WO2006013523A2

WO2006013523A2 - Display structures with improved contrast using saturable absorbers

Info

Publication number: WO2006013523A2
Application number: PCT/IB2005/052454
Authority: WO
Inventors: Levinus Pieter Bakker; Adrianus Johannes Stephanes Maria De Vaan; Gert Wim Hooft
Original assignee: Koninklijke Philips Electronics N.V.; U.S. Philips Corporation
Priority date: 2004-07-27
Filing date: 2005-07-21
Publication date: 2006-02-09
Also published as: WO2006013523A3

Abstract

A projection system (100, 200) includes a light source (101, 201) and an imaging surface (104, 207). The projection system also includes a saturable absorber (105, 208) between the light source and the imaging surface. In one example embodiment, the projection system is a scanning projection system and in another example embodiment the projection system includes a light valve such as a liquid crystal (LC) panel.

Description

DISPLAY STRUCTURES WITH IMPROVED CONTRAST USING SATURABLE

ABSORBERS

Scanning light and light- valve projection systems projection displays may be used in projection televisions, computer monitors, point of sale displays, head-up displays, microdisplays and electronic cinema, to mention only a few applications.

Scanning light projection systems often include a source of red light, a source of green light and a source of blue light. These sources are continuously scanned based on information of an image in order to project that image onto a screen.

Another type of projection screen is based on a light valve, such as a liquid crystal (LC) panel. As is well-known, the LC panel optically modulates the light incident thereon, based on image information. By selectively preventing light from reaching the imaging surface and allowing other light to reach the surface, based on the image information, an image is formed from the plurality of picture elements (pixels) of the LC panel.

Yet another type of light- valve projection system incorporates a digital micro- mirror device (DMD) as the light- valve, rather than an LC light- valve. A digital micro- mirror device (DMD) is based on an array of micro-mirrors. Each pixel consists of a single mirror that can be rotated about an axis. In operation, each mirror is rotated to a first position or a second position. In the first position, light incident on the mirror is reflected from the mirror to a projection lens, and to the imaging surface (viewing screen). In the second position, light incident is reflected by the mirror and is not coupled to the projection lens. Thereby, in the first position, a bright-state pixel is formed at the imaging surface, and in the second position, a dark-state pixel is formed at the imaging surface. Grey scales may be made by sub-field addressing. In single-panel DMD projectors, color is obtained by color sequential techniques. From these basic principles, images may be formed at the imaging surface.

While the projection systems alluded to above are useful in display applications, there are drawbacks and shortcomings encountered in known projection systems. For example, in many projection systems, image contrast is marred by the ambient light that is substantially diffusely reflected from the image screen to the viewer. Additionally, contrast issues arise elsewhere in known systems, and serve to degrade the image quality. What is needed therefore is a method and apparatus that addresses at least the shortcomings of known systems described above.

In accordance with an example embodiment, a projection system includes a light source and an imaging surface. The projection system also includes a saturable absorber between the light source and the imaging surface.

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. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Fig.1 is a schematic diagram of a projection system in accordance with an example embodiment.

Fig. 2 is a schematic diagram of a projection system in accordance with another example embodiment.

Figs. 3a and 3b are graphical representations of the absorbed and transmitted intensity versus incident intensity, respectively, of a saturable absorber in accordance with an example embodiment.

Fig. 4 is a tabular representation of saturable absorber dyes and their characteristics.

Fig. 5 is a cross-sectional view of a saturable absorber 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 present invention. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as to not obscure the description of the present invention. Wherever possible, like numerals refer to like features throughout.

Briefly, in accordance with example embodiments described in detail herein, image display devices and systems include a light source, an image display and a saturable absorber disposed between the light source and the image display. The saturable absorber transmits substantially all light of above a threshold intensity and absorbs substantially all light below a particular intensity threshold. In this manner, the darker portions or pixels of an image have a reduced intensity to the viewer and the bright pixels, by contrast, have a higher light intensity. Overall, thereby, the image has an improved contrast. It is noted that in certain example embodiments the image display system is a scanned laser projection system, while in other example embodiments, the display system is a light- valve based system. In accordance with illustrative embodiments, the light valve may be an LC panel or a DMD.

Fig. 1 is a schematic representation of a projection display system 100 in accordance with an example embodiment. The projection display system 100 includes a light source 101 that illustratively emits light of red, blue and green wavelengths. For example, the light source may include a red wavelength laser, a blue wavelength laser and a green wavelength laser. The light emitted from the source 101 is incident on a light scanner 102. The scanner 102 is useful in scanning the light from the light source 101. As is well-known to one of ordinary skill in the art, the sequential scanning of the light along with the modulation of the light provided at the source 101 results in the forming of an image. As the details of scanning in projection display devices are well known to one of ordinary skill in the art, details are omitted so as to avoid obscuring the description of the example embodiments.

After the light is scanned, it is incident on system optics 103, which include, among other elements projections optics, which project the light onto an imaging surface 104. The system optics 103 are also well-known in the art, and are thus no discussed in detail so as to avoid obscuring the description of the example embodiments.

As saturable absorber (SA) 105 is disposed between the system optics and the surface 104. As will become clearer as the present description continues, the SA 105 usefully reduces the lower intensity light that is reflected from the surface 104 back toward the viewer (not shown). In this manner the higher intensity light, which corresponds to bright pixel light, is transmitted nearly unattenuated; and the lower intensity light, which corresponds to dark pixel light, is attenuated by the SA, thereby improving the contrast of the image. In accordance with certain example embodiments, the SA may be disposed on or over the screen 104. In the example embodiment of Fig. 1, the SA 105 is useful in improving the contrast between the dark pixels and the bright pixels of the image at the image surface 104. This example embodiment is particularly useful in reducing the adverse impact on ambient light on the image reflected from the imaging surface 104. To this end, the imaging surface 104 is usefully a substantially diffusive light reflector. As such, ambient light as well as light from the image projected by the system are reflected from the imaging surface 104. Thus, the ambient light can deleteriously impact the contrast of the image provided.

However, compared to the intensity of the bright pixels, the intensity of the reflected ambient light is small. This differential in the intensity between the ambient light and bright state light is emphasized in the example embodiments. To wit, the SA 105 usefully passes the higher intensity light of the bright pixels substantially unattenuated; and substantially absorbs the ambient light. Of course, dark state pixels are also attenuated by the SA. Finally, it is noted that in sequential scanning laser applications, the contrast provided through the example embodiments is significantly due to the high intensity light provided by the lasers of the light source 101.

Fig. 2 shows a projection display 200 in accordance with another example embodiment. The projection display 200 is illustratively a light- valve based system, which is well known to one of ordinary skill in the art. The display 200 includes a light source 201, which includes a reflector and a source of white light. In many applications the source of white light is an ultra-high pressure (UHP) gas lamp, which is well known to one of ordinary skill in the display arts. Light from the light source 201 is incident on optics 202, which provides more homogeneous light and may further focus the light. The system 200 optionally includes a first polarizer that either absorbs or reflects light from the light source so that light of substantially only one state of polarization is transmitted to a light valve 204. The light valve modulates the light in order to form an image of dark and bright pixels. In an example embodiment, the light valve 204 may be an LC panel or a DMD. As will be appreciated by one of ordinary skill in the art, in embodiments in which a DMD is implemented as the light valve 204 the first polarizer would not be needed.

Light that is transmitted by the light valve 204 is then incident on an analyzer 205, which is normally an absorptive polarizer. Light transmitted by the analyzer 205 is incident on a system or projection optic 206 and thus onto an imaging surface 207. It is noted that the details and variations of light valve-based displays are well known and thus not described so as to avoid obscuring the disclosure of the example embodiments. In the example embodiment shown in Fig. 2, an SA 208 is disposed adjacent to the light valve 204. As described in further detailed herein the SA 208 provides selective attenuation to the dark state light that emerges from the light valve 208, and provides substantially no attenuation to the bright state light that emerges from the light valve 208. As such, the contrast of the image provided at the imaging surface 207 is improved by the SA 208 of the example embodiment, because the intensity of the dark state light is reduced.

Before discussing illustrative materials useful as SAs, it is useful to qualitatively describe their functions. To wit, Figs. 3a and 3b are graphical representations of the transmission and transmittivity versus illumination intensity of an SA useful in the example embodiments. As can be appreciated from a review of Figs. 3a and 3b, at lower intensity levels, the SA significantly precludes the transmission of light, especially compared with the transmission/transmittivity of the incident light of higher intensity. For example, compared to the laser light, very little ambient light will be transmitted by the SA. Finally, it is noted that the saturation intensity occurs at the intersection of the dashed lines of Fig. 3 a. As can be appreciated, at intensity levels above the saturation point, the light is substantially completely transmitted by the SA.

In keeping with the example embodiments, a variety of different types of saturable absorber materials and structures may be used as the SA 105 or the SA 208. For example, as is known, saturable absorbers are frequently used in dye- lasers. These absorbers are usually supplied as powder, which has to be mixed with a solvent in the appropriate concentration, as would be readily understood by one having ordinary skill in the art. This concentration determines the maximum absorption of the SA-layer. It is also possible to have the absorbers spin-coated on a substrate, which may be a transparent substrate such as glass. Spin-coating is beneficial because the dyes that are not solved in a solvent do not show any fluorescence.

It is noted that the saturable absorbers are usually relatively broadband (width in the order of 100 run). When the SA is illuminated with monochromatic light, the saturation takes place in a band around the wavelength of the incident light. Thus for light outside this saturated band, there is no saturation effect. The spectral properties of the saturable absorbers need to be tuned to the spectra of the incident light in order not to have a color shift due to the absorption and due to saturation of the absorption. For displays this means that the absorbers should be tuned to the spectra of the display's primary colors. Fig. 4 is a table showing certain saturable absorber materials that are useful in providing the SA of certain example embodiments described herein. These materials are available from first Lambda Physik (Lambdachrome® Laser-grade Dyes) and are provided for merely illustrative purposes. The saturation intensity of organic dyes incorporated into a solid state matrix is very low, in the order of approximately 0.01 W/cm² to approximately 1 W/cm². This saturation intensity is suitable light projection applications such as those described in connection with the example embodiments of Figs. 1 and 2. A potential shortcoming of SA dye materials is their susceptibility to degradation when exposed to ultraviolet light. Therefore, ultraviolet light should be properly removed by for instance encapsulating the absorbers. Furthermore, present dye materials have an unacceptable lifetime. If this drawback can be overcome, SA dye materials would be an attractive option for the saturable absorbers of the example embodiments.

One viable option for the material and structure of the SA's of the example embodiments is quantum dots. To this end, inorganic materials such as quantum dots of II- VI semiconductors can be used as saturable absorbers 105 and 208 of the example embodiments. These materials are cost effective and very stable.

The absorption spectrum of quantum dots depends on their size. In general, the quantum dots act as high-pass filters, absorbing light with a wavelength below a certain threshold. This threshold wavelength shifts to higher wavelengths when the size increases. For purposes of illustration, the absorption threshold wavelength of CdSe quantum dots of size 1.7nm is around 400 nm, while for 15nm dots it is around 600 nm. This makes CdSe quantum dots a suitable absorber for use in the example embodiments described herein. It is noted that in order for the quantum dots to function as a suitable saturable absorber, the saturation intensity should be achieved during illumination in a display. The saturation intensity of quantum dots is in the order of approximately 10 kW/cm² to approximately 100 kW/cm². As such, CdSe quantum dot are a suitable option for use in projection screens for laser-scanning projectors such as those previously described in connection with the example embodiments. In scanning laser projectors, the intensity in the laser spot can be this high. Other quantum dots materials could be ZnSe and CdTe. Quantum dots are high-pass filters, i.e. below a certain cut-off wavelength light is absorbed. When the incident intensity (at wavelengths shorter than the cut-off wavelength) is higher than the saturation intensity, then the absorption near the cut-off wavelength decreases.

More recently discovered materials that can act as saturable absorbers. For example, carbon nanotubes (CNT) have been recently reported as having suitable characteristics for use as the SA in the example embodiments of Figs. 1 and 2. In addition to their characteristics as an SA, CNTs have the additional benefit, when properly aligned, of functioning as a polarizer.

In accordance with an example embodiment, a multi-layer quantum dot structure is used to effectively selectively attenuate the low intensity light in projection displays such as those of the example embodiments of Figs. 1 and 2. In the example embodiments, each layer is useful in providing a saturable absorber for a respective one of the primary color (RGB) wavelength ranges.

Fig. 5 is a cross-sectional view of a saturable absorber 500 in accordance with an example embodiment. The SA 500 includes a first layer 501, which provides attenuation for low intensity red light; a second layer 502, which provides attenuation for low intensity green light; and a third layer 503, which provides attenuation for low intensity blue light. Each of the layers 501-503 includes a suitable quantum dot material that is incorporated in a host material (e.g., glass) and applied to a projection screen or disposed adjacent to an LC panel. It is noted that the quantum dots are high-pass filters and not band-pass filters. The result is that the green quantum dot layer 502 will absorb not only green light, but also blue light. Furthermore, the red quantum dot layer 501 will absorb light in the red, green, and blue wavelength ranges. Illuminating a mixture of these quantum dots with blue light with an intensity higher than the saturation intensity will saturate the absorption of the blue quantum dot for blue light. However, the red and green quantum dots are not saturated for this blue light. A similar phenomenon happens with green light.

In accordance with certain example embodiments, this potential shortcoming is addressed by incorporating the quantum dots in powder particles. These powder particles (i.e., light scattering centers) scatter the incident radiation, with the size and shape of the powder particles (independent of the size of the quantum dots incorporated in the powder particles) determining their scattering properties. The size of the quantum dots and their density in the powder particles determine the absorption of the powder particles. Hence, it is possible to design a powder layer consisting of powder particles with quantum dots with the proper scattering and absorption properties.

In an example embodiment, the blue absorbers are incorporated into powder particles of size approximately 100 nm, having a density of approximately 5><10¹³m^"2, and an index of refraction of 1.5. It is noted that the host material of the powder is non- absorbing. Without the quantum dots, the transmission of such a layer is 10% at 400 nm, 50% at 550 nm, and 80% at 700 nm and calculated using the Rayleigh scattering cross- section. The rest of the light is scattered. The green quantum dot layer 502 is incorporated in a similar 100 nm powder particle layer or in a white-diffusive reflecting layer (i.e., with particle size larger than 100 nm). The red quantum dot layer 503 is incorporated into a white-diffusive reflecting layer.

The blue quantum dot layer 503 scatters blue, when it is not absorbing due to saturation, and it just absorbs when it is unsaturated. The blue light, which is transmitted through the blue quantum dot layer 503 into the green layer, is absorbed by the green quantum dot layer 502. The green layer 502 absorbs blue light. Green light is only absorbed when the absorbed intensity is low. When the light is not absorbed, the green and red light are partly scattered. The red layer absorbs blue and green light. The green layer 502 absorbs red light when the incident intensity is low, and it scatters red light when the absorbed intensity is high. In an example embodiment, a relatively high (above threshold) intensity incident light is that approximately 100, 70, and 100% of the red, green, and blue light is diffusely reflected by the structure 500. On the other hand, for low intensity red, green and blue light only 0%, 25%, and 9%, respectively, is transmitted by the structure 500. Thus, the presence of dark state light at a screen is more readily discerned, without substantial loss of bright state light.

In an example embodiment, the layers 501-503 are easy to apply to a projection screen, or are provided on a substrate (not shown) for use in LCD applications. In accordance with example embodiments, the substrate may be a flexible material or a non- flexible material. For example, the substrate may be an organic material (e.g., a plastic) or a glass material. The scattering material in each of the layers may be one or more of a variety of glass materials; epoxy materials; organic materials such as polystyrene, PMMA, polycarbonate; or silicone materials; or silica or titania; or a combination thereof. Finally, the materials useful as quantum dots include CdSe, ZnSe and CdTe, to name only a few.

In view of this disclosure it is noted that the various methods and systems described herein can be implemented using a variety of optical components and in a variety of applications. Further, the various methods, devices, systems and parameters are included by way of example only and not in any limiting sense. Therefore, the embodiments described are illustrative and are useful in improving the amount of light transmitted to a display. In view of this disclosure, those skilled in the art can implement the various example devices and methods in determining their own techniques and needed equipment to effect these techniques, while remaining within the scope of the appended claims.

Claims

CLAIMS:

1. A projection system (100, 200), comprising: a light source (101, 201); an imaging surface (104, 207); and a saturable absorber (105, 208) between the light source and the imaging surface.

2. A projection system as recited in claim 1, wherein the light source includes a red light source, a green light source and a blue light source.

3. A projection system as recited in claim 1, wherein the light source is a white light source.

4. A projection system as recited in claim 3, wherein the light source is an ultra high pressure (UHP) gas discharge lamp.

5. A projection system as recited in claim 1, comprising a scanner.

6. A projection system as recited in claim 1, comprising a liquid crystal (LC) panel.

7. A projection system as recited in claim 1, wherein the imaging surface (104) is a diffusive reflector, and the saturable absorber (105) is disposed over the imaging surface.

8. A projection system as recited in claim 2, wherein the light sources are lasers.

9. A projection system as recited in claim 6, wherein the saturable absorber is disposed adjacent the LC panel.

10. A projection system as recited in claim 1, wherein the saturable absorber is comprised of a plurality of saturable absorber layers (500).

11. A projection system as recited in claim 10, wherein each of the layers (501, 502, 503) includes a saturable absorber material and a plurality of light scattering centers.

12. A projection system as recited in claim 11, wherein the saturable absorber material comprises a plurality of quantum dots.

13. A projection system as recited in claim 12, wherein the quantum dots are one or more of CdSe, ZnSe or CdTe.

14. A projection lens system as recited in claim 11, wherein the light scattering centers are diffusive particles.

15. A projection system as recited in claim 10, wherein the plurality of layers includes a saturable absorber layer for blue- wavelength light, a saturable absorber layer for green- wavelength light and a saturable absorber layer for red- wavelength light.

16. A projection system as recited in claim 14, wherein the particles have a diameter of approximately 100 nm.

17. A projection system as recited in claim 14, wherein the particles have a density of approximately 5^χl0¹³m^'2 and an index of refraction of 1.5.

18. A projection system as recited in claim 12, wherein the quantum dots have a saturation intensity on the order of approximately 10 kW/cm² to approximately 100 kW/cm².

19. A projection system as recited in claim 11, wherein the scattering centers may be one or more of: a glass material; an epoxy material; an organic material; a silicone material; silica; or titania.

20. A projection system as recited in claim 1, wherein the projection system is disposed in one of: a television; a computer monitor; a point of sale display; a head-up display; a microdisplay; or an electronic cinema.