技术功效语段：
[0008]The measures according to the invention have the effect that the shape of the second part of the discharge vessel of the high-pressure discharge lamp forms a refractive element, or lens. Light transmitted by the second part is redirected due to the refractive character of the second part. The shape of the second part is chosen to be such that the refractive character of the second part reduces an angular distribution of the light impinging on the light exit window. In the known illumination systems, the high-pressure discharge lamp is constituted by two substantially identical parts. The two parts generally have a conical shape and are produced by using the same production molds so that they are identical within the accuracy parameters of the production process used. At the narrow end of the conical shape, an electrode protrudes through the wall of the part of the discharge vessel. The known high-pressure discharge lamp is produced by connecting the wide ends of two conically shaped parts of the discharge vessel. Light emitted by a known high-pressure discharge lamp and impinging on the back reflector at a distance from the optical axis typically propagates towards the light exit window at a relatively large angle to the normal axis of the light exit window, and will thus impinge on the light exit window at a substantially large angle. This creates a relatively large angular distribution of the light impinging on the light exit window around the normal axis of this window. Due to this relatively large angle of the impinging light, part of the light may not be able to propagate through the remainder of the optical system which typically accepts only a limited range of angles of incidence. This will reduce the efficiency of the known illumination system. In the illumination system according to the invention, the first and the second part have a different shape. The shape of the second part is chosen to form a refractive element redirecting the light emitted by the discharge arc towards the back reflector so that the redirected light impinges on the back reflector at an angle closer to a normal axis of this reflector. The subsequently reflected light will propagate towards the light exit window and impinges on the light exit window at an angle closer to the normal axis of this window, thus reducing the angular distribution of the light impinging on the light exit window. Due to the reduction of the angular distribution of the impinging light on the light exit window, less of the reflected light may be lost, which enhances the brightness of the illumination system according to the invention.
[0012]The inventors have found that the efficiency of the known illumination system is mainly limited by two different effects. A first effect is the relatively large angular distribution at the light exit window, which is mainly caused by light transmitted by the second part of the discharge vessel. A second effect is the relatively large magnification of the image of the discharge arc at the light exit window, which may cause loss of light. This second effect is mainly caused by the light which is transmitted by the first part of the discharge vessel. By choosing a specific shape of both the first and the second part of the discharge vessel in the high-pressure discharge lamp of the illumination system according to the invention, both the angular distribution at the light exit window and the magnification of the image of the discharge arc are reduced. As a result, the efficiency of the illumination system according to the invention is increased.
[0014]In an embodiment of the illumination system, the discharge vessel comprises a wall having an outer surface and an inner surface, a shape of the outer surface of the second part being substantially identical to the shape of the outer surface of the first part, and a shape of the inner surface of the second part being different from the shape of the inner surface of the first part, thereby forming the refractive element in the second part. This embodiment has the advantage that it is relatively easy to produce. Generally, the discharge vessel is constituted by two halves each having substantially cylindrical inner walls. By pushing the two halves together at a high temperature so as to obtain the discharge vessel, the inner wall is pushed out to form an inner curved wall. By simply altering the pressure at which the two halves are pressed together during the production process, the curvature of the inner wall may thus be adapted and controlled.
[0016]In an embodiment of the illumination system, the inner diameter of the second part at a range of distances from the focal point is at least 10% larger than the inner diameter of the first part at matching distances in a matching range of distances from the focal point on the opposite side of the focal point. An asymmetry of at least 10% results in a measurable improvement of the efficiency and typically exceeds the production process window of contemporary production processes.
[0017]In an embodiment of the illumination system, the inner wall of the first part and/or the inner wall of the second part of the discharge vessel in a cross-sectional view along a plane comprising the optical axis is convexly shaped towards the discharge arc, or is concavely shaped towards the discharge arc, or is linearly shaped. When the first part and/or the second part are convexly shaped, the wall of the discharge vessel is relatively far remote from the discharge arc, resulting in a relatively low temperature of the wall of the discharge vessel and thus limiting the strain in the discharge vessel material between a situation in which the high-pressure discharge lamp is switched on and a situation in which the high-pressure discharge lamp is switched off. A substantially linear shape of the first part and/or the second part has the advantage that the asymmetric discharge vessel can be manufactured relatively easily because the initial shape of a quartz tube before shaping is a substantially hollow cylinder shape with straight inner walls. During manufacture of the discharge vessel, the inner wall of the discharge vessel may not become hot enough to produce a convex or concave shape.
权利要求：
1. An illumination system (100) comprising a high-pressure discharge lamp (80, 82, 84, 86) at least partially surrounded by a back reflector (30) which is capable of reflecting light emitted by the high-pressure discharge lamp (80, 82, 84, 86) towards a light exit window (50) of the illumination system (100), - the back reflector (30) having an optical axis (55), - the high-pressure discharge lamp (80, 82, 84, 86) comprising a discharge vessel (90, 92, 94, 96) enclosing a discharge space and comprising two electrodes (98, 99) between which, during operation, a discharge arc is produced, the discharge arc being located substantially at a focal point (40) of the back reflector (30) on the optical axis (55), - the discharge vessel (90, 92, 94, 96) comprising a first part (10, 14) arranged at least partially between the discharge arc and the back reflector (30), and a second part (20, 22, 24) arranged at least partially between the discharge arc and the light exit window (50), the second part (20, 22, 24) having a different shape compared to the first part (10, 14), thereby forming a refractive element in the second part (20, 22, 24) for reducing an angular distribution at the light exit window (50) of the light emitted from the discharge arc and refracted by the second part (20, 22, 24), characterized in that the back reflector (30) is an ellipsoidal back reflector (30) having the focal point (40) and a further focal point (45), wherein the ellipsoidal back reflector (30) comprises spherical aberrations for redirecting the light transmitted by the second part (20, 22, 24) and/or the first part (10, 14) towards the further focal point (45).
2. An illumination system (100) as claimed in claim 1, wherein the first part (10, 14) of the discharge vessel (90, 92, 94, 96) forms a further refractive element for reducing a size of an image of the discharge arc, the image being produced by light refracted by the first part (10, 14) and reflected from the back reflector (30).
3. An illumination system (100) as claimed in claim 1, wherein the spherical aberrations comprise first-order aberrations and/or second-order aberrations and/or third-order aberrations.
4. An illumination system (100) as claimed in claim 1, 2, or 3, wherein the discharge vessel (90, 92, 94, 96) comprises a wall having an outer surface (60, 62) and an inner surface (70, 72, 74, 76, 78), a shape of the outer surface (62) of the second part (20, 22, 24) being substantially identical to the shape of the outer surface (60) of the first part (10, 14), and a shape of the inner surface (72, 74, 78) of the second part (20, 22, 24) being different from the shape of the inner surface (70, 76) of the first part (10, 14), thereby forming the refractive element in the second part (20, 22, 24).
5. An illumination system (100) as claimed in claim 4, wherein an inner diameter (d20) of the second part (20, 22, 24) at a distance (x) from the focal point (40) is at least 10% larger than an inner diameter (d10) of the first part (10, 14) at the same distance (-x) from the focal point (40) on an opposite side of the focal point (40), the inner diameter (d10, d20) of the first part (10, 14) and the second part (20, 22, 24) being defined in a direction substantially perpendicular to the optical axis (55).
6. An illumination system (100) as claimed in claim 5, wherein the inner diameter of the second part (20, 22, 24) at a range (Δx) of distances from the focal point (40) is at least 10% larger than the inner diameter of the first part (10, 14) at matching distances in a matching range (-Δx) of distances from the focal point (40) on the opposite side of the focal point (40).
7. An illumination system (100) as claimed in any one of the preceding claims, wherein the inner wall (70, 76) of the first part (10, 12, 14, 16) and/or the inner wall (72, 74, 78) of the second part (20, 22, 24, 26) of the discharge vessel (90, 92, 94, 96) in a cross-sectional view along a plane comprising the optical axis (55) is convexly shaped towards the discharge arc, or is concavely shaped towards the discharge arc, or is linearly shaped.
8. A high-pressure discharge lamp (80, 82, 84, 86) comprising all the characteristics of the high-pressure discharge lamp as defined in the illumination system (100) as claimed in claims 1, 4 and 5 or 1, 4 and 6.
9. An image projection system (110) comprising the illumination system (100) as claimed in claims 1 to 7.