WO2006054253A1

WO2006054253A1 - Color drum filter for projection system including light-absorbing baffles

Info

Publication number: WO2006054253A1
Application number: PCT/IB2005/053788
Authority: WO
Inventors: Aldegonda Lucia Weijers; Martijn Riemeijer
Original assignee: Koninklijke Philips Electronics, N.V.; U.S. Philips Corporation
Priority date: 2004-11-19
Filing date: 2005-11-16
Publication date: 2006-05-26

Abstract

A light-valve projection system (100) includes a source of light (101), which includes a plurality of wavelength components. The light- valve projection system also includes a color- drum (107, 200), which includes a plurality of color filters. The color drum also includes at least one baffle, and the baffle substantially absorbs light over a visible spectrum of wavelengths.

Description

US040457

COLOR DRUM FILTER FOR PROJECTION SYSTEM INCLUDING LIGHT-AB S ORB ING

BAFFLES

Color display projectors or projection televisions may be designed as three -primary color systems such as an RGB system (for example, red, green and blue), or as multi-primary color systems having more than three primary colors, such as an RGBYC system (red, green, blue, yellow and cyan). In such color projection systems, the image carrying panel is illuminated or scrolled by the different colors sequentially in time. The scrolling colors can be generated by projecting white light from a source through a color wheel, a rotating prism or a color drum.

As is known, the light source in a color projection system is a white light source comprising light components having wavelengths across the visible spectrum. For example, the light source may be an ultra-high pressure (UHP) gas light, which emits white light. Discontinuities between the color filters of the drum can result in unfiltered light from the light source being transmitted to the light- valve. Furthermore, the optical elements that are used to provide light from the light source to the color drum, as well as the color filters themselves may cause stray light (e.g., scattered light) to be incident on the light-valve and be projected onto the imaging surface (screen) of the display. Among other drawbacks, this stray light may result in picture elements (pixels) at the light valve being illuminated by the wrong color, or the stray light's being imaged onto the screen at an improper location, or both. These and other affects can degrade the image quality. Moreover, as the number of pixels increases for increased resolution, the affects of stray light can be more pronounced.

What is needed therefore is an apparatus that overcomes at least the shortcomings of the known color projection systems.

In accordance with an example embodiment, a color drum includes a plurality of color filters and at least one baffle. The baffle substantially absorbs light over a visible spectrum of wavelengths.

In accordance with another example embodiment, a light- valve projection system includes a source of light, which includes a plurality of wavelength components. The light- US040457

valve projection system also includes a color-drum, which includes a plurality of color filters. The color drum also includes at least one baffle, and the baffle substantially absorbs light over a visible spectrum of wavelengths.

The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale, and in fact may be arbitrarily increased or decreased for clarity of discussion.

Fig. 1 is a perspective view of a color projection system in accordance with an example embodiment. Figs. 2a-2c are cross-sectional views of a color drum having color filters, shields and baffles in accordance with an example embodiment.

Fig. 3 is a perspective view of a color drum in accordance with an example embodiment.

In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of the example embodiments. However, upon review of this application, other embodiments that do not depart from the concepts of the described example embodiments will be apparent to one having ordinary skill. Moreover, descriptions of well-known devices, elements, methods and systems may be omitted so as to not obscure the description of the present invention. Nonetheless, such devices, methods, systems and protocols that are within the purview of one of ordinary skill in the art may be used in accordance with the example embodiments. Finally, wherever practical, like reference numerals refer to like features.

Fig. 1 is a perspective view of a color projection system 100 in accordance with an example embodiment. It is emphasized that many of the components of the system 100 are known to one skilled in the art. These known components may not be described in significant detail, so as to avoid obscuring the description of the illustrative embodiments.

The system 100 includes a light source 101, which is a source of white light. The light source may be a UHP gas lamp or similar device. The light source 101 includes a parabolic reflector such as shown. The light emitted by the source 101 traverses a polarization conversion system (PCS) 102, which transforms light incident thereon from the light source US040457

101 from its unpolarized state to a substantially polarized state. Illustratively, the light emerges from the PCS 102 into a substantially linearly polarized state (e.g., S or P polarized light).

The light that is emitted from the PCS 102 is incident on a condenser lens 102, and traverses other known lens elements 103, 104, 105. Ultimately, the light is coupled from a lens 106 to the color drum 107. Illustratively, the color drum 107 is rotating about an axis through a rod lens 108, which may be an element of the color drum 107. Light incident on the color drum 107 is selectively transmitted by the color drum 107, through optical elements 109, 110 and 111, and is incident on a polarization beam splitter (PBS) 112. The PBS 112 separates the light into its orthogonal polarization components, and transmits light of one polarization state to a light valve 113. In an example embodiment, light-valve 113 may be a liquid crystal (LC) panel, such as a liquid crystal on silicon (LCOS) panel. In alternative embodiments, other types of microdisplay devices may be used, such as a digital light processing (DLP™) panel. In operation, white light is emitted from the light source 101 and is incident on the color drum 107. The color drum rotates and provides a sequential scrolling of the colors of the color filters onto the light valve 112. Ultimately, and in ways well-known to those skilled in the art, a color image is provided at a display surface (not shown) through the scrolling of light across the light valve. In another example embodiment, rather than being disposed at an image location, the color drum 107 may be disposed in a pupil location. In such an embodiment, the color drum 107 is adapted for use in a color sequential mode, but without scrolling.

As described in further detail herein, in an example embodiment the color drum 107 includes a plurality of color filters, namely twice as many as the desired number of primary colors desirably projected onto the light valve 112. For example, if the desired colors to be scrolled onto the light valve 112 and thus the display surface (not shown) are R, G, B there are six color filters, two for each primary color. Likewise, if a greater degree of color quality is desired and the system includes five colors (e.g., R, G, B, C, Y) that are scrolled, there are 10 color filter; again two for each color.

White light from the source 101 may be outside the optimal cone of light, and/or may US040457

be incident on imperfections or aberrations in the optical elements 102, 104, 105 and 106. Moreover, and as described more fully herein, light from the source 101 that is incident on the drum 107 may not be directly transmitted out of the distal side of the drum, but rather scattered within the drum before being transmitted out the distal side. Finally, reflections can occur at many surfaces of the optical elements of the system. These and other effects can result in stray light in the system 100.

The stray light may be incident on a color filter of the color drum 107 at a relatively high angle of incidence, and thus outside the tolerance angle of the color filter upon which the light is incident. As a result, the white light may be filtered but the light emerging from the color filter may not be of the intended color. Naturally, this may result in poor image quality. Moreover, light that enters the color drum 107 may be reflected from the interior surfaces of the drum. After the reflections within the drum, the light may traverse the wrong color filter, or maybe incident at a relatively high angle of incidence on the filter or on the pixel of the light- valve, or both. If the stray light is incident on the wrong color filter, the light may be transmitted on a pixel of the light- valve, and thus provide the wrong color to the pixel. If the stray light is incident at a high angle of incidence, the wrong color may be emitted. Also, further reflections of light within the drum may occur, further exacerbating the problem.

Finally, it is noted that because of one or more affects described above, stray light may be incident on the light- valve 112 at a relatively high angle of incidence. If this light is incident at a high angle of incidence on the light-valve, the light may be transmitted incorrectly onto the screen via the projection optics (not shown) of the display system 100. Beneficially, the example embodiments significantly reduce, if not substantially eliminate the affects of stray light.

Fig. 2a is a cross- sectional view of a color drum 200 in accordance with an example embodiment. The color drum 200 is substantially a ten-sided structure. The color drum 200 includes a rod lens 211 disposed substantially concentrically about the center of the drum 200. In the present example embodiment, the drum 200 is useful for a five color system. Of course, this is merely illustrative, and it is contemplated that more or fewer colors may be transmitted by the color drum in a scrolling projection application. To wit, the color drum 200 may include any number of different color segments; such as three color segments for use in a US040457

three-color (RGB) projection system, four color segments for use in a four-color projection system, or six color segments for use in a six color projection system.

The color drum 200 includes a plurality of color filters 201-210. In an example embodiment, the color filters are chosen and arranged for a five-color projection system (RGBYC) with a color filter for each color being disposed substantially directly opposite to a substantially identical color filter. For example, color filter 204 may be a red color filter and is disposed directly opposite color filter 209, which is substantially identical to color filter 204. It is noted that in order to accommodate deficiencies in one or more wavelength bands (colors) in the light from the light source, the relative size of the corresponding color filters may be made comparatively larger. Similarly, in order to accommodate for a relative abundance of light of one or more wavelength bands (colors), the corresponding color filters may be comparatively smaller.

It is noted that the arrangement of the color drum described in connection with Fig. 2a is merely illustrative. For example, it may be useful to have a transmissive element that is not a color filter disposed substantially opposite at least one, if not each of the color filters, which substantially transmit one of the chosen primary colors. For example, a glass element may be substituted for red color filter 209. Futhermore, the transparent elements may have an anti- reflective (AR) property (e.g., via an AR coating) to reduce reflections. In another example embodiment, rather than providing substantially identical color filters in directly opposite positions, one or more of color filters disposed directly opposite to a color filter may have wavelength transmission characteristics that are spectrally shifted relative to the characteristics of its counterpart. For example, color filter 209 may have transmission characteristics (and thus, reflection or absorption characteristics) that are shifted spectrally relative to those of color filter 204. In yet another example embodiment, the out-of-band blocking characteristics of one or more color filters disposed directly opposite may be different from its counterpart. For example, the color filter 209 may have out-of-band blocking characteristics that are different from those of color filter 204. These characteristics may make such a filter more favorable for various reasons. Of course, the above variations are merely illustrative and other variations will be US040457

readily apparent to one of ordinary skill in the art having had the benefit of the present disclosure.

In the present example embodiment, the color filters and transmissive or AR elements (if applicable) 201-210 are individual elements. Thus, there may be gaps (e.g., gaps 226 shown in Fig. 2c) that may allow light to pass through. The light that passes through these gaps may not traverse a color filter. For example, light may traverse the lens 105 and be incident on a gap between two color filters (e.g., filters 203 and 204) of the drum 200 and enter the drum as unfiltered light, which is substantially white light. On the distal side of the drum 200, this light may be incident on another gap, and emerge from the color drum 200 substantially unfiltered. As such, light from the light source 101 may be transmitted to the light- valve unfiltered, and ultimately may have a deleterious impact on the quality of the image projected onto the screen.

In accordance with an example embodiment, in order to substantially prevent the transmission of unfiltered light through the gaps, shields 211 are disposed between one or more of the filters 201-210. It is noted that the shields 211 may be disposed in the gaps, in front on the gaps (i.e., outside the color drum 200), or behind the gaps (i.e., inside the color drum). Shields 211 in front of and behind the gap 226 are shown between filter 202 and filter 203. In an example embodiment, the shields 211 comprise a material that is optically absorptive over the visible spectrum. Illustratively, a black or red silicon glue material may be used. Alternatively, black aluminum stripes may be used. The shields 211 prevent the transmission of unfiltered light from the light source to the light- valve.

In the example embodiments described thus far in connection with Figs. 1 and 2, color filters/ AR elements 201-210 are substantially flat. Alternatively, the color filters, or AR elements, or both may be curved, providing a substantially cylindrical color drum. While the gaps between the color filters and AR elements may be reduced in such an embodiment, they will likely remain. As such, the shields 211 may be disposed to fill these gaps in order to reduce the stray light through the gaps.

Of course, the shapes of the color drum 200 may be other than those described thus far. To wit, the color drum may be of a variety of other regular and irregular shapes and number of sides depending on the number of primary colors being transmitted to the light valve. US040457

Moreover, a variety of color filters may be used. In example embodiments, the color filters are dichroic filters or multi-layer dielectric stack filters. Alternatively, at least one of the color filters may be another type of filter that transmits certain wavelengths and reflects undesired wavelengths. In yet another example embodiment, the color filters substantially absorb the light of all wavelengths except the wavelength(s) or wavelength band desirably transmitted. It is useful to include a cooling feature (e.g., a fan) to mitigate the thermal effects associated with absorptive filters.

As referenced previously, light from the light source 101 may be transmitted via the lens 105 to the color drum 200 and be reflected from the distal end of the color drum, undergo further reflections before being transmitted to the light valve. For example, the light may be incident on the color drum 200 at a relatively high angle of incidence. As stated, filters 201- 210 may be dichroic filters or multi-layer filters. As is well known, dichroic filters or multi¬ layer dielectric stack filters transmit light of a particular wavelength or wavelength range that is incident at nearly normal incidence (approximately a 0° angle of incidence) to the surface of the filter. These filters normally have an angular tolerance so that light of the wavelength/wavelength range that is incident within the tolerance is transmitted. However, if the light over the particular wavelength range is incident at an angle of incidence outside the angular tolerance of the color filter, it is substantially reflected rather than substantially transmitted. Moreover, if light from the source is incident on, for example, a blue color filter at a high angle of incidence, the blue filter may (undesirably) reflect the blue light and (again undesirably) transmit light of other wavelengths (colors).

As can be appreciated, from the affects described above, as well as other affects, such as reflections within the color drum 200, some of the light that is incident on the distal portion of the drum may be reflected back into the drum, where it undergoes a number of further reflections and then is transmitted through the distal side. Moreover, the light of the wrong color may be transmitted through the distal side of the drum. In either event, light of the wrong color may be incident on the pixels of the light- valve. This will ultimately impact the quality of the image provided at the screen.

In accordance with an example embodiment, the stray light that is reflected within the within the drum is significantly reduced, if not substantially eliminated, by a plurality of US040457

baffles 212 that are disposed within the drum. These baffles, which remain substantially stationary along with a rod lens 213 when the drum rotates, are substantially absorbing of all light across the visible spectrum. Stray light that is transmitted to the drum 200, but is reflected within the drum as it rotates is incident on one of the baffles and is absorbed. As such, the stray light that would otherwise be transmitted to the light valve and then to the screen is substantially absorbed by the baffles.

In operation, white light 214 from a light source (e.g., light source 101) is incident on the rotating color drum 200. The white light 214 traverses the color filters on the near side of the color drum, the rod lens 213, and the color filters on the distal end of the color drum as the drum rotates. The primary colors are transmitted to the light-valve and then to the screen, via the filtering using the color filters of each respective color. The rotation of the drum effects a scrolling of the colors. However, light 215 that is incident on one of the shields 211 is prevented from entering the color drum 200 and thus the issues of stray unfiltered light are significantly reduced, if not substantially eliminated. Fig. 2b illustrates the use of the baffles 212 for absorbing stray light from a variety of sources. For example, light 216 is incident on the drum at color filter 204. The light traverses the color filter 204, traverses the rod lens 213 and is incident on the color filter 209 at a relatively high angle of incidence, illustratively outside its tolerance angle. Thus, due to the relationship of the wavelength and angle of incidence, and the transmission/reflection characteristics of the filter 209 (e.g., a dielectric stack), some of the light 217 is reflected at the surface of the filter 209, and some is reflected from within the filter 209 (e.g., through known optical phenomena). These reflected (stray) light components 217, 218 are substantially absorbed by the baffle 212 as shown, and are thereby prevented from being reflected within the color drum 200 and exiting the drum 200 at another point on the distal side. Thus, these stray components 217, 218 are prevented from being transmitted to the light- valve, where they may impact the scrolling sequence of the primary colors needed to create the image at the screen.

Similarly, light 221, which is partially reflected by the color filter 209 and partially reflected by the lens 106 at two surfaces, is prevented from reaching the light-valve by baffles 212. Moreover, similar to the stray light 216, light 220, which is also reflected by the color US040457

filter 209 by similar physics, is prevented from reaching the light- valve as shown.

Fig. 2b also shows how light 219, which is stray light from an aberration or other defect on the lens 205 is prevented from reaching the distal portion of the color drum 200 by virtue of the example embodiments. To this end, a portion of light 216 is reflected from the lens 105 at numerous locations as shown, emerging as light 219. This light 219 would otherwise be transmitted to the distal portion of the color drum 200 and would emerge as stray light, creating deleterious effects at the screen. However, the baffle 212 prevents this from occurring.

Fig. 2c shows the prevention of stray light 222 from being transmitted to the light- valve in accordance with an example embodiment. In the present example, stray light 222 is reflected within the lens 105 and is incident on the color filter 203 illustratively at an edge 224. This light 222 propagates through the color filter 203 being reflected by total internal reflection (TIR). For example, the color filter may act as a waveguide. At the end surface opposite edge 224 of the color filter 203, or at another point(s) along the surface, the light emerges from the color filter 203, and otherwise may be transmitted to the light-valve.

However, the baffle 212 absorbs this stray light and prevents its being transmitted to the light- valve.

Moreover, as shown in Fig. 2c, light 223 is light that is incident on the outer surface of the lens and is thus on the outer portion of the cone of light desirably transmitted from the light source. This light may not be properly scrolled and is beneficially absorbed by the baffle 212. Finally, light 225 has components that are reflected at least partially by elements of the drum 200, or the lens 106, or both. These reflected portions can be stray light to the system, and may be deleterious to the image. These reflected portions are thus absorbed by the baffles 212 as shown. It is emphasized that the light rays described in connection with the example embodiments of Figs. 2a- 2c are merely illustrative. Clearly, the deleterious stray light may have different trajectories and sources. Such stray light may also be substantially eliminated or significantly reduced by virtue of the example embodiments.

Fig. 3 shows a color drum 300 in accordance with an example embodiment. The color drum 300 is similar to that described in the example embodiment of Figs. 2a-2c, however, is US040457

tailored for a four color system. Illustratively, the drum 300 includes a plurality of color filters 301-308, wherein each filter is disposed substantially directly opposite a filter that is substantially identical in dimensions and color filtration characteristics. For example, color filter 301 is substantially the same as color filter 305, and is comparatively larger most other of the filter elements. Thus, filters 301 and 305 usefully increase the portion of a primary color (one of the four colors projected to the light valve) that is relatively deficient in the emission spectrum of the light source. As described previously, the color drum may include substantially arcuate color filters, rather than the eight-sided drum shown. This provides a substantially cylindrical drum. In the example embodiment of Fig. 3, like the drum 200 of Figs. 2a- 2c, the drum 300 may include one or more substantially transparent elements selectively substituted for one or more color filters. Moreover, the color filters do not have to be as described immediately above or arranged as described immediately above. For example, the color filters may be the same size and it is not essential that a color filter is disposed substantially directly opposite another substantially identical filter.

As referenced previously, located a substantially hollow interior of color drum 200 is the cylindrical or rod lens 211. It is noted that the choice of a cylindrical/rod lens is merely illustrative. Other types and shapes of lens elements are also envisioned. Baffles 212 extend radially outward from rod lens 213 to the outer wall of color drum 200, as illustrated in Fig. 2, leaving enough clearance between the baffles and the drum to allow the drum to rotate freely, while the rod lens 213 and the baffles 212 remain substantially stationary. Finally, it is noted that in the example embodiments described in connection with Figs. 1-3, there are four baffles 212 disposed in a pseudo-juxtaposed relationship. This is merely illustrative. Clearly, more or fewer baffles may be used. One consideration is to ensure that the baffles do not interfere with light that traverses the rod lens, as this light is unlikely to be deleterious stray light.

It should be understood that the various embodiments are described by way of example only and should not be construed in a limiting sense. In view of this disclosure, those skilled in the art can implement the various example devices and variations thereof while remaining within the scope of the appended claims. US040457

Claims

US040457CLAIMS:

1. A color drum (107, 200, 300), comprising: a plurality of color filters (201-210); and at least one baffle (212), which substantially absorbs light over a visible spectrum of wavelengths.

2. The color drum of claim 1, wherein each of the plurality of color filters is substantially flat.

3. The color drum of claim 1, further comprising at least one antireflective element.

4. The color drum of claim 1, further comprising at least one element, which transmits light of substantially all visible wavelengths.

5. The color drum of claim 1, wherein each of the color filters transmits substantially one color of light.

6. The color drum of claim 5, wherein each color filter is a dichroic filter.

7. The color drum of claim 1, wherein the plurality of color filters include at least one red (R) filter, at least one green (G) filter and at least one blue (B) filter.

8. The color drum of claim 1, wherein the plurality of color filters include at least one cyan (C) color filter, at least one blue (B), at least one green (G) color filter, at least one red (R) color filter and at least one yellow (Y) color filter.

9. The color drum of claim 1, wherein a plurality of primary colors are transmitted through the color drum and the plurality of color filters is twice the plurality of primary colors.

10. The color drum of claim 1, wherein the at least one baffle is substantially absorptive of US040457

light across the visible wavelength spectrum.

11. The color drum of claim 1, further comprising a gap (226) between at least two of the plurality of color filters, and a shield (211) disposed in or over the gap.

12. A light-valve projection system (100), comprising: a source of light (101), which includes a plurality of wavelength components; and a color-drum (107), which includes a plurality of color filters (201-210) and at least one baffle (212), wherein the baffle substantially absorbs light over a visible spectrum of wavelengths.

13. The light-valve projection system of claim 12, wherein each of the plurality of color filters is substantially flat.

14. The light-valve projection system of claim 12, further comprising at least one antireflective element.

15. The light-valve projection system of claim 12, further comprising at least one element, which transmits light of substantially all visible wavelengths.

16. The light-valve projection system of claim 12, wherein each of the color filters transmits substantially one color of light.

17. The light-valve projection of claim 16, wherein at least one of the plurality of color filters is a dichroic filter.

18. The light-valve projection system of claim 12, wherein the light-valve (113) is one of a liquid crystal device (LCD), a liquid crystal on silicon (LCOS) device or a digital light processor (DLP). US040457

19. The light-valve projection system of claim 12, wherein at least one of the plurality of color filters is a multi-layer dielectric filter.

20. The light-valve projection system of claim 12, further comprising a gap between at least two of the plurality of color filters, and a shield disposed in or over the gap.