A prism assembly for use in a digital light processing system

Abstract

According to an embodiment a prism assembly (100) is disclosed to project an image by prisms comprising dichroic reflectors (202, 203) coated on respective surfaces for reflecting a spectral band and transmitting other spectral bands, and a set of dedicated digital micromirror devices, DMDs (201, 204, 205), the prism assembly assembled to split an incoming light beam (103) into a set of spectral bands, direct each spectral band to a respective DMD (201, 204, 205), redirect from the respective DMDs (201, 204, 205) the respective spectral bands into an on-state (102) for reaching a screen respectively an off-state (101) for absorbing inside the assembly, whereby the spectral bands redirected into the on-state (102) are recombined to form the image, wherein at least one dichroic reflector (202, 203) is divided into a first and second zone wherein a coating composition of the first zone differs from that of the second zone.

Description

A PRISM ASSEMBLY FOR USE IN A DIGITAL LIGHT PROCESSING SYSTEMField of the Invention

[0001] The present invention relates to the field of digital light processing systems, and in particular to a prism assembly for use in such a digital light processing system.Background

[0002] Digital light processing, DLP, is a set of chipsets based on optical micro-electro- mechanical technology that uses digital micromirror devices, DMDs. A DMD comprises microscopically small mirrors for modulating an incoming light beam into either an on-state by deflecting the light beam towards a screen or an off-state by deflecting it away thereof.

[0003] Said DLP technology is used in digital projection and imaging technology. For projecting an image in colour a prism assembly is used to split a light-beam into each primary colour of light, which is then routed to its dedicated DMD chip. Next, by redirecting incident light in the on-state respectively the off-state the different colours can be recombined and routed out through a lens. For best light-output and saturated colours, it is therefore recommended using three DMDs, one for the red colour, one for the green colour, and one for the blue colour.

[0004] The prism assembly is thus configured to split an incoming light beam, for example originating from a lamp, into a set of spectral bands, and further direct each spectral band to a respective DMD. Finally, by use of the respective DMDs for eachspectral band redirected spectral bands in the on-state are combined to form the image and projected onto the screen. The spectral bands that are redirected into the off-state are absorbed in the system housing the prism assembly.

[0005] In EP1632805B1 a prism assembly is disclosed configured to be used in a DLP projector. The prism assembly comprises six prisms configured to redirect an incoming light beam and to further form an outgoing light beam as explained above. To this end, surfaces of dedicated prisms are coated with respective dichroic reflectors adapted to a respective spectral band.

[0006] In EP1632804A1 another prism assembly is disclosed likewise configured to be used in a DLP projector.

[0007] A disadvantage of prism assemblies known in the art is that the light beams redirected in the off-state and absorbed within the system cause unwanted effects like stray light and dissipated heat. Special cooling techniques are therefore needed to avoid overheating and thermal stresses.

[0008] It is therefore an object of the present invention to alleviate the above drawback and to provide an improved prism assembly configured to project an image on a screen when used in a DLP projector.Summary of the Invention

[0009] This object is achieved, in a first aspect, by a prism assembly according to the first claim, the prism assembly configured to project an image on a screen by way of prisms comprising dichroic reflectors coated on respective surfaces thereof for reflecting a respective spectral band and transmitting other spectral bands, and a set of dedicated digital micromirror devices, DMDs, the prism assembly assembled to:- split an incoming light beam into a set of spectral bands;- direct each spectral band to a corresponding DMD;- redirect from each DMD the corresponding spectral band into an on-state for reaching the screen and an off-state away from the screen, whereby the spectral bands redirected into the on-state are recombined to form the image on the screen; characterized in that at least one dichroic reflector is divided into a first and second zone wherein a coating composition of the first zone differs from a coating composition of the second zone.

[0010] The prism assembly comprises a number of prisms, for example a 5-element prism assembly when the assembly is a Philips prism assembly, or a 6-element prism assembly. It should thus be noted that the number of prisms present in the assembly is not restrictive in view of the invention.

[0011] The prism assembly is configured in such a way that it splits, directs, and redirects an incoming light beam in different spectral bands. These spectral bands are further recombined thereby composing an image for projecting it on a screen. Therefore, one or more prisms comprise respective dichroic reflectors coated on respective surfaces of the one or more prisms. The dichroic reflectors are configured to reflect a specific colour and letting through other colours. These colours correspond to said different spectral bands, and preferably comprises a red band, a green band, and a blue band. In a preferred embodiment, a first dichroic reflector comprises a coating composition for reflecting the red band and letting through the green and the blue band. Next, a second dichroic reflector comprises a coating for reflecting the blue band and letting through the green band.

[0012] The prisms and dichroic reflectors coated in respective surfaces thereof are thus assembled in such a way that an incoming light beam is split into the set of spectral bands. Next, each spectral band is directed to a dedicated DMD. The dedicated DMD in turn is configured to either redirect the respective spectral band into an on-state or on off-state. As already explained, the redirected spectral bands in the on-state are recombined to form the image that is projected on a screen. The redirected spectral bands in the off-state can be absorbed by the system.

[0013] Further, a dichroic reflector comprises multiple layers of optical materials of different refractive indices and thicknesses deposited onto a glass substrate. Internal reflections between these refractive index boundaries define the characteristics of the dichroic reflector in the sense of which wavelengths are mainly transmitted, and which ones are mainly reflected. Further, when light passed through the reflector at a given angle of incidence, AOI, a portion will be reflected by each of the thin layers within a cavity. Due to the optical path length difference between the reflective layers, reflected light will interfere with incident light, resulting in only certain wavelengths being mainly reflected, and the rest being mainly transmitted. Since the exact optical path length differences are dependent on the AOI of the incoming light, the characteristics of dichroic reflectors are angle-dependent as well.

[0014] As will further be discussed, these characteristics can be illustrated by reflection curves, whereby at a determined wavelength a switch occurs from being mainly reflective to mainly transmissive or vice versa. In practice, said reflection curve comprises a wavelength at which the switch between having a higher reflectivity to having a higher transmissivity occurs, and this wavelenght can further be identified by the 50%-point. The 50%-point is therefore also dependent on the AOI as a consequence of the optical path length differences between the reflective layers as explained above. In other words, there is a shift of the 50%-point depending on the AOI.

[0015] Due to the difference in AOI for light propagating in the direction of the DMDs and light returning from the DMDs, and due to the shift of the 50%-point, a spectral band with light in the wavelength band of the shift will see the coating as mainly reflective in the incoming state of the light. But, when the light returns from the DMD in the off-state, the light will be transmitted, or vice versa. The first occurrence happens for a first coating, where cyan light falls within said wavelength band around the 50% point. The second occurrence happens for a second coating whereby yellow light falls within said wavelength band around the 50%-point. This light is called off-state dichroic shift light, off-state DSL, which is light not directed according to a desired manner thereby causing unwanted effects. One of these unwanted effects is that said light goes to zones where mechanical items are glued to the prisms, for example to fix aDMD to the prism assembly. Due to the off-state DSL these glue zones heat up which may result in glue getting loose.

[0016] The glue zones are symmetric and need to be large enough to withstand external forces, such as vibrations, shocks, and transportation. Furthermore, the locations as well as the sizes of the glue zones are mechanically fixed, or less strict formulated may not be adapted that easy.

[0017] According to some embodiment, at least one dichroic reflector is divided into two zones, a first and a second zone, wherein a coating composition of the first zone differs from a coating composition of the second zone. Alternatively, each dichroic reflectors are divided into a respective first and respective second zone, and wherein a respective coating composition of the respective first zone differs from a respective coating composition of the second zone.

[0018] The first zone corresponds to a surface area with a first dichroic coating applied and the second zone corresponds to a surface area with a second dichroic coating applied . A spectral band of the illumination beam is directed to the first zone by the coating to the DMD and the beam coming from the DMD in the on state with the same spectral band (on-state beam) is directed by the same coating to the screen. Different from the first coating, the beam coming from the DMD in the off-state with the same spectral band (off-state beam) is reflected in the second zone by the second coating away from the screen.

[0019] The AOI of the on-state beam on the first coating applied to the first zone differs from the AOI of the off state beam on the second coating applied to the second zone Therefore, it has been observed that by dividing the dichroic reflectors in said two zones, the off-state DSL is minimized. The defining of zones solves or reduces the problem of having the unwanted effects as discussed above and as will be further explained.

[0020] Further, the coating composition of the first zone is adapted to the AOI of the on-state beam with a spectral band. The coating composition of the second zone is adapted to the AOI of the off-state beam with the same spectral band. The AOI of thecoating of the said second zone will be increased by a predefined angle based on a tilt angle of the respective DMD associated with said spectral band vs the AOI of the coating of the said first zone. Preferably, the predefined angle corresponds to double or twice said tilt angle.

[0021] Alternatively or additionally, the second zone further corresponds to a zone shielding redirected spectral bands in the off-state towards the glue areas.

[0022] According to a second aspect of the invention, a projector is disclosed comprising the prism assembly according to the first aspect of the present disclosure. The projector may, for example, be a DLP projector.Brief Description of the FiguresThe invention will be further illustrated with reference to the figures wherein,

[0023] Fig. 1 illustrates a side view of a Philips prism with incoming, on and off state optical axes;

[0024] Fig. 2 illustrates schematically the working principle of a Philips prism with two dichroic coatings;

[0025] Fig. 3A and 3B illustrate glue areas in the Philips prism;

[0026] Fig. 4 illustrates yellow off-state dichroic shift light hitting a top blue glue area;

[0027] Fig. 5 to Fig. 10 illustrate reflection curves of respective coatings; and

[0028] Fig. 11 and 12 illustrate the energy load on a coating of the state-of-the-art respectively a coating according to the invention.Detailedof Embodiments

[0029] The present invention will be described with respect to certain embodiments and with reference to certain figures, but the invention is not limited thereto and is defined only by the claims. The figures described are only schematic and non-limiting. In the figures, the size of certain elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and relative dimensions do not necessarily correspond to actual practical embodiments of the invention.

[0030] In addition, the terms first, second, third and the like are used in the specification and in the claims to distinguish between like elements and not necessarily to describe a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the invention may be used in sequences other than those described or illustrated herein.

[0031] Furthermore, the terms top, bottom, over, below and the like in the specification and claims are used for illustrative purposes and not necessarily to describe relative positions. The terms so used are interchangeable under appropriate circumstances, and the embodiments of the invention described herein may be used in orientations other than those described or illustrated herein.

[0032] Further, although referred to as "preferred embodiments", the various embodiments are to be construed as exemplary in which the invention may be practiced rather than as a limitation on the scope of the invention.

[0033] The term "comprising", used in the claims, should not be construed as being limited to the means or steps set forth below; the term does not exclude other elements or steps. The term should be interpreted as specifying the presence of the named features, elements, steps, or components referred to, but does not exclude thepresence or addition of one or more other features, elements, steps or components, or groups thereof. The scope of the expression "a device comprising means A and B" should therefore not be limited to devices consisting only of the components A and B. The meaning is that with respect to the present invention only the components A and B of the device are listed, and the claim is further to be interpreted as including equivalents of these components.

[0034] In three chip projection systems using micromirror array type imagers, a Philips prism is commonly used as colour splitting and recombining system. Fig. 2 schematically illustrates the working principle of such a Philips prism with two dichroic coatings. Fig. 1 likewise illustrates a side view 100 of such a Philips prism with incoming 103, on 102 and off 101 state optical axes.

[0035] In a Philips prism there is at least one dichroic coating surface reflecting a wavelength band and transmitting another wavelength band of an incoming light beam. Alternatively, the Philips prism has two dichroic coating surfaces reflecting a wavelength band and transmitting another wavelength band of an incoming light beam. For example, green is transmitting through two dichroic coatings to a green dedicated micromirror array, blue is transmits through one of the dichroic coatings and is reflected by another dichroic coating, and the red wavelength is reflected by a first dichroic coating.

[0036] In Fig. 2 this first dichroic coating 203 is illustrated, as well as the second dichroic coating 202.

[0037] With again reference to Fig. 2, when looking from a side of the prism 200, one can see the optical axis of the incoming beam 103 entering the prism 200 from the bottom and going towards the red 204, green 205, and blue 201 micromirror array. With reference to Fig. 1 , the on-state optical axis 102 escaping the prism horizontally, and the off-state beam 101 escaping under a different angle is illustrated. The on-state optical axis 102 is also illustrated in Fig. 2.

[0038] From the illustration in Fig. 1 , it can be noticed that the incident angles of the light on the coatings are very different for the incoming 103, the on-state 102, and off- state 101 beam. The angles can vary from 0° to more than 60°.

[0039] In some embodiments, the dichroic coatings and wide angular variations presents coating characteristics shift with changing angles. The average angle of the light beams is, for example, for incoming light 103 going to the micromirror array at an angle of 34.7° for blue and 39.9° for red, for the on-state light 102 coming from the micromirror array at an angle of 13° for blue and 19.4° for red, and for the off-state light 101 coming from the micromirror array at an angle of 56° for blue and 60° for red.

[0040] These characteristics are further illustrated by the reflection curves in the graphs of Fig. 5 to Fig. 10. More particularly, Fig. 5 illustrates the characteristics of the blue coating 202 for the incoming beam 103, Fig. 6 illustrates the characteristics of the blue coating 202 for the on-state beam 102, and Fig. 7 illustrates the characteristics of the blue coating 202 for the off-state beam 101. Next, Fig. 8 illustrates the characteristics of the red coating 203 for the incoming beam 103, Fig. 6 illustrates the characteristics of the red coating 203 for the on-state beam 102, and Fig. 7 illustrates the characteristics of the red coating 203 for the off-state beam 101.

[0041] The graphs illustrated in Fig. 5 to 10 depict reflection curves, which represent the intensity reflection on the Y-axis represented as a function of the wavelength in pm on the X-axis. Each figure showcases three distinct curves, namely the reflection curves for S-polarization 502, 602, 702, 801 , 901 , 1001 , P-polarization 501 , 601 , 701 , 802, 902, 1002, and an average polarization 503, 603, 703, 803, 903, 1003, respectively.

[0042] As can be observed in the reflection curves, there is a clear transition between reflectivity and transmissivity depending on the corresponding percentage. This transition is identified by the 50%-point that determines whether the surface mainly reflects or mainly transmits light. In other words, the reflection curve comprises a wavelength at which the switch between having a higher reflectivity to having a higher transmissivity occurs, and this wavelenght can further be identified by the 50%-point. The 50%-point is therefore also dependent on the AOI as a consequence of the opticalpath length differences between the reflective layers. In other words, the shift of the 50%-point may further depend on the AOL In an exemplary embodiment, the first coating 202 may be a blue coating and a second coating 203 may be red coating. Vice versa, in some other embodiments, the first coating 202 may be a red coating and a second coating 203 may be blue coating, depending on the sequence of colour lights for processing. In the case of the blue coating 202, there is a shift around the 50%- point, affecting light in the wavelength band of the shift, which corresponds to cyan light. Consequently, when the light enters the surface, it will perceive the coating as reflective in the incoming state, but after reflection by the micromirror array, when coming back in the off-state, it will be transmitted by the coating. For the red coating 203, the shift around the 50% point affects light within the wavelength range of yellow. As a result, the light will perceive the coating as transmissive in the incoming state. But when coming back from the micromirror array in the off-state, the light will be reflected by the coating. This light is called off-state dichroic shift light, off-state DSL, and is not escaping the prism 100 in a desired way and gives rise to unwanted effects.

[0043] One of these effects is that the light, which fails to escape in a controlled manner, reaches areas where mechanical components of the prism 100 are glued to fix the micromirror array devices to the prism assembly. If these glue zones where glue is applied experience excessive heat due to the off-state DSL, the glue may get loose, leading to instability of the assembly. In a Philips prism 100, there can be numerous glue zones, for example twenty-three in the current example, but possibly more. In Fig. 3A, another view 301 of the prism assembly is illustrated, with references 310- 315 representing the glue zones. Similarly, Fig. 3B provides another view 302 is given with references 305-309 representing the glue zones. References 303-305 pertain to the micromirror array devices.

[0044] To withstand external forces, such as vibrations, shocks, transportation, and heat from the light in harsh conditions, e.g. when the prism is used in a projector standing in a room with an ambient temperature of up to 40°C, the glue zones 305- 315 need to be kept symmetric and sufficiently large. Therefore, the respective locations of the glue areas are fixed and cannot be easily removed around and / or made smaller. Degradation and / or delamination effect of the glue may pose risk to the overall quality of the end-product.

[0045] These unwanted effects are further illustrated in Fig. 4. Fig. 4 illustrates yet another view 400 of the prism with light 404 passing through. At reference 401 , the blue top glue area is hit. While at reference 402, the yellow light 404 is reflected by the red coating 203. At reference 403 the yellow light 404 goes through the red coating 203. And, at reference 405, there is yellow light with wavelength in the range of the 50%-point, as already explained above. In other words, the off-state DSL that is hitting the green micromirror array passing the red 203 and blue 202 coating towards the micromirror array but coming back from the micromirror array in the off-state is not passing the red coating 203 anymore. This off-state DSL hit the blue glue area.

[0046] To address this issue, two distinct zones are established on each coating surface 202, 203 to accommodate the incoming 103 and on-state 102 beams separately from the off-state beam 101 . Each zone is coated with an optimized composition suited to achieve the desired optical properties. The zones enable the definition of the 50%-point for each incident angle and significantly reduces the presence of off-state DSL. The amount of off-state DSL in the prism 100 can be reduced by a factor of three to ten on the coating surfaces. Consequently, this leads to a more stable prism assembly and improves its cooling capabilities.

[0047] The aspect of the present disclosure is directed to partitioning the coating surfaces 202, 203 into two zones: a first zone and a second zone. The first zone comprises coatings optimized for the incoming 103 and on-state 102 light beams, while the second zone encompasses coatings optimized for the off-state light beam 100. This approach effectively minimizes the heat load on bonding points and, overall, reduces the occurrence of stray light within the prism 100.

[0048] The verification of the energy load of the 2-coating solution on one of the established prism geometries for a three chip SST DMD projector are presented in Figures 1 1 and 12. The detected energy load on the top blue glue point is displayed. In figure 1 1 , the DSL on the glue point is depicted using the conventional state-of-the- art approach, which involves using one coating per surface. On figure 12, the DSL is shown using the double and split coating solution per surface. The improvement on the energy load on the glue points goes from a factor three to a factor ten, which is anenormous step to support a good and long-lasting functionality of this prism with the DMDs as imaging devices connected to it, for instance with regard to the convergence stability and cooling features of the prism.

[0049] This solution offers several improvements that will enhance the image quality. Firstly, it will significantly enhance convergence stability, ensuring that the projected image remains aligned and sharp over time. This is a critical issue, particularly in high lumen projectors, and the present solution effectively addresses it.

[0050] Additionally, the present invention provides long-term stability for the image, ensuring consistent performance and preventing degradation over extended periods of use. This reliability of the prism assembly provides distinct advantage to the projector using said prism assembly.

[0051] Furthermore, by offering better control of temperature, the embodiments of the present disclosure reduce the need of specialized cooling techniques. Thus, the coating arrangement of these embodiments not only allow simplification of the product design but also results in a lower DCP (Development Cost Per Projector) for the projector.

[0052] In summary, the present disclosure not only improves image quality by enhancing convergence stability and long-term performance but also offers benefits such as reduced cooling requirements and lower development cost.SUBSTITUTE SHEET (RULE 26)

Claims

CLAIMS1.- A prism assembly (100) configured to project an image on a screen by way of prisms comprising dichroic reflectors (202, 203) coated on surfaces of the prisms for reflecting a spectral band and transmitting other spectral bands, and a set of digital micromirror devices, DMDs (201 , 204, 205), dedicated to each spectral band, the prism assembly being configured to:- split an incoming light beam (103) into a set of spectral bands;- direct each spectral band to a dedicated DMD (201 , 204, 205);- wherein each DMD (201 , 204, 205) is configured to reflect the corresponding spectral bands into an on-state (102) for reaching the screen and an off-state (101 ) away from the screen, whereby the spectral bands redirected into the on- state (102) are recombined to form the image on the screen; characterized in that at least one dichroic reflector (202, 203) is divided into a first and second zone wherein a coating composition of the first zone differs from a coating composition of the second zone.2.- The prism assembly (100) according to claim 1 , wherein each dichroic reflector (202, 203) is divided into a first zone and a second zone, and wherein the coating composition of the first zone differs from the coating composition of the second zone.3.- The prism assembly (100) according to any one of the preceding claims, wherein, for at least a spectral band of the set of spectral bands of the incoming light beam, the first zone corresponds to a surface area which is configured to receive the spectral band of the incoming light beam to direct the light beam towards the dedicated DMD, such that when generating the on state beam reflected by the DMD, the same surface area is configured to guide the on state beam to form the image (102) on the screen,SUBSTITUTE SHEET (RULE 26)and when generating the off state beam reflected by the DMD, the second zone corresponds to a surface area configured to guide the off state beam (101 ) away from the screen.4.- The prism assembly (100) according to claim 3, wherein the coating composition of the first zone is adapted to the first angle of incidence, AOI1 , of the on state beam with a spectral band directed to form the image (102), and wherein the coating composition of the second zone is adapted to a second AOI, AOI2, of the off state beam with the same spectral band directed to the off-state (101 ), wherein the AOI2 is the first AOI increased by a predefined angle based on a tilt angle of the respective DMD associated with said spectral band.5.- The prism assembly (100) according to claim 4, wherein the predefined angle corresponds to the double of the tilt angle.6.- The prism assembly (100) according to any one of the claims 3 to 5 further comprising a set of glue areas (305-315) for mechanically connecting the prisms and DMDs (201 , 204, 205), and wherein the second zone further corresponds to a zone shielding redirected spectral bands in the off-state (101 ) towards the glue areas (305- 315).7.- A prism assembly (100) according to claim 6, wherein a first dichroic reflector comprises a coating composition for reflecting the red band and letting through the green and the blue band.8.- A prism assembly (100) according to claim 7, wherein a second dichroic reflector comprises a coating composition for reflecting the blue band and letting through the green band.SUBSTITUTE SHEET (RULE 26)9.- A prism assembly (100) according to any one of the preceding claims, wherein the prism assembly is a Philips prism assembly.10.- A projector comprising a prism assembly (100) according to any of the preceding claims.1 1.- A projector according to claim 10, wherein the projector is a digital light processing, DLP, projector.SUBSTITUTE SHEET (RULE 26)

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