Technical Field of the Invention

This invention relates to a projector for projecting images onto a screen.

Background of the Invention

Projectors based on a light valve which controls the transmission or not of light therethrough by a scattering effect are known. See, e.g., Fergason, US 4,693,557 (1987) and Jones et al., US 5,075,789 (1991). A characteristic of such projectors is that brightness and contrast are both a function of the projection aperture. As the projection aperture decreases the contrast ratio increases but the brightness decreases, so that an improvement in one parameter is offset by a decrement in the other parameter. The situation is further complicated by the fact that projectors are not necessarily used in wholly darkened rooms, but, rather, under a variety of ambient lighting conditions. For instance, it may be desirable to have some minimal ambient lighting present to permit viewers to take notes of a presentation. So, a contrast ratio-brightness combination which may be optimized for a particular set of ambient lighting conditions may not be suitable for a different set of ambient lighting conditions.

Summary of the Invention

The instant invention provides a projector which overcomes the foregoing Hmitations and permits adjustment of its brightness and contrast ratio to adapt to a variety of ambient lighting conditions.

Accordingly, this invention provides a projector, comprising: (a) a source of light; (b) a light valve which comprises a plurality of elements, each of which can be independently switched between a state in which incident light incident from the source is substantially transmitted or reflected and a state in which incident light from the source is substantially scattered to impart an image to the incident light; (c) an illumination light controller which is disposed between the source and the light valve and is adjustable to provide a variable illumination aperture for controlling the degree of collimation of light reaching the light valve from the light source;

(d) projection lens for projecting onto a screen imaged light received from the light valve; and

(e) a projection aperture controller which is adjustable to provide a variable projection aperture for controlling the amount of imaged light transmitted by the light valve which is projected to the screen by the projection lens.

In a preferred embodiment, the projector further comprises a light sensor which senses the intensity of the ambient and the projected light and responsively adjusts the projection aperture of the projection aperture controller such that the real contrast ratio is a maximum. The real contrast ratio may be defined as

off + amb

where P _on and P ₀ ff are the screen brightness values for the "on" and "off states of the light valve, respectively, and can be adjusted by controlling the projection aperture. Pamb is the screen brightness due to ambient light.

Furthermore, the illumination aperture is adjusted to be less than the projection aperture.

In another embodiment, there is provided a projector, comprising: (a) a source of light having a characteristic image size; (b) a light valve which comprises a plurality of elements, each of which can be independently switched between a state in which incident light incident from the source is substantially transmitted or reflected and a state in which incident light from the source is substantially scattered to impart an image to the incident light; (c) projection lens for projecting onto a screen imaged light received from the light valve; and (d) a projection aperture controller which is adjustable to provide a variable projection aperture for controlling the amount of imaged light transmitted by the light valve which is projected to the screen by the projection lens; the image size of the light source being smaller than the smallest aperture to which the projection aperture controller can be adjusted.

Brief Description of the Drawing(s)

Fig. la- lb show schematically projectors of this invention.

Fig. 2 shows in magnified detail several pixels of the light scattering light valve of the projector.

Fig. 3a-3d show the mode of operation of a liquid crystal-polymer composite suitable for use in the light valve of the projector.

Fig. 4 shows how the apertures in a projector of this invention can be adjusted for optimal contrast ratio.

Fig. 5a-5b show how the image of the Hght source can have an effect on the necessity for aperture control.

Description of the Preferred Embodiments

A projector 10 of this invention is shown schematically in Fig. la. Light from light source 11 is collimated and directed by parabolic mirror 12 and lens arrangement 13 towards a Hght-scattering light valve 14. (By creating a secon- dary image of the arc on the aperture plane using lens arrangement 13, the arc size can be reduced by adjusting the illumination aperture. Light valve 14 com¬ prises a plurality of elements or pixels, each of which can be independently switched from a state in which incident Hght arriving from the light source is substantially transmitted (i.e., the element is substantially transparent) or is substantially scattered. Light valve 14 is connected to a controller 15, which may be for example a microcomputer and controls which elements are transmissive or scattering. Disposed between light valve 14 and light source 11 is an illumi¬ nation aperture controller 16, which can be adjusted to provide a variable illumi¬ nation aperture, thereby controlling the degree of collimation of light reaching light valve 14 from Hght source 11. Light transmitted by light valve 14 is pro¬ jected by lens arrangement 17 onto a screen 18 where the image can be viewed. A projection aperture controller 19 is disposed between light valve 14 and screen 18. The aperture of controller 19 is variable, thereby permitting control of the amount of light which is projected by lens arrangement 17 onto screen 18.

Optionally projector 10 includes a Hght sensor 20 which senses the am¬ bient light intensity as well as projected image intensity (denoted by ray 21). Sensor 20 is connected to and controls projection light controller 19 and respon¬ sively adjusts the latter's projection aperture such that the real contrast ratio is maximized under various ambient light conditions. Further the illumination aperture is adjusted such that the illumination aperture is less than the projection aperture as the projection aperture is adjusted by sensor 20. If sensor 20 is not used to automatically control the illumination and projection apertures, the apertures may be controlled manually, mechanically or electromecha- nically, the viewer adjusting them to the settings which provide him or her with the maximum real contrast, i.e., best image quality. Generally, this will occur where the projection aperture is greater than or equal to the illumination aperture. Reducing the projection aperture can increase the contrast ratio when the illumination aperture is less than the projection aperture.

Fig. 2 shows in magnified detail four elements 30a-d of Hght valve 14. Elements 30a, 30c, and 30d are shown in their substantially transparent states, so that incident light is transmitted therethrough. Element 30b is shown in its Hght scattering state, so that it scatters incident light. The scattering may be either forwardly (ray 31a) or backwardly (ray 31b) or, as is normally the case, both. By controlling which elements are in the transparent or scattering states, an image can be imparted to the incident light.

In Fig. la and 2 light valve 14 has been shown as operating in a transmis- sive mode, but for the sake of compactness or other design considerations, light valve can be made to operate in a reflective mode (that is, incident light is either reflected or not), so that the projector may have a folded-up compact design. Such a design is shown schematically in Fig. lb, where numerals repeated from Fig. la indicate like elements. Light passing through aperture controller 16 is reflected by mirror 16 towards lens 26, which focuses the light onto reflective

Hght valve 14'. Light which is reflected by Hght valve 14' is then directed towards projection light controller 19 and lens 17' and projected onto screen 18.

In a preferred embodiment, the light valve comprises a liquid crystal-poly- mer composite as the electrooptically active component. In a liquid crystal-poly¬ mer composite, discrete volumes of a liquid crystal material are encapsulated, dispersed, embedded or otherwise contained in a matrix polymer. The volumes

are not necessarily limited to spherical or substantially spherical ones. They may be irregularly shaped, and even interconnected. The amount of intercon¬ nection between volumes may be to an extent such that the liquid crystal mate¬ rial appears to form a continuous phase. "Liquid crystal material" denotes a material having liquid crystalline properties, whether that composition consists of a single discrete liquid crystalline compound, a mixture of different liquid crystalline compounds, or a mixture of liquid crystalline and non-liquid crystal¬ line compounds. Preferably, the liquid crystal material is nematic or opera¬ tionally nematic. More preferably, it also has a positive dielectric anisotropy.

Suitable matrix polymers include but are not limited to poly( vinyl alcohol) ("PVA") and its copolymers, gelatin, polyurethane, latexes, polyethylene oxide), polyCvinyl pyrrolidone), cellulosic polymers, natural gums, acrylic and metha- crylic polymers and copolymers, epoxies, polyolefins, vinyl polymers, and the like. PVA is a preferred containment medium.

Suitable liquid crystal-polymer composites and their method of preparation are disclosed in U.S. Pat. Nos. 4,435,047 (1984), 4,606,611 (1986), 4,616,903 (1986), and 4,707,080 (1987), all to Fergason; published European patent appHcation EP 156,615 (1985), by Pearlman et al.; U.S. Pat. No. 4,671,618 (1987), to Wu et al.; U.S. Pat. Nos. 4,673,255 (1987) and 4,685,771 (1987), to West et al.; U.S. Pat. No. 4,688,900 (1987) to Doane et al.; and published European patent application EP 0,313,053 (1989), by Dainippon Ink and Chemicals; the disclo¬ sures of each which are incorporated herein by reference.

Fig. 3a-3b show the mode of operation of a Hquid crystal-polymer com¬ posite light valve suitable for use in projectors of this invention. Light valve 60 comprises a Hquid crystal-polymer composite 61 in which droplets or volumes 62 of nematic Hquid crystal material 63 having a positive dielectric anisotropy are dispersed in a matrix polymer 64. Composite 61 is sandwiched between first and second electrodes 65a and 65b, made from a transparent conductor such as indium tin oxide ("ITO"). The application or not of a voltage across electrodes 65a and 65b from power source 66 is controlled by switch 67, shown in Fig. 3a in the open position ("off-state"). As a result, no voltage is impressed across com- posite 61 and the electric field experienced by Hquid crystal material 63 is effectively zero. Due to surface interactions, the Hquid crystal molecules prefe¬ rentially lie with their surfaces parallel to the curved interface with matrix poly-

mer 64, resulting in a generally curvilinear alignment within each droplet. The curvilinear axes in different droplets 62 are randomly oriented, as symbolized by the differing orientations of the curvilinear patterns. Liquid crystal material 63 has an extraordinary index of refraction n _e which is different from and but an ordinary index of refraction n ₀ which is the substantiaUy the same as index of refraction n _p of the matrix polymer. Incident light ray 68 traveling through composite 61 has a high statistical probability of encountering at least one inter¬ face between matrix polymer 64 and Hquid crystal material 63 in which the Hquid crystal index of refraction with which it operatively interacts is n _e . Since n _e is different from the index of refraction n of the containment medium, there is refraction, or scattering of Hght ray 68, both forwardly and backwardly, causing composite 61 to have a translucent or frosty appearance.

Fig. 3b shows light valve 60 in the on-state, with switch 67 closed. An electric field is applied between electrodes 65a and 65b and across composite 61, with a directionality indicated by arrow 69. Liquid crystal material 63, being positively dielectrically anisotropic, aligns parallel to the electric field direction. (The required voltage is dependent inter alia on the thickness of the composite and typicaUy is between 3 and 50 volts.) Further, this alignment with the field occurs in each droplet 62, so that there is order among the directors from droplet to droplet, as shown symbolically in Fig. 3b. When the Hquid crystal molecules are aligned in this manner, the Hquid crystal index of refraction with which incident light ray 68 operatively interacts is no. Because n ₀ is substantially the same as n _p , there is no scattering at the Hquid crystal-containment medium interface. As a result, ray 18 is transmitted through composite 61, which now appears transparent. Transmission rates of at least 50 %, and preferably on the order of 70 % or higher may be attained.

The Hght valve of Fig. 3a-3b can be readily modified for operation in the reflective mode. Two such modifications are shown in Fig. 3b and 3c (numerals like those in Fig. 3a-3b denoting like elements). In the embodiment of Fig. 3c, rear electrode 65b' is not transparent, but reflective. Thus, when composite 61 is in its light transmissive state, the Hght transmitted therethrough is reflected by rear electrode 65b' and again through composite 61. In the embodiment of Fig. 3d, rear electrode 65b is transparent, but a reflector 70 is positioned therebehind, with similar operational effect.

In evaluating the performance of a projector, it is common to measure an ideal contrast ratio that is measured in a totally dark room. However, when a projector is used in a lighted room, picture quality is determined not by this ideal contrast ratio but by a real contrast ratio, which factors in the amount of ambient lighting. If the ambient light significantly increases the black (off- state) light level in the projected image, the real contrast ratio may be much lower than the ideal contrast ratio. The projection onto the screen of stray light which has been forwardly scattered by the light valve may also contribute to decreasing the blackness of the off-state. Therefore, to maximize the real contrast ratio, the projector needs to be operated at an optimum aperture (f-stop) for a given ambient light condition. The (f-stop) is optimal when the real contrast ratio, given by the relationship

off + amb

becomes a maximum. A projector which has a fixed aperture, which is "fac¬ tory-set," as it were, to provide good contrast for a given set of ambient light con¬ ditions, may provide unsatisfactory contrast ratio under a different set of am¬ bient Hght conditions. For example, referring now to Fig. 4, it can be seen that to maximize the real contrast ratio, the projection aperture can be adjusted to f/2.5, f/4, and 177.5, for ambient Hght levels of 100, 10, and 1 lumens, respectively.

Generally, the desirability of having a controllable illumination aperture is linked to the necessity of reducing the effective light source image size in order to achieve high contrast ratios. This is shown schematically in Fig. 5a, where numerals repeated from prior figures denote like elements. In the figure Hght source 11 is of a relatively large size, such that its effective image size 71 is larger than the smallest aperture to which the projection aperture controller can be set. In such an instance, an adjustable illumination aperture controller 16 is needed to control the illumination aperture so that it is as large as possible (for enhancing brightness) but yet smaller than the projection aperture (for enhancing contrast).

A different situation is shown in Fig. 5b, where Hght source 11' is of a relatively small size, such that it has an image size 72 smaller than the smallest settable projection aperture. In such an instance, an illumination aperture

controller 16 is not needed to control the size of the illumination aperture and hence controller 16 is shown as a dotted line.

Preferably the light valve is of the active matrix type, in which each pixel (or picture element) is driven (switched from one visual state to another) by an active switching element such as a thin film transistor ("TFT"), varistor, diode, or metal-insulator-metal element ("MIM"). The switching element helps elimi¬ nate cross-talk and maintain an initially applied voltage across the corres¬ ponding pixel, even when it is not being actively addressed, so that the pixel stays "on" while other pixels are addressed. The longer the pixels holds the ini¬ tially applied voltage, the longer it can be maintained in the "on" state until it is next addressed, permitting the construction of displays having a larger number of pixels. If the matrix contains a sufficiently large number of switching ele¬ ments of sufficiently small size, high resolution displays are possible.

Active matrix driven projectors of this invention are useful as television, computer, or instrument screens or monitors. Active matrix displays and light valves employing Hquid crystal-polymer composites are disclosed in Becker et al., US 5,124,822 (1992), and Kamath et al., US 5,233,445 (1993), the disclosures of which are incorporated herein by reference.

The foregoing detailed description of the invention includes passages which are chiefly or exclusively concerned with particular parts or aspects of the invention. It is to be understood that this is for clarity and convenience, that a particular feature may be relevant in more than just passage in which it is disclosed, and that the disclosure herein includes all the appropriate com¬ binations of information found in the different passages. Similarly, although the various figures and descriptions thereof relate to specific embodiments of the invention, it is to be understood that where a specific feature is disclosed in the context of a particular figure, such feature can also be used, to the extent appropriate, in the context of another figure, in combination with another feature, or in the invention in general.