Abstract:
An optical device for an image projector for providing a projected wide field of view image, includes a first dichroic mirror or splitter, arranged in an input path for receiving light of a plurality of wavelength bands, and adapted to output by reflection on a first path respective first portions of selected ones of the wavelength bands and by transmission on a second path respective second portions of the selected ones of the wavelength bands. Light modulating devices, arranged in respective ones of the first and second paths to receive the first and second portions, generate and output on respective first and second intersecting output paths modulated ones of the first and second portions. A second dichroic mirror or combiner, adapted to reflect one and transmit the other of the first and second portions, is arranged in an area of intersection of the first and second intersecting output paths, to output, by reflection and transmission, respectively, the modulated ones of the first and second portions on a common output path.

Description:
FIELD OF THE INVENTION 
     The invention relates to an optical image projector, more specifically an electronic projector having an extended field of image projection wherein the image in the image plane is made by imaging several partial images generated by an apparatus having a plurality of light modulating devices. 
     BACKGROUND OF THE INVENTION 
     A wide range of different electronic projectors employ light modulating panels of different kinds to generate the image which by using lens optics is projected onto a screen. Currently the mostly used panels are of types LCD (“Liquid Crystal Display”) or DMD (“Digital Modulating Device”). 
     An LCD-panel is made of a number of light valves arranged side by side in a matrix, wherein each light valve is individually controllable and generates a point or image element, frequently referred to as a pixel, in an image assembled by the pixel light valves of the matrix. By controlling the light valve of each pixel to let through more or less light from a rewards located light source, seen from the direction in which the light flows from the light valve of the pixels, an image will be formed assembled by the pixels which let through light in mainly the same direction. 
     A DMD-panel is mainly generated by a number of tiltable micro mirrors, arranged side by side in a matrix, wherein each micro mirror is individually controllable and generates a point or a picture element, frequently referred to as a pixel, in an image consisting of a large number of such pixels. By controlling individually the mirrors of the various pixels to reflect light from a forward located light source in a particular direction, seen from the direction into which light is reflected, an image is generated assembled by the points that reflect light mainly in the same direction. The optical efficiency of a DMD-panel is frequently improved by combining the DMD-panel with a TIR-prism (total internal reflection prism). 
     By the commonly employed light source which emits light with a spatial distribution, such as for example an incandescent lamp or an arc lamp, which may include a kollimator, the light flowing from each pixel exhibits a divergence corresponding to the divergence of the light provided by the light source to the panel. 
     To project onto a screen the image generated by the light modulating panel, a lens is arranged in the path of the light exiting from the panel, such that light is collected, i.e. focused, in a plane of projection, which represents an imaging of the panel itself onto the screen. 
     Due to limitations in the production technology, light modulating panels are delivered in s different and limited matrix sizes and dimensions, preferably adapted to standardized picture formats that often are used in other image acquisition technology or screen technology. The available matrix sizes and dimension thus control the image resolution that by one single light modulating panel is achievable in a projected image. As an example, with a panel which includes 800 pixels horizontally and 600 pixels vertically, a projected image to fill a cinema screen having dimensions 4000 mm horizontally and 3000 mm vertically, would exhibit pixels that each measures 5 mm by 5 mm. For a viewer located close to the screen, such as for example a viewer located in one of the first rows in the movie theatre, each pixel will be clearly visible and would destroy the viewing experience of the picture being projected. 
     To obtain a better resolution, a plurality of panels are employed in an arrangement for their projection side by side in the projection plane as mosaic elements that together form the desired, projected image. With the movie theatre screen example mentioned above, an image mosaic obtained using a two by two panel arrangement of the same panel type indicated above, would require only one half of the image magnification in the projection optics, and would provide an image which exhibits pixels each having the dimensions 2.5 mm by 2.5 mm. In other words, thereby a reduction by a factor of two of the pixel size in the projected image is achieved, which may prove to be acceptable for use in a movie theatre. 
     In the technical field of electronic image projection a number of different solutions are know for generating a projected image that includes a mosaic of imagings of a plurality of light modulating panels. Known solutions employ partly reflecting mirrors or polarization dependent optical elements to deliver light from light sources or for combining light exiting from a number of light modulating devices to generate an image mosaic in the image projection plane. 
     The technical field of the invention particularly relates to electronic image projectors of a type that make use of a single lens for imaging a number of light modulating panels for creating the image mosaic, and which preferably also includes a single light source for providing light to the light modulating panels of the projector. 
     A solution for a projector of the type referred to in the section above, is illustrated using the example of the accompanying  FIG. 1 . A light source OS 1  transmits a ray bundle in a direction towards the partly transmittive mirror DMS and the mirror MR 1 , which in turn each direct their part of the ray bundle towards each of their light modulating devices OM 1  and OM 2 , respectively. The pixels of the light modulating devices OM 1  and OM 2  forward the light exiting from the devices in a direction by the deflection of mirrors MR 3  and MR 2 , respectively, which control the outwards flowing light towards and through the imaging lens LE 1  and forwards towards the image plane IP where images IM 1  and IM 2 , respectively, are generated side by side, by the respective panels of the light modulating devices OM 1  and OM 2 . To generate colour images, a rotating colour filter CF 1  is used, which is synchronised with the controlling of the light modulating panels of the light modulating devices OM 1  and OM 2 . 
       FIG. 1  also illustrates that the lights which flow from the light modulating devices is OM 1  and OM 2 , respectively, to generate an image mosaic comprised of the respective image parts IM 1  and IM 2  that are located immediately adjacent to each other, would only partly fall onto the mirrors MR 3  and MR 2 , respectively, where the mirrors come together with the optical central axes of the lens. Such an arrangement would require strict control of the design of the mirror combinations MR 3 , MR 2 , to avoid a visible transition in the imagings of IM 1 , IM 2  in the image mosaic. Furthermore, due to the spreading of the light coming from the light modulating devices OM 1  and OM 2 , a considerable part of the light that is to generate the respective parts of the images IM 1  and IM 2  that are located close to the transition, would pass below the mirrors MR 3 , MR 2 , and lead to a significant vignetting. 
     A corresponding problem is also present in a projector that is designed according to the exemplary alternative that is illustrated in  FIG. 2 . 
     A possible solution to the above mentioned problems of the transition between the mirrors MR 3  and MR 2 , and also to the vignetting which in the accompanying  FIGS. 1 and 2  in part is indicated by the broken line, is to image the mosaic elements IM 1  and IM 2  with some overlap, and to control in a special way those parts of the panels of the light modulating devices OM 1  and OM 2  that contribute to generating the images in the overlapping area. Such a solution is, however, complicated as the control of the panels of the light modulating devices OM 1  and OM 2  would have to be adapted individually and be maintained adjusted and controlled for each and every projector optics. 
     A further possible solution, which may be understood by studying the solution suggested in  FIG. 3 , is to replace the mirror MR 3  that is included in  FIGS. 1 and 2 , with an ordinary partly reflective mirror DM 2  without any selectivity with regard to the wavelength of the light. A disadvantage of this solution wherein DM 2  is an ordinary partly reflective mirror or beam splitter without any wave length selectivity, is that DM 2  introduces a loss of light, as light exiting from OM 1  that in part will be coupled through DM 2 , cannot be transmitted towards the lens LE 1 , and light exiting from OM 2  that only in part will be reflected by DM 2  also will not be forwarded towards the lens LE 1 . The light loss from OM 1  is illustrated by the broken line U 1  and the light loss from OM 2  is illustrated by the broken line R 2 . 
     SUMMARY OF THE INVENTION 
     According to the present invention, the above mentioned and other problems are solved by providing an optical image projector having a colour separating mirror or beam splitter that provides light to respective ones of several light modulating devices, colour combining mirrors or beam splitters that combine light from respective ones of the several light modulating devices, and an imaging optics arranges in the light parts of the combined light such that several light modulating devices are imaged side by side. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a prior art solution for a projector of the type for generating a projected linage that includes a mosaic of imagings of a plurality of light modulating panels. 
         FIG. 2  illustrates an exemplary alternative solution in the prior art. 
         FIG. 3  illustrates a projector according to the present invention. 
         FIG. 4  illustrates a projector according to the present invention that employs two light modulating devices 
         FIG. 5  illustrates a projector according to the present invention that employs three light modulating devices. 
         FIG. 6  illustrates the relative distribution of reflectivity RF and transmittivity TR of the primary colour bands that results from a dichroic mirror or beam splitter adapted for the invention shown in  FIG. 4 . 
         FIG. 7  illustrates the relative distribution of reflectivity RF and transmittivity TR of the primary colour bands that results from a dichroic mirror or beam splitter adapted for the invention shown in  FIG. 5 . 
         FIG. 8A  illustrates exemplary characteristics of a light source by the curve labeled  100 , of the colour wheel filters by the curves labelled  120 ,  130  and  140 , and of an “ideal” dichroic mirror or beam splitter (DM 1 , DM 2 , DM 3 ) by the curve labelled  110 . 
         FIGS. 8B and 8C  illustrate a magnified version of those curves that are illustrated in  FIG. 8A , and the vertical broken line indicates a common point of reference. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Firstly, the invention is explained by the example being illustrated in  FIG. 3 . 
     The projector of the invention includes a light source OS 1  and, possibly, its associated optics, adapted such that multicoloured light is directed to fall onto a dichroic, transmittive first mirror or beam splitter DM 1 . This dichroic, transmittive first mirror or beam splitter DM 1  is adapted to split light, as energy in or more RF of the band of one or more selected colours M, Y, C generally is fully reflected and the energy of the other part T of the band of the selective colours generally is entirely transmitted (or, may be divided for the energy of further parts of the band, using a further splitter DM 3 ). 
     Examples of DM-characteristics of a dichroic mirror or beam splitter for the inventions is shown schematically in the accompanying  FIGS. 6 ,  7 ,  8 A-C, illustrating that the dichroic, transmittive first mirror or beam splitter DM 1  is adapted for dividing each colour into three band, designated herein by “low”, “middle” and “high”, respectively, such that central RF parts of the primary colour bands for “R”, “G” and “B” are reflected in the band designated “middle”, and the surrounding parts TR of each of the primary colour bands “R”, “G” and “B” are transmitted in bands designated “low” and “high”, or vice versa. By a subsequent combination of light that has been transmitted, or as an alternative, reflective, in the bands named “low” and “high”, a colour coordinate is generated that corresponds to the colour coordinate of light in the band referred to as “middle”. The relative distribution of reflectivity RF or transmittivity TR is in the  FIGS. 6 and 7  somewhat different, and is meant to illustrate the principle differences between the mirror or beam splitter that is adapted for a solution employing two light modulating devices such as for example the solution illustrated in  FIG. 4 , and for a solution employing three light modulating devices such as for example the solution illustrated in  FIG. 5 , respectively. It should be noted that in the solution being illustrated in  FIG. 4  having two light paths, one would preferably dimension DM 1  in such a way that the total light energy in the two bands “low” and “high” corresponds to the light energy in the one band “middle”. In comparison, for the solution illustrated in  FIG. 5  wherein light is distributed onto three light paths, one would preferably design DM 1  in such a way that the total light energy in the two bands “low” and “high” corresponds to one third of the light energy in the one band “middle”, and furthermore dimension the splitter DM 2  such that the reflective light energy corresponds to the is transmitted light energy. The aforementioned examples of dimensioning DM 1 , and possibly also of DM 3 , are from a colour combination perspective based on mainly equal contributions from, or possibly loss in, each of the respective light modulating devices, but the dimensioning could also be adapted to solutions wherein the contribution of the individual light modulating devices, or possibly also losses, are different. The characteristics of the dichroic mirror or beam splitter DM 1 , DM 3  that are provided in the light path could advantageously also be dimensioned to take into consideration the energy distribution of the light from the light source being employed. 
     According to the present invention, the dichroic mirror or beam splitter could be dimensioned together with or in relation to the characteristics of the colour filter wheel and/or the characteristics of the light source, thereby achieving the desired colour space or other property with regard to colour, intensity or other optical properties of the mosaic images that are created in the image plane IP by use of a projector implementing the present invention. 
       FIG. 8A  provides exemplary characteristics of a light source by the curve labelled  100 , of the colour wheel filters by the curves labelled  120 ,  130  and  140 , and of an “ideal” dichroic mirror or beam splitter (DM 1 , DM 2 , DM 3 ) by the curve labelled  110 . The vertical axis indicates for filters and mirrors or beam splitters a characteristic value for their transmission, and for the light source a relative power characteristic value. In particular for the curve labelled  110  which exemplifies the characteristics of the “ideal” dichroic mirror for beam splitter DM 1 , DM 2 , and possibly also DM 3 , indicates in the vertical axis with a percentage that low transmittivity implies a correspondingly high reflectivity. The  FIGS. 8B and 8C  include a magnified version of those curves that are illustrated in  FIG. 8A , and the vertical broken line indicates a common point of reference at about 550 nanometers along the horizontal axis for indication of the wave length. 
     Of the exemplary characteristics being illustrated by  FIGS. 8A ,  8 B and  8 C, a skilled person in the art in the field of the present invention is taught that an advantageous total characteristic, corresponding to the “ideal” characteristics that are explained by reference to the principle examples that are illustrated in  FIGS. 6 and 7 , may be achieved by dimensioning the reflected, or the transmittive parts, respectively, of the characteristics of the dichroic mirror or beam splitter DM 1 , with regard to both the shape and widths of the “low”, “middle” and “high” bands, and their positioning in relation to the respective ones of the characteristics of the filters of the wheel colour wheel and/or the characteristic of the light source. In this perspective, the objects of the present invention are achieved by an embodiment of the invention wherein the provision of light in one band “middle” and provision of light in both bands “low” and “high” for the respective, different light modulating devices are made through a preferable total characteristic achieved by a combination of the characteristics of the dichroic mirror or beam splitter DM 1  and a further filter, as the further filter as an example preferably could be realised in one or more of the filters of the colour wheel CCF 1 . 
     In a projector according to the present invention as being illustrated by the example in  FIG. 3 , is including further optics adapted such that the light reflected from DM 1  is directed into a first path to a first light modulating device OM 1  located onto a first optical axis being displaced in relation to the optical axis of the focusing imaging lens LE 1 . 
     The projector of the invention further includes optics adapted such that the transmitted light T is provide into a second path towards a second light modulating device OM 2  that is located onto a second optical axis, shifted with regard to the optical center axis of the focusing imaging lens LE 1 . 
     As indicated above, an embodiment of the invention may include one or more further light dividers, light combiners or light modulating devices, or possibly plurality of imaging optics devices, to generate a corresponding image into a common image field, which corresponding image then constitutes a partial image of an image comprised by the corresponding partial images generated each by a corresponding respective light modulating device. Such further splitters, combiners and light modulating devices are shown in the accompanying  FIG. 5 , wherein light is provided in a further third path to a third light modulating device OM 3  located on a third optical axis shifted in relation to the optical axis that are decided by the locations of the first and second light modulating devices OM 1  and OM 2  in relation to a common imaging lens LE 1 . 
     A corresponding dichroic, transmittive second mirror or beam splitter DM 2  is located in the area where the first and second paths of modulated light exiting from OM 1  and OM 2  (and possibly also the third path of modulated light exiting from OM 3 ) intersect, and effects a combination of the modulated light from the light modulating devices as DM 2  mainly provides a total reflection towards the lens LE 1  of energy in the selected bands that were provided to OM 1  and mainly in total transmits towards the lens LE 1  the energy in the selected bands that were provided to OM 2 . 
     As the first and second, and possibly also the third, as illustrated in  FIG. 5 , light modulating devices are located on different optical axis in relation to the optical center axis of the focusing image creating lens, they are imaged in different places, preferably as partial images located side by side, in an image plane IP.  FIG. 5  illustrates a solution including three light modulating devices, OM 1 , OM 2  and OM 3 , respectively, which by their imaging through lens LE 1  and possibly other optical elements, such as for example the mirrors MR 2 , MR 3  and DM 2 , create an image mosaic that include three partial images IM 1 , IM 2  and IM 3  located side by side in the image plane IP. By a suitable selection of directions for two, possibly also three or more, axis, the light modulating devices are imaged side by side in the image plane, possibly with a complete or only partial overlap. The characteristics of the dichroic second mirror or beam splitter DM 2  is preferably corresponding to or complementary to the characteristics of the dichroic first mirror or beam splitter DM 1 , depending on whether the image is to be generated on the first or second side of the outcoupling element that is illustrated as DM 2  in  FIGS. 3 ,  4  and  5 . 
     In a variant of the solution disclosed above, other light modulating devices than the DMD is employed. As an example, LCD-panels may be used as the light modulating devices, that may be of a reflective or a transmitted type, with or without light polarizing elements in form of mirrors or prisms in the corresponding light paths. 
     The first and second, and possibly also the third, dichroic mirror or beam splitter may in a further variant be provided as dichroic filters, with or without the forms of mirrors, prisms or other optical elements for altering the direction of the beams, and may be selected in consequence of the locations of the further optical elements relative to each other. However, such combinations of dichroic filters and other optical additional elements should be selected such that one achieves both a distribution of light to the various light modulating devices, and a combination of light from the same light modulating devices, that in total provides the solution corresponding to the solution provided by the inventive principles of the solutions disclosed herein. 
     The use of dichroic mirrors or beam splitters for splitting the light at the input, and for collecting the light at the output, respectively, provides according to the present invention a generally loss less splitting and combination of light, as no light is lost in polarizing optics or by reflection or transmission into other directions by semi-transparent mirrors or beam splitters. At the same time, the number of optical surfaces to which or by which the light must pass from the light source to the image plane can be kept at a minimum. 
     In a variant of the solution of the invention, the arrangement of the optical elements are such that the partial images in the image plane provided by respective ones of the light modulating panels represent respective ones of images of an object as seen from different camera positions, such as typically is provided by a stereoscopic camera arrangement. In use for projection of a stereoscopic set of partial images, control signals corresponding to left and right images would then be provided to respective ones of the light modulating devices OM 1  and OM 2 . For a viewer being suitably located with respect to the plane in which the partial images IM 1  and IM 2  are formed, where image separation for the eyes possibly may be obtained by the provision of shutter glasses or other means that allows the viewer to observe one of the partial images by one eye and the other of the partial images by the other eye, a 3D image experience may be provided. Considering an application of the arrangement of the invention for a 3D movie experience, the arrangement of the invention as illustrated in  FIG. 3  or  FIG. 4  could advantageously adapted for a multi viewer situation by allowing the partial images IM 1  and IM 2  to be formed in the same area of the image plane, while for a single viewer 3D viewing experience the partial images IM 1  and IM 2  could be formed in different parts of the image plane to allow 3D viewing by geometrical separation of the images for the right and left eyes, without necessitating the use of shutter glasses or similar means. 
     In case of an adaptation of the invention for providing a 3D viewing experience as disclosed in the paragraph above, with fully overlapping partial images IM 1  and IM 2  in the image plane IP, employment of the three path solution illustrated in  FIG. 5  adapted to provide the third partial image IM 3  in a fully overlapping constellation with the partial images IM 1  and IM 2  allows for an introduction of a further optical enhancement or augmentation of the total image formed by the partial images projected in the image plane IP.