Xenon ceramic lamp with integrated compound reflectors

A xenon ceramic lamp comprising a short-arc lamp with two integral reflectors disposed around the cathode arc ball to collect a wide range of elevation angles of light relative to the center longitudinal axis. The two integral reflectors and the cathode arc ball are within the same sealed volume of the lamp. A first reflector, generally below a common first focus, is a concave elliptical type for projecting light out through a sapphire window to a second focus. A second reflector, generally above the first focus, is a concave spherical type having its focus just offset from the first focus. Therefore, light rays may be emitted at nearly all angles from the cathode arc ball that will be reflected or back reflected by the elliptical and spherical reflectors.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates generally to xenon short-arc ceramic lamps and
 specifically to such lamps which incorporate a spherical-elliptical
 reflector combination in a compound system to improve efficiency.
 2. Description of the Prior Art
 Short arc lamps provide intense point sources of light that allow light
 collection in reflectors for applications in medical endoscopes,
 instrumentation and projection. Also, short arc lamps are used in
 industrial endoscopes, for example in the inspection of jet engine
 interiors.
 A typical short arc lamp comprises an anode and a cathode positioned along
 the longitudinal axis of a cylindrical, sealed concave chamber that
 contains a gas pressurized to several atmospheres. U.S. Pat. No.
 4,633,128, issued Dec. 30, 1986, to Roy D. Roberts, the present inventor,
 and Robert L. Miner, describes such a short arc lamp in which a copper
 sleeve member is attached to the reflecting wall to conduct heat from the
 reflecting wall through to the exterior wall and eventually to circulating
 ambient air.
 U.S. Pat. No. 4,305,099, describes a light collection system for
 projectors, such as light valve projectors, which have a compound
 reflector associated with an arc lamp. The compound reflector includes an
 ellipsoidal reflector positioned to collect a portion of the light from
 the arc lamp and reflect a direct image of the light in a beam to an image
 forming plane of the projector and a spherical reflector positioned to
 collect another portion of the light from the arc lamp and reflect it back
 through the gap of the arc lamp to the ellipsoidal reflector to be
 reflected as a secondary image of the light from the lamp in the beam. The
 ellipsoidal and spherical reflectors are formed as full, uninterrupted
 surfaces of revolution. To provide uniform light distribution, the beam is
 directed through a pair of spaced lens plates, each having corresponding
 arrays, in rows and columns, of rectangular lenticules. The adjacent focus
 of the ellipsoidal reflector is centered in the arc, while the center of
 curvature of the spherical reflector, in order to avoid transmission loss
 through the arc, is displaced to a portion of the gap of the lamp which is
 relatively free of the arc. For maximum light efficiency, the direct image
 is focused just to one side, and the secondary image is focused just to
 the other side of the image forming plane. Such patents are all
 incorporated herein by reference.
 Conventional lamps with parabolic collector/reflectors have the advantage
 of good collection and distribution efficiency when used in conjunction
 with a lens for focusing. However, such combinations can be too expensive
 for many applications. Conventional lamps with elliptical
 collector/reflectors have a different kind of problem. In order to collect
 a large polar angle of the lamp output, a wide spread of arc
 magnifications are automatically generated at the second focus. The rays
 with the smallest angles have the largest magnification. And the rays with
 the largest angles have the smallest magnification.
 The collection efficiency of conventional elliptical collector/reflectors
 is good, but the distribution efficiency is often poor. In a compound
 reflector geometry that combines reflector types, the elliptical part is
 usually a rather shallow dish that provides a small spread of arc
 magnifications over a select spread of ray angles. But the polar angle
 collection of such a lamp's output is rather poor from the ellipse.
 SUMMARY OF THE PRESENT INVENTION
 It is therefore an object of the present invention to provide a xenon
 ceramic lamp that is more efficient than conventional designs.
 Briefly, a ceramic lamp embodiment of the present invention comprises a
 short arc lamp with two integral reflectors disposed around the cathode
 arc ball to collect a wide range of elevation angles of light relative to
 the center longitudinal axis. The two integral reflectors and the cathode
 arc ball are all within the same sealed volume of the lamp. A first
 reflector, generally below a common first focus, is a concave elliptical
 type that projects light out through a sapphire window to a second focus.
 A second reflector, generally above the first focus, is a concave
 spherical type that has its focus just offset from the first focus.
 Therefore, light rays emitted at nearly all angles from the cathode arc
 ball will be reflected or back reflected by the elliptical and spherical
 reflectors.
 An advantage of the present invention is that a ceramic lamp is provided in
 which no lamp envelope exists to interfere with the optimum reflection of
 rays from the spherical back reflector.
 Another advantage of the present invention is that a ceramic lamp is
 provided which is more efficient than the quartz lamps or other types of
 separate envelopes and compound reflectors.
 These and other objects and advantages of the present invention will no
 doubt become obvious to those of ordinary skill in the art after having
 read the following detailed description of the preferred embodiment which
 is illustrated in the drawing figure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 FIG. 1 illustrates a xenon short-arc lamp embodiment of the present
 invention, referred to herein by the general reference numeral 10. A
 principle purpose and use of the lamp 10 is to illuminate a small-aperture
 light valve 12, e.g., as are used in projection television receiver
 systems. The lamp 10 comprises a ceramic body 14 forming a concave
 elliptical reflector 16, a metal envelope 18 forming a concave spherical
 back reflector 20, a sapphire window 22, a cathode 24, an anode 26, and a
 bulk-copper anode base 28. In operation, a light beam 30 is brought to a
 "second" focus 32. A cathode arc ball 34 is shown for reference. The
 envelope 18 is preferably made of metal because metal is more readily
 fashioned and less expensive compared to ceramic materials. A multilayer
 dichroic cold mirror coating over an absorbing layer for spherical back
 reflector 20 may be preferable to reduce the infrared output of the lamp
 10 and to reduce heat delivered back to the cathode arc ball 34.
 The arc can be optimized to extend lifetime over source radiance or
 brightness. Xenon lamps have more than adequate source brightness to
 satisfy most video light valve apertures. A compound reflector improves
 the effective source brightness. So with more than enough brightness, the
 reflector and overall size can be reduced for the benefit of lifetime.
 Lower pressures and larger cathode tip radii will thereby reduce cathode
 tip erosion and improve lifetime.
 Conventional xenon short-arc lamps with integral ceramic reflectors are
 often made of alumina. The ceramic body 14 is preferably constructed of an
 alumina "toughened" with zirconia. Such zirconia toughened alumina (ZTA)
 is marketed by Coors Ceramics. A transformation toughened aluminum oxide
 (GTC-TA) is similarly marketed by Diamonite Products. The advantage of
 these toughened aluminas is their greater resistance to thermal shock,
 their tensile strength, and their flexural strength, compared with
 ordinary alumina used in prior art ceramic lamps. The use of
 zirconia,toughened alumina significantly improves thermal management,
 which is one of the biggest design challenges for a short arc lamp. In
 alternative embodiments of the present invention, aluminum nitride is used
 in the construction of the ceramic body 14.
 In embodiments of the present invention, an integrated spherical back
 reflector 20 provides for a wider collection angle in elevation from the
 cathode arc ball 34. The center of curvature of the sphere in the
 integrated spherical back reflector 20 is preferably coincident at the
 focus of the ellipse reflector 16. Preferably, any light rays emitted at
 elevation angles that are not collected by the ellipse reflector 16 will
 be captured by the spherical back reflector 20 and reflected back through
 the cathode arc ball 34 to the ellipse reflector 16 and on to the second
 focus 32. The efficiency of the rays collected by the spherical back
 reflector 20 is less than the ellipse reflector 16 since they are
 reflected by the sphere and must also pass through the arc. Light is
 passed back through the cathode ball of the arc, the absorption can be as
 high as eighty percent or perhaps more. The center of curvature of the
 spherical back reflector 20 is not collocated with the focal point of the
 ellipse reflector 16, but is offset about 0.015 inches. The exact amount
 of offset depends on the arc conditions of pressure, current, cathode tip
 radius, and reflector coatings. However, the optimum offset is empirically
 determinable.
 In an alternative embodiment of the present invention, the ceramic body 14
 with concave elliptical reflector 16, and metal envelope 18 with concave
 spherical back reflector 20 can be coaxially placed outside a sealed gas
 tube housing just the anode-cathode combination. In such a case, the
 sapphire window 22 becomes unnecessary.
 Although the present invention has been described in terms of the presently
 preferred embodiments, it is to be understood that the disclosure is not
 to be interpreted as limiting. Various alterations and modifications will
 no doubt become apparent to those skilled in the art after having read the
 above disclosure. Accordingly, it is intended that the appended claims be
 interpreted as covering all alterations and modifications as fall within
 the true spirit and scope of the invention.