WO2005091646A1

WO2005091646A1 - Rotatable transmissive optical element and color display system with such element

Info

Publication number: WO2005091646A1
Application number: PCT/IB2005/050882
Authority: WO
Inventors: Robertus J. Heeman
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-03-19
Filing date: 2005-03-11
Publication date: 2005-09-29

Abstract

A transmissive, rotatable optical element for use in a scrolling color display system, the optical element being arranged to provide, when rotated, varying color filtering of incident light. The optical element has at least one position in which incident light will pass two different color filters, in combination forming a third filter. By substituting at least one color filter with a combination of two other filters, the number of different components in the optical element can be reduced. A display system using such an optical element is also disclosed.

Description

Rotatable transmissive optical element and color display system with such element

The present invention relates to a scrolling color display system comprising a light source and a color scanner for generating a light beam with a plurality of scrolling color fields, which beam is arranged to illuminate a light modulator to produce an image generated by the light modulator. The invention also relates to a rotatable transmissive element intended for use in such a system.

Such a scrolling color display system is disclosed in e.g. US 5,532,763, and is particular in that light from a light source is divided into a plurality of beams (typically three; red, blue and green), which are sequentially scrolled over a display device, e.g., a reflective LCD, and then projected by means of a lens. The bars are scrolled, e.g., from top to bottom, and are synchronized with the display so that they complete a scrolling sequence within one picture frame. An improved display system was disclosed in US 6,361,172, according to which the scanner is a rotating transmissive element in the form of a rotating drum having transmissive surface portions in the form of filters with different transmission bands. The surface portions opposite each filter can either be filters of the same color, or just clear portions. When the drum rotates, it will sequentially filter an incident light beam into the different colors. Recently, use of multiple (more than three) primary colors has been introduced in the field of scrolling color display systems, and drums of the mentioned type have been found suitable for implementing such projection systems. A problem with any optical element used as a scanner to generate a plurality of primary colors, is that the filters, especially the green filter which is a band-pass filter, are expensive. This, of course, is particularly a problem in systems with more than three primary colors, where the number of different components becomes relatively large. An object of the present invention is to mitigate this problem, and provide an optical element capable of generating multiple primary colors, which is more cost efficient to manufacture. This object is achieved by a rotatable optical element and a color display system according to claim 1 and 12. A first aspect of the invention relates to a rotatable optical element for use in a scrolling color display system, the optical element being arranged to provide, when rotated, varying color filtering of incident light. The element has at least one position in which incident light will pass two different color filters, in combination forming a third filter. By substituting at least one color filter with a combination of two other filters, the number of different components in the optical element can be reduced. Preferably, the two color filters with different properties are single sided cutoff filters, in combination forming a band pass filter. This enables for example avoiding the more expensive green band-pass filter, by using a yellow and a cyan cut-off filter in combination. As known per se, the optical element can further comprise at least one drum having a plurality of pairs of oppositely located surface portions, arranged so that light incident on the drum will pass both surface portions in a pair, at least one of the surface portions in each pair being provided with a color filter. Such rotating drums have been found advantageous in color scrolling display systems. The two different filters can be arranged on oppositely located surface portions of a single drum, or be arranged on opposite sides of one surface portion. Alternatively, or in combination, the optical element can comprises two or more drums. In case of a drum type optical element, a lens can be arranged coaxially inside the drum(s) for focusing light passing through one surface portion of a drum onto another surface portion of the drum. Instead of, or in combination with, a drum, the optical element can comprise at least one disc having a plurality of sectors provided with color filters. This type of optical element may also be advantageous in a scrolling color display system. The two different color filters can be arranged on opposite sides of one such disc, or be arranged on separate discs. In case of several discs, a lens can be arranged to focus light passing through the first disc onto the second disc. A second aspect of the invention relates to a color display system comprising a light source and a color scanner for generating a light beam with sequentially varying color, which beam is arranged to illuminate a light modulator, thereby generating an image, wherein the scanner is an optical element according to the first aspect of the invention. The display system can be a projection system, further comprising means for projecting the image onto an image surface. The projecting means can be arranged behind the image surface compared to a viewer (rear projection), or arranged in front of the image surface compared to a viewer (front projection). Alternatively, the display system is a direct view system, i.e. the light modulator acts as a viewing screen.

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention. Fig 1 is a schematic view of a color projection scanner according to an embodiment of the present invention. Fig 2 is a perspective view of a first embodiment of the optical element in fig 1. Fig 3 is a schematic plane view of a first alternative of the optical element in fig 2. Fig 4 is a schematic plane view of a second alternative of the optical element in fig 2. Fig 5 is a schematic plane view of a second embodiment of the optical element in fig 1. Fig 6 is a perspective view of a third embodiment of the optical element in fig

1. Fig 7 and 8 are schematic plane views of a fourth embodiment of the optical element in fig 1.

The present invention can advantageously be used in a projection system of the kind illustrated in fig 1. The system comprises an display driver 1, arranged to receive a data input stream 2, from e.g. a personal computer or a video cassette recorder (not shown), and to drive a light modulator 3, such as a reflective LCD. A light source 4, preferably a UHP lamp followed by an integrator, is controlled by a lamp driver 5 to generate a light beam 6a. The beam is imaged by a first lens 7 onto a rotatable optical element 8, which converts the light beam 6a from the lamp 4 into a colored light beam 6b shifting color as the optical element 8 rotates. In the present example, the optical element generates five primary colors (red, green, blue, cyan and yellow) one after the other. The beam 6b is focused by a second lens 9 into a polarizing beam splitter (PBS) 10 and further onto the light modulator 3. As the optical element rotates, the fields of the beam 6b are brought to continuously scroll across the display device, to sequentially illuminate the display device with the different primary colors. An image generated by the by the light modulator 3 is reflected back into the PBS 10, which directs the reflected image to a projection lens 11, for projection onto a suitable screen (not shown). The projection lens can be arranged behind the screen as seen from a viewer (rear projection) or in front of the screen (front projection). The invention is not limited to use in a projection display system. On the contrary, it may advantageously be used in many display system, such as a direct view system. In this case, the light modulator 3 can be a larger, transmissive LCD, arranged at a larger distance from the optical element, by itself acting as the display screen. A first embodiment of the optical element 8, is shown in more detail in fig 2, and comprises a drum 12 surrounding a cylindrical lens 13, coaxial with the drum. The drum has a plurality of pairs of oppositely located surface portions, typically one pair for each primary color used by the display system. With five primary colors (red, green, blue, cyan and yellow), as in this case, the drum is decagon, i.e. a polygon with ten sides. Each surface portions can be formed by a transmissive substrate, and at least some surface portions are provided with color filters. Figs 3 and 4 show how a beam of incident light 6a is focused by lens 7 onto a surface portion 16a; 18a of the drum 12, then is focused by the lens 13 onto the opposite surface portion 16b; 17b, and finally is refocused by the lens 9. According to a first alternative of the drum in fig 2, two opposite surface portions 16a, 16b are provided with different color filters 17a, 17b, in combination realizing a third color filter. For example, as is shown in fig 3, the drum 6 may have four pairs of opposite surface portions 14a, 14b, each pair provided with two color filters 15a, 15b of the same color (i.e. red, blue, cyan and yellow) filters respectively. Obviously, one of the surface portions (e.g. 14b) in each of these four pairs may instead be clear (i.e. without filter) (see fig 4). The surface portions 16a, 16b of the fifth pair are provided with two different color filters, here single sided cut-off filters (e.g. yellow and cyan), so that a third filter is formed, here a green band-pass filter. Note that the thickness of the filters in fig 3 (as well as in the following figures) for reasons of clarity has been exaggerated compared to the thickness of the substrate. According to a second alternative of the drum in fig 2, one surface portion 18a is provided with two different filters 19a, 19b. For example, as is shown in fig 4, the drum may have four pairs of opposite surface portions 14a, 14b, each pair comprising one clear section 14b (i.e. without filter), and one section 14a provided with a color filter 15a (i.e. red, blue, cyan and yellow). Obviously, both surface portions in each pair can also be provided with identical filters (see fig 3). The fifth pair comprises one clear surface portion 18b (i.e. without filter), and one surface portion 18a provided with two different color filters 19a, 19b, e.g. one yellow and one cyan. These two filters can be arranged on opposite sides of the transmissive substrate forming the surface portion 18a. According to a second embodiment, shown in fig 5, the optical element 8 comprises two drums 21, 22, similar to the one in fig 3, which are arranged coaxially around a central lens 23. The drums may be rotated together or separately. In this case, incident light 6a will pass through four surface portions 24a, 24b, 24c, 24d, and the drums are arranged and rotated in such a way that for at least one position of the drums 21, 22, two of these four surface portions will be provided with filters 25a, 25b with different properties, e.g. a yellow and a cyan filter as mentioned above. These two different filters may be arranged on surface portions on the same side of the double drum, e.g. on portions 24a and 24b or 24c and 24d. According to a third embodiment, shown in fig 6, the optical element 8 has the form of a rotatable disc 31 , comprising a transmissive substrate 32, e.g. made of glass, provided on one side with a plurality of sector shaped filters 33, 34a. On the other side of the substrate 32 is arranged at least one additional filters 34b, so that the disc in this sector presents incident light with two different filters, 34a and 34b. According to a fourth embodiment, shown in figs 7 and 8, two discs 31a, 31b similar to the one in fig 6 are arranged coaxially, one after the other. The discs can be rotated together or separately. In this case, no additional filter is provided, and instead the discs 31a, 3 lb are arranged and rotated in a way so that for at least one position of the discs, incident light will pass through two different filters 35a, 35b. In fig 7, the discs are arranged close enough to each other to eliminate the need of any additional focusing element. In fig 8, the discs are arranged at such a distance from each other that a focusing element, e.g. a lens 36, is required to focus light passing through the first disc onto the second. Measured spectral data of currently used filters in a five color drum show that the 50% points of the filter edges of the yellow and cyan filters are approximately at the same wavelength resulting in transition widths approximately as wide as the green band pass filter. Such yellow and cyan filters should thus be possible to combine into one green filter as described above. The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the optical system preceding and succeeding the optical element 8 are only schematically illustrated by lenses 7 and 9. In reality, and as the skilled man will realize, several lenses may be required to achieve the desired focusing.

Claims

CLAIMS:

1. A transmissive, rotatable optical element (8; 12; 21, 22; 31; 31a, 31b) for use in a scrolling color display system, said optical element being arranged to provide, when rotated, varying color filtering of incident light, characterized in that said optical element has at least one position in which incident light will pass two different color filters (17a, 17b; 19a, 19b; 25a, 25b; 34a, 34b; 35a, 35b), in combination forming a third filter.

2. An optical element according to claim 1, wherein said two different color filters are single sided cut-off filters, in combination forming a band -pass filter.

3. An optical element according to claim 1, comprising at least one drum (12) having a plurality of pairs of oppositely located surface portions (14a, 14b; 16a, 16b; 18a, 18b), arranged so that light incident on the drum will pass both surface portions in a pair, at least one of the surface portions in each pair being provided with a color filter (15a, 15b, 16a, 16b; 19a, 19b; 25a, 25b).

4. An optical element according to claim 3, wherein said two different filters (17a, 17b) are arranged on oppositely located surface portions (16a, 16b).

5. An optical element according to claim 3, wherein said two different filters (19a, 19b) are arranged on opposite sides of one surface portion (18a).

6. An optical element according to claim 3, comprising two coaxially arranged drums (21, 22).

7. An optical element according to claim 3, further comprising a lens (13) arranged coaxially inside the drum(s) (12; 21, 22), for focusing light passing through one surface portion of a drum onto another surface portion of the drum.

8. An optical element according to claim 1, comprising at least one disc (31) having a plurality of sectors provided with color filters (33, 34a, 34b; 35a, 35b).

9. An optical element according to claim 8, wherein said two different color filters (34a, 34b) are arranged on opposite sides of one of said sectors.

10. An optical element according to claim 8, wherein said two different color filters (35 a, 35b) are arranged on two separate discs (31a, 31b).

11. An optical element according to claim 10, further comprising a lens (36) for focusing light passing through the first disc onto the second disc.

12. A scrolling color display system comprising a light source (4) and a color scanner (8) for generating a light beam with sequentially varying color, which beam is arranged to illuminate a light modulator (3), thereby generating an image, characterized in that the scanner (8) is an optical element according to one of the preceding claims.

13. A display system according to claim 12, further comprising means (11) for projecting the image onto an image surface.

14. A display system according to claim 13, wherein said projection means (11) are arranged behind the image surface compared to a viewer.

15. A display system according to claim 13, wherein said projection means (11) are arranged in front of the image surface compared to a viewer.

16. A display system according to claim 12, wherein said light modulator (3) is adapted to act as a viewing screen.