Prism assembly for digital light processing system

Abstract

In accordance with an embodiment, there is disclosed a prism assembly (100) to project an image through a prism, the prism assembly comprising dichroic reflectors (202, 203) plated on each surface to reflect a spectral band and transmit other spectral bands, and a set of dedicated digital micromirror devices (DMD) (201, 204, 205), the prism assembly being assembled to split an incident light beam (103) into a set of spectral bands, the at least one dichroic reflector (202, 203) is divided into first and second regions, each spectral band is directed to a respective DMD (201, 204, 205), the respective spectral band is redirected from each DMD (201, 204, 205) to an on-state (102) to reach a screen, respectively, and to an off-state (101) to be absorbed inside the assembly, whereby the spectral bands redirected to the on-state (102) are recombined to form an image, and wherein the at least one dichroic reflector (202, 203) is divided into first and second regions, wherein the coating film composition of the first area is different from the coating film composition of the second area.

Description

Technical Field

[0001] This invention relates to the field of digital light processing systems, and more particularly to a prism assembly for use in such digital light processing systems. Background Technology

[0002] Digital light processing (DLP) is a chipset based on optical microelectromechanical systems (MEMS) technology, using digital micromirror devices (DMDs). DMDs include small mirrors under a microscope, used to modulate an incident light beam into a conducting state by deflecting it toward the screen, or to modulate it into a cutoff state by deflecting it away from the screen.

[0003] DLP technology is used in digital projection and imaging. For projecting color images, a prism assembly splits the light beam into individual primary colors, which are then transmitted to their dedicated DMD chip. The different colors are then recombine and output through a lens by redirecting the incident light to either an on or off state. Therefore, to obtain optimal light output and saturated colors, it is recommended to use three DMDs: one for red, one for green, and one for blue.

[0004] Therefore, the prism assembly is used to split an incident light beam, such as from a lamp source, into a set of spectral bands, and further guide each spectral band to a corresponding DMD. Finally, by using the DMDs corresponding to each spectral band, the redirected spectral bands in the conducting state are combined to form an image and projected onto a screen. The spectral bands redirected to the cutoff state are absorbed in the system housing the prism assembly.

[0005] In EP1632805B1, a prism assembly is disclosed configured for use in a DLP projector. The prism assembly comprises six prisms configured to redirect an incident beam and further form an outgoing beam as described above. For this purpose, the surfaces of the dedicated prisms are coated with corresponding dichroic reflectors, each dichroic reflector adapted to a corresponding spectral band.

[0006] Another prism assembly for DLP projectors is also disclosed in EP1632804A1.

[0007] A drawback of known prism assemblies in the art is that beams redirected to the cutoff state and absorbed within the system can lead to undesirable effects such as stray light and heat dissipation. Therefore, special cooling techniques are required to avoid overheating and thermal stress.

[0008] Therefore, the object of the present invention is to mitigate the above-mentioned disadvantages and to provide an improved prism assembly configured for projecting images onto a screen by a DLP projector. Summary of the Invention

[0009] In a first aspect, the objective is achieved by a prism assembly according to the first claim, the prism assembly being configured to project an image onto a screen via a prism, the prism assembly comprising dichroic reflectors coated on various surfaces of the prism for reflecting one spectral band and transmitting other spectral bands, and a set of dedicated digital micromirror devices (DMDs), the prism assembly being assembled as follows:

[0010] - Split the incident beam into a set of spectral bands;

[0011] - Orient each spectral band to the corresponding DMD;

[0012] - The corresponding spectral bands from each DMD are redirected to the on state to reach the screen, and redirected to the off state to move away from the screen, so that the spectral bands redirected to the on state are recombined to form an image on the screen;

[0013] The feature is that at least one dichroic reflector is divided into a first region and a second region, wherein the coating composition of the first region is different from that of the second region.

[0014] The prism assembly includes multiple prisms, such as a pentagonal prism assembly or a hexagram prism assembly when the assembly is a Philips prism assembly. Therefore, it should be noted that, in view of the present invention, the number of prisms present in the assembly is not limiting.

[0015] The prism assembly is configured such that it splits, directs, and repositions the incident light beam into different spectral bands. These spectral bands are further recombine to form an image for projection onto a screen. Therefore, one or more prisms include dichroic reflectors coated on their respective surfaces. The dichroic reflectors are configured to reflect a specific color and transmit other colors. These colors correspond to the different spectral bands, preferably including a red spectral band, a green spectral band, and a blue spectral band. In a preferred embodiment, a first dichroic reflector includes a coating composition for reflecting the red spectral band and transmitting the green and blue spectral bands. A second dichroic reflector then includes a coating composition for reflecting the blue spectral band and transmitting the green spectral band.

[0016] Therefore, the prism and the dichroic reflectors coated on its surfaces are assembled in such a way that the incident beam is split into a set of spectral bands. Each spectral band is then directed to a dedicated DMD. The dedicated DMD is sequentially configured to redirect its respective spectral band to either an on or off state. As previously described, the redirected spectral bands in the on state are recombined to form an image projected onto the screen. The redirected spectral bands in the off state can be absorbed by the system.

[0017] Furthermore, dichroic reflectors comprise multiple layers of optical materials with varying refractive indices and thicknesses deposited on a glass substrate. The internal reflections between these refractive index boundaries determine the characteristics of the dichroic reflector—specifically, which wavelengths are primarily transmitted and which are primarily reflected. Further, when light passes through the reflector at a given angle of incidence (AOI), a portion of it is reflected by the individual thin layers within the cavity. Due to the optical path difference between the reflective layers, the reflected light interferes with the incident light, resulting in only specific wavelengths being primarily reflected, while the remaining wavelengths are primarily transmitted. Since the precise optical path difference depends on the angle of incidence (AOI), the characteristics of a dichroic reflector are also angle-dependent.

[0018] As will be discussed further, these characteristics can be illustrated by a reflection curve, where a transition from predominant reflection to predominant transmission occurs at a specific wavelength, and vice versa. In fact, this reflection curve includes a wavelength at which the transition between higher reflectivity and higher transmittance occurs, and this wavelength can be further determined by the 50% point. Therefore, as mentioned earlier, the 50% point also depends on the angle of incidence (AOI) due to the optical path difference between the reflective layers. In other words, the 50% point shifts with changes in the angle of incidence.

[0019] Because of the difference in the angle of incidence between the light incident on and returning from the DMD, and due to the 50% point offset, spectral bands with light falling within the offset wavelength band will be primarily reflected by the coating in the incident state. However, when this light returns from the DMD in a cutoff state, it will be transmitted, and vice versa. The first case occurs on a first coating, where cyan light falls within the wavelength band near the 50% point. The second case occurs on a second coating, where yellow light falls within the wavelength band near the 50% point. This light, referred to as cutoff state dichroic offset light, or cutoff state DSL, is light that is not oriented as intended, thus causing adverse effects. One such adverse effect is that the light can illuminate areas where mechanical components are bonded to the prism, such as the bonding area that secures the DMD to the prism assembly. Due to the cutoff state DSL, these bonded areas will heat up, potentially causing the adhesive to loosen.

[0020] The glued area is symmetrical and needs to be large enough to withstand external forces such as vibration, impact, and transportation. Furthermore, the position and size of the glued area are mechanically fixed, or, more precisely, difficult to adjust easily.

[0021] According to some embodiments, at least one dichroic reflector is divided into two regions, namely a first region and a second region, wherein the coating composition of the first region is different from that of the second region. Alternatively, each dichroic reflector is divided into its own first region and its own second region, wherein the coating composition of each first region is different from that of each second region.

[0022] The first region corresponds to the surface region coated with a first dichroic coating, and the second region corresponds to the surface region coated with a second dichroic coating. A spectral band of the illumination beam is directed to the coating in the first region and then directed towards the DMD. A beam from the DMD in the conducting state (the conducting beam) with the same spectral band is directed to the screen by the same coating. Unlike the first coating, a beam from the DMD in the cutoff state (the cutoff beam) with the same spectral band is reflected away from the screen by the second coating in the second region.

[0023] The incident angle of the conducting beam applied to the first coating in the first region is different from the incident angle of the cutoff beam applied to the second coating in the second region. Therefore, it can be observed that by dividing the dichroic reflector into the two regions, the cutoff state DSL can be minimized. This division of regions resolves or reduces the adverse effects discussed above and will be explained further.

[0024] Furthermore, the coating composition of the first region is adapted to the incident angle of a beam in a conducting state having a certain spectral band. The coating composition of the second region is adapted to the incident angle of a beam in a cutoff state having the same spectral band. The incident angle of the coating in the second region, relative to the incident angle of the coating in the first region, is increased by a predetermined angle determined based on the tilt angle of the corresponding DMD associated with the spectral band. Preferably, this predetermined angle corresponds to twice the tilt angle.

[0025] Alternatively or additionally, the second region also corresponds to a region for shielding the redirected spectral bands that are in a cut-off state and facing the glued region.

[0026] According to a second aspect of the present invention, a projector is disclosed that includes the prism assembly described in the first aspect of the present disclosure. The projector may, for example, be a DLP projector. Attached Figure Description

[0027] The invention will be further illustrated with reference to the accompanying drawings, wherein,

[0028] Figure 1 A side view of a Philips prism with optical axes in incident, conducting, and cut-off states is shown.

[0029] Figure 2 The working principle of a Philips prism with two dichroic coatings is illustrated schematically.

[0030] Figure 3A and 3B The glued area in the Philips prism is shown;

[0031] Figure 4This shows the dichroic shifted light in the yellow light cutoff state incident on the upper blue light bonding area;

[0032] Figures 5 to 10 The reflection curves of each coating are shown; and

[0033] Figure 11 and Figure 12 The energy load on the most advanced coating according to the present invention is shown respectively. Detailed Implementation

[0034] This invention will be described in conjunction with some embodiments and accompanying drawings, but the invention is not limited thereto, only defined by the claims. The described drawings are merely illustrative and non-limiting. In the drawings, for illustrative purposes, the size of some elements may be exaggerated and not drawn to scale. Dimensions and relative dimensions do not necessarily correspond to actual embodiments of the invention.

[0035] Furthermore, the terms "first, second, third, etc." are used in the specification and claims to distinguish similar elements without necessarily describing the order or chronological order. Where appropriate, the terms are interchangeable, and embodiments of the invention may be used in an order different from that described or illustrated herein.

[0036] Furthermore, the terms top, bottom, above, below, etc., used in the specification and claims are for illustrative purposes and do not necessarily describe relative positions. Where appropriate, the terminology used is interchangeable, and the embodiments of the invention described herein may be used in orientations different from those described or illustrated herein.

[0037] Furthermore, although referred to as “preferred embodiments,” the various embodiments are to be understood as exemplary embodiments through which the invention can be practiced, rather than as limitations on the scope of the invention.

[0038] The term "comprising" as used in the claims should not be construed as limited to the means or steps described below; the term does not exclude other elements or steps. The term should be interpreted as indicating the presence of a said feature, element, step, or component, but does not exclude the presence or addition of one or more other features, elements, steps, components, or combinations thereof. Therefore, the scope of the statement "a device comprising A and B" should not be limited to a device that only includes components A and B. It means that, relative to the invention, only components A and B of the device are listed, and the claims should be further interpreted to include equivalent components of these components.

[0039] In three-chip projection systems that use micromirror array imagers, Philips prisms are typically used as the color separation and reconstruction system. Figure 2 The working principle of this Philips prism with two dichroic coatings is illustrated schematically. Figure 1A side view of the Philips prism 100 with optical axes in the incident, conduction, and cutoff states 103, 102, and 101 states is also shown.

[0040] In a Philips prism, at least one dichroic coating surface reflects one wavelength band of the incident light beam and transmits another wavelength band. Alternatively, the Philips prism has two dichroic coating surfaces that reflect one wavelength band of the incident light beam and transmit another wavelength band. For example, green light is transmitted to a dedicated green light micromirror array through two dichroic coatings, blue light is transmitted through one dichroic coating and reflected by the other, and red light is reflected by the first dichroic coating.

[0041] exist Figure 2 The image shows a first dichroic coating 203 and a second dichroic coating 202.

[0042] Combined again Figure 2 When viewed from one side of prism 200, the optical axis of the incident beam 103, which enters prism 200 from the bottom and faces the red micromirror array 204, the green micromirror array 205, and the blue micromirror array 201, can be seen. Combined with... Figure 1 This illustrates the optical axis 102 escaping the prism horizontally in the on state, and the beam 101 escaping at a different angle in the off state. The optical axis 102 in the on state also... Figure 2 As shown in the image.

[0043] from Figure 1 As can be seen from the diagram, the incident angles of the incident beams illuminating the coating are very different for the incident beam 103, the beam in the on state 102, and the beam in the off state 101. The angles can vary from 0° to over 60°.

[0044] In some embodiments, the dichroic coating and wide-angle variation exhibit coating characteristics that change with angle. The average angle of the beam is, for example, 34.7° for the incident beam 103 directed towards the micromirror array and 39.9° for the red light; 13° for the on-state beam 102 from the micromirror array and 19.4° for the red light; and 56° for the off-state beam 101 from the micromirror array and 60° for the red light.

[0045] These characteristics are achieved through Figures 5 to 10 The reflection curve in the image further illustrates this. More specifically, Figure 5 The characteristics of the blue light coating 202 for the incident light beam 103 are shown. Figure 6 The characteristics of the blue light coating 202 for the conductive beam 102 are shown. Figure 7 The characteristics of the blue light coating 202 for the cutoff beam 101 are shown. Next, Figure 8The characteristics of the red light coating 203 for the incident light beam 103 are shown. Figure 6 The characteristics of the red light coating 203 for the conductive beam 102 are shown. Figure 7 The characteristics of the red light coating 203 for the cut-off beam 101 are shown.

[0046] Figures 5 to 10 The graphs depict reflection curves, with the Y-axis representing intensity reflection, which is expressed as a function of wavelength on the X-axis, with wavelength measured in μm. Each graph shows three different curves: the reflection curves for S-polarization (502, 602, 702, 801, 901, 1001), P-polarization (501, 601, 701, 802, 902, 1002), and the reflection curves for average polarization (503, 603, 703, 803, 903, 1003).

[0047] As can be seen in the reflectance curve, there is a clear transition between reflectance and transmittance, depending on the corresponding percentage. This transition is identified by the 50% point at which the surface primarily reflects or transmits light. In other words, the reflectance curve includes a wavelength at which the transition from higher reflectance to higher transmittance occurs, and this wavelength can be further determined by the 50% point. Therefore, the 50% point also depends on the angle of incidence (AOI), due to the optical path difference between the reflective layers. In other words, the offset of the 50% point may further depend on the angle of incidence. In one exemplary embodiment, the first coating 202 may be a blue light coating, and the second coating 203 may be a red light coating. Conversely, in some other embodiments, depending on the order of color light processing, the first coating 202 may be a red light coating, and the second coating 203 may be a blue light coating. For the blue light coating 202, there is an offset near its 50% point, affecting light within the offset wavelength band, which corresponds to cyan light. Therefore, when light enters the surface, the coating is considered reflective in the incident state, but after being reflected by the micromirror array, it will be transmitted through the coating when returning in the cutoff state. For the red light coating 203, the offset near its 50% point affects light in the yellow light wavelength range. Therefore, in the incident state, the light will perceive the coating as transmissive. However, when returning from the micromirror array in the cutoff state, the light will be reflected by the coating. This light is called cutoff-state dichroic shift light, or simply cutoff-state DSL (Dichroic Shift Light), which will not escape from prism 100 in the intended manner and will cause adverse effects.

[0048] One of these effects is that light rays that cannot escape in a controlled manner can reach the mechanical parts of prism 100 used to bond and secure the micromirror array devices to the prism assembly. If these bonded areas coated with adhesive are subjected to excessive heat due to the cutoff state DSL, the adhesive may loosen, leading to assembly instability. In the Philips prism 100, there may be multiple bonded areas, for example, twenty-three in this example, but there may be more. Figure 3A Another view 301 of the prism assembly is shown, where markings 310-315 indicate the glued areas. Similarly, Figure 3B Another view 302 is provided, in which labels 305-309 indicate the glued areas. Labels 303-305 relate to the micromirror array device.

[0049] To withstand external forces such as vibration, impact, and transportation, as well as heat from light under harsh conditions, such as when the prism is used in a projector in a room with an ambient temperature as high as 40°C, the adhesive areas 305-315 need to remain symmetrical and sufficiently large. Therefore, the positions of each adhesive area are fixed and cannot be easily removed or reduced. Degradation and / or delamination of the adhesive can pose a risk to the overall quality of the final product.

[0050] Figure 4 This further illustrates these adverse effects. Figure 4 Another view 400 shows light 404 passing through the prism. At mark 401, the upper blue-coated region is illuminated. At mark 402, yellow light 404 is reflected by the red-coated layer 203. At mark 403, yellow light 404 passes through the red-coated layer 203. And at mark 405, as previously explained, there is yellow light with wavelengths within the 50% point range. In other words, the cutoff state DSL illuminating the green micromirror array passes through both the red-coated layer 203 and the blue-coated layer 202 when it is directed toward the micromirror array, but no longer passes through the red-coated layer 203 when returning from the micromirror array in the cutoff state. This cutoff state DSL illuminates the blue-coated region.

[0051] To address this issue, two distinct regions are formed on each coated surface 202, 203 to accommodate the incident beam 103 and the on-state beam 102, separating them from the off-state beam 101. Each region is coated with an optimized composition to achieve the desired optical properties. These regions define 50% points for each incident angle and significantly reduce the presence of the off-state DSL. The amount of off-state DSL in the prism 100 can be reduced by three to ten times on the coated surface. This makes the prism assembly more stable and improves its cooling capacity.

[0052] One aspect of this disclosure involves dividing the coated surfaces 202, 203 into two regions: a first region and a second region. The first region includes a coating optimized for the incident beam 103 and the on-state beam 102, while the second region includes a coating optimized for the off-state beam 101. This approach effectively minimizes the thermal load at the bonding points and reduces stray light generation within the prism 100 overall.

[0053] Figure 11 and Figure 12 Energy load verification of a dual-coating solution was demonstrated, applied to a prism geometry for a three-chip SST DMD projector. The energy load detected at the upper blue light bonding point is shown. Figure 11 The image depicts a DSL on adhesive points using conventional existing techniques, involving the use of a single coating on each surface. Figure 12 The image showcases a DSL with a dual-zone coating solution on each surface. Energy load at the bonding points is improved by three to ten times, representing a significant advancement that enables the prism to perform well and sustainably with the DMD as an imaging device, for example, regarding the prism's convergence stability and cooling characteristics.

[0054] This solution offers several improvements that enhance image quality. First, it significantly improves convergence stability, ensuring that the projected image remains aligned and sharp over time. This is a critical issue, especially in high-lumen projectors, and this solution effectively addresses it.

[0055] Furthermore, this invention provides long-term image stability, ensuring consistent performance and preventing degradation during long-term use. The reliability of the prism assembly offers a significant advantage to projectors using this assembly.

[0056] Furthermore, by providing better temperature control, the embodiments of this disclosure reduce the need for dedicated cooling technologies. Therefore, the coating arrangements of these embodiments not only allow for simplified product design but also reduce the DCP (Development Cost Per Projector) of the projector.

[0057] In summary, this disclosure not only improves image quality by enhancing convergence stability and long-term performance, but also provides benefits such as reduced cooling requirements and development costs.

Claims

1. A prism assembly (100) configured to project an image onto a screen via a prism, the prism assembly comprising dichroic reflectors (202, 203) coated on the surface of the prism to reflect one spectral band and transmit 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 the incident beam (103) into a set of spectral bands; - Orient each spectral band to a dedicated DMD (201, 204, 205); -in, Each DMD (201, 204, 205) is configured to reflect a corresponding spectral band to the on state (102) to reach the screen, and to reflect to the off state (101) to move away from the screen, so that the spectral bands redirected to the on state (102) are recombined to form an image on the screen; The feature is that at least one dichroic reflector (202, 203) is divided into a first region and a second region, wherein the coating composition of the first region is different from that of the second region.

2. The prism assembly (100) according to claim 1, characterized in that, Each of the dichroic reflectors (202, 203) is divided into a first region and a second region, wherein the coating composition of the first region is different from that of the second region.

3. The prism assembly (100) according to any one of the preceding claims, characterized in that, For at least one of the spectral bands in a set of spectral bands of the incident beam, the first region corresponds to a surface region configured to receive the incident beam to direct the beam to the dedicated DMD, such that when a conduction beam reflected by the DMD is generated, the same surface region is configured to guide the conduction beam to form an image on the screen (102). When a cutoff beam reflected by the DMD is generated, the second region corresponds to a surface region configured to guide the cutoff beam (101) away from the screen.

4. The prism assembly (100) according to claim 3, characterized in that, The coating composition of the first region is adapted to a first incident angle AOI1 of the on-state beam having a spectral band oriented to form the image (102), and wherein the coating composition of the second region is adapted to a second incident angle AOI2 of the off-state beam having the same spectral band oriented to the off-state (101), wherein the second incident angle AOI2 is the first incident angle AOI1 plus a predetermined angle determined based on the tilt angle of the corresponding DMD associated with the spectral band.

5. The prism assembly (100) according to claim 4, characterized in that, The predetermined angle corresponds to twice the tilt angle.

6. The prism assembly (100) according to any one of claims 3 to 5, characterized in that, It also includes a set of glued regions (305-315) for mechanically connecting the prism and the DMD (201, 204, 205), wherein the second region also corresponds to a region that shields a redirected spectral band, which is in a cut-off state (101) and faces the glued region (305-315).

7. The prism assembly (100) according to claim 6, characterized in that, The first dichroic reflector consists of a coating that reflects the red spectral band and transmits the green and blue spectral bands.

8. The prism assembly (100) according to claim 7, characterized in that, The second dichroic reflector comprises a coating for reflecting the blue spectral band and transmitting the green spectral band.

9. The prism assembly (100) according to any one of the preceding claims, characterized in that, The prism assembly is a Philips prism assembly.

10. A projector, characterized in that, Includes the prism assembly (100) as described in any of the preceding claims.

11. The projector according to claim 10, characterized in that, The projector is a digital light processing (DLP) projector.

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