wavelength; inputting the first beam in first sides of first prisms of a beam combiner and internally reflecting the second beam by the first sides of the first prisms; inputting the second beam in second sides of the first prisms of the beam combiner and internally reflecting the first beam by the second sides of the first prisms; and refracting the first and second beams by first sides of second prisms of the beam combiner for outputting parallel on a spatial light modulator or a screen.

In a first possible implementation form of the method according to the third aspect, the method comprises the step of inputting a third beam into a flat side the beam combiner.

In a second possible implementation form of the method according to the first implementation form of the third aspect, the method comprises the step of refracting the third beam by second sides of second prisms of the beam combiner for outputting the third beam parallel to the first and second beam.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the invention will be described with respect to the following figures, in which:

Fig. 1 shows a schematic diagram of a beam combiner having first and second prisms according to an implementation form;

Fig. 2 shows a schematic diagram of the beam combiner having first and second prisms according to an implementation form;

Fig. 3 A shows a schematic diagram of the beam combiner having first and second prisms according to an implementation form; Fig. 3B shows a schematic diagram of the beam combiner having first and second prisms and according to an implementation form;

Fig. 4 shows a light distribution of the beam combiner;

Fig. 5 shows an optical system for combining three beams and forming uniform shape of beams; and

Fig. 6 shows an intensity profile of the beam combiner.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Fig. 1 shows a schematic diagram of a beam combiner 105 having a plurality of first and second prisms 107 and 109 according to an implementation form. 10. The beam combiner 105 is for combining a first and a second beam 103-1, 103-2. The first triangular prisms 107 have first and second sides 107-1 and 107-2. The first sides 107-1 of the first prisms 107 serve for inputting the first beam 103-1 into the beam combiner 105 and internally reflecting the second beam 103-2. The second sides 107-2 of the first prisms 107 serve for inputting the second beam 103-2 and internally reflecting the first beam 103-1. The first sides 107-1 and the second sides 107-2 are equal in lengths. The first prisms 107 have a cross section in the form of an equilateral or an isosceles triangle. The first sides 107-1 of the first prisms 107 and the second sides 107-2 of the first prisms 107 are arranged in an acute angle with respect to each other.

Further the beam combiner 105 comprises a plurality of second triangular prisms 109 with first and second sides 109-1 an 109-2. The first sides 109-1 of the second prisms 109 serve for outputting and refracting the first and second beam 103-1, 103-2 onto a spatial light modulator or a screen 200. The second sides 109-2 of the second prisms 109 are arranged perpendicularly with respect to an output side 117 of the beam combiner 105. The second sides 109-2 of the second prisms 109 serve for refracting a third beam 103-3 parallel to the first and second beam 103-1 and 103-2 that are refracted by the first sides 109- 1 of the second prisms 109. The second prisms 109 have a cross section in the form of a right triangle.

The first and second prisms 107 and 109 are integrally formed on parallel faces of block of transparent material. Therefore, there is no need for manually need adjusting the first and second prisms 107 and 109 relatively to each other.

Fig. 2 shows a schematic diagram of the beam combiner 105 having first and second prisms 107 and 109 according to an implementation form. The first and second prisms 107 and 109 are parallel linear, triangular prisms.

The block of the beam combiner 105 comprises a flat side face 11 1 for inputting a third beam 103-3 of a third wavelength. The flat side face 11 1 is integrally formed in a lateral side of the beam combiner block between an input face 119 for inputting the beams and an output face 117 for outputting the beams. The part of the beam combiner 105 having the flat side face 111 protrudes from the combiner block in triangular fashion.

The input face 119 of the beam combiner 105 and the output face 117 of the beam combiner 105 are parallel to each other. The first input face 119 with first prisms 107 uses total internal reflection (TIR) for beam combining. At the output face 119 beams are refracted by the plurality of second prisms 109 into the same direction.

The two light beams 103-1 and 103-2 are combined in the first comb having the first prisms 107 using total internal reflection. A light beam with two mixed colors is obtained. This beam and the third beam 103-3 are combined at the second comb having the second prisms 109. In this case refraction is used. The first comb and the second comb are an integral part of a transparent block which forms the beam combiner 105. This block has a special entrance surface for the third light beam 103-3, which is used for increasing transmission.

The beam combiner redirects three light beams 103-1, 103-2 and 103-3 parallel in the same direction. Thus, the beam combiner 105 allows redirecting three light beams with different colors in the same direction. The beam combiner 105 consists of one single transparent block without optical coatings.

Fig. 3 A shows a schematic diagram of the beam combiner 105 having first and second prisms 107 and 109 according to an implementation form.

The incidence angle a to the second prisms 109, i.e. comb or prismatic array, should be

a = arcsin ((1 + (1 + Sn ² ) ^m )/4n ² ),

b = arcsin (1 + (1 + Sn ² ) ^V2 )/4n,

where n is the refraction index of the combiner material, as the third beam 103-3 should be in the same direction as the first and the second light beams 103-1 and 103-2.

The input surface between AB of the flat side face 111 should be perpendicular to the third light beam 103-3. Consequently, an angle between points BAM should be equal ±5° so that losses for the third light beam 103-3 are reduced.

If diameter of incident first and second light beams 103-1 and 103-2 is d. The diameter of the third light beam between points NB is calculated from

NB = 2d x sin (a)

Fig. 3B shows a schematic diagram of the beam combiner 105 having first and second prisms 107 and 109 according to an implementation form. In this implementation form the dimensions of the beam combiner 105 are reduced, since the distance between points CA is CA = 0. In this case light sources are used which produce narrow beams.

Fig. 4 shows a light distribution of the beam combiner 105 when it is illuminated with the three beams 103-1, 103-2, and 103-3. Fig. 4 shows results of a simulation of the beam combiner 105. In order to employ the beam combiner 105 in an optical projector a white image with a uniform shape should be formed at the spatial light modulator (SLM). ^~

Fig. 5 shows an optical system 100 for combining a first, a second beam, and a third beam 103-1, 103-2 and 103-3 on a screen or a spatial light modulator 200. The optical system 100 comprises a first light source 101-1 for outputting a first beam 103- 1 of a first wavelength, a second light source 101-2 for outputting a second beam 103-2 of a second wavelength, and a third light source 101-3 for outputting a third beam 103- 3 of a third wavelength. The light sources 101-1, 101-2 or 101-3 are light emitting diodes or lasers. The light beams 103-1, 103-2 and 103-3 created by the light sources 101-1, 101-2 or 101-3 have a different wavelength each.

The first sides 107-1 of the first prisms 107 face the first light source 101-1 for inputting the first beam 103-1 and for internally reflecting the second beam 103-2. The second sides 107-2 of the first prisms 107 face the second light source 101-2 for inputting the second beam 103-2 and for internally reflecting the first beam 103-1.

The first and second beam 103-1 and 103-2 are incident at the first side 109-1 of the second prisms 109. The third beam 103-3 is incident on the second side 109-2 of the second prisms 109.

The first sides 109-1 of the second prisms 109 serve for outputting and refracting the first and second beams 103-1, 103-2 parallel onto the spatial light modulator 200. The second sides 109-2 of the second prisms 109 are arranged for refracting and outputting the third beam 103-3 parallel to the first and second beam 103-1, 103-2 refracted by the first sides 109-1 of the second prisms 109.

The third beam 103-3 is input in the protruding flat side face 111. The third light source 101-3 is arranged for perpendicularly inputting the third beam 103-3 into the flat side face 111. In similar fashion the first light source 101-1 is located such that the first beam 103-1 is input into the first sides 107-1 of the first prisms 107 perpendicularly. Also the second light source 101-2 is located such the second beam 103-2 is input into the second sides 107-2 of the first prisms 107 perpendicularly.

The beams 103-1, 103-2 and 103-3 are output on a common side of the beam combiner 105. This side integrally forms the plurality of parallel linear, triangular second prisms 109. The output side of the beam combiner 105 combine the directions of the beams 103-1, 103-2 and 103-3 with different colors.

After refraction at the output side of the beam combiner 105 all beams 103-1, 103-2 and 103-3 propagate in the same direction.

The optical system comprises a homogenizer 121 for creating a beam 115 with uniform shape from the first, the second and the third beam 103-1, 103-2, 103-3 output by the beam combiner 105. The homogenizer 121 can be a multispectral homogenizer. The homogenizer 121 is not only used for creating a single beam with uniform shape but also for finally combining beams with different colors. .

In the optical system 100 three beams 103-1, 103-2, 103-3 having different wavelength are combined in order to create a white uniform shape at the spatial light modulator 200 (LCoS, DLP). To that end a homogenizer 121 and a lens 123 are installed after the beam combiner 105. The optical system 100 can be used in a projector for projecting images onto a screen. Using the optical system 100 images with different colors can be created in series. The angle b after the beam combiner 105 can be calculated as:

b = arcsin (1 + (1 + Sn ² ) ^m )/4n.

The optical system 100 can be used in a projector, where it exhibits a good transmission and is implemented in a small way.

Fig. 6 shows intensity profile of the beam combiner 105. Fig. 6 shows a modeling and corresponding simulation results. The difference between central and side parts is 24% in column profile and 15% in row profile. The image color is white.

The beam combiner has the advantage that the beam combiner does not need to have an optical coating. This simplifies the construction and manufacturing of the beam combiner. Merely total internal reflection and refraction are used for combining different beams. Since the beam combiner consists of one continuous transparent block, in which the prisms are integrally formed, it is not necessary to manually adjust or align optical elements with each other.

From the foregoing, it will be apparent to those skilled in the art that a variety of methods, systems, and the like, are provided. Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the invention beyond those described herein. While the present inventions has been described with reference to one or more particular embodiments, those skilled in the art recognize that many changes may be made there to without departing from the scope of the present invention. It is therefore to be understood that within the scope of the — appended claims and their equivalents, the inventions may be practiced otherwise than as specifically described herein.