Illumination – Plural light sources – With modifier
Reexamination Certificate
1999-03-31
2001-03-06
Sember, Thomas M. (Department: 2875)
Illumination
Plural light sources
With modifier
C362S247000, C362S301000, C362S298000, C362S346000
Reexamination Certificate
active
06196699
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an illumination system, and more particularly relates to a collimated-light illumination system suitable for use in color projection display systems employing one or more electro-optic light modulators.
Most color projection display systems employing one or more electro-optic light modulators, also employ a single-lamp white light illumination source, a first se of dichroic filters arranged to separate the white light from the source into primary (red, blue and green) color components, and a second set of dichroic filters to recombine the components after modulation.
The light modulators usually are active matrix liquid crystal display panels, either transmissive or reflective, and the light source is a high intensity discharge lamp with a small arc length. This small arc length results in a relatively compact illumination beam having a low etendue. Etendue is a term used in optics to define the geometrical limits of an illumination beam, in terms of both area and angular extent. The low etendue of the illumination beam enables the display panel to utilize a large portion of the beam despite the relatively small panel size (approximately 1.3 inches or less).
Considerable effort has been made to further reduce the size, and consequently the cost, of these complex display panels. As the size of such panels are reduced, the arc length of the lamp must also be reduced, in order to reduce the etendue of the illumination beam, thereby maintaining an adequate lumen output of the projection system.
Unfortunately, as arc length is reduced in size, the current density in the lamp electrodes increases until a practical limit is reached. Unfortunately, at the present state of the art of lamp technology, this practical limit dictates arc lengths which result in projected images of marginal brightness.
In the published abstract 6-242397 of Japanese Patent Application No. 5-29320, there is shown a projection display device having two light sources
20
and
21
, whose respective illumination beams are condensed on opposed reflection surfaces
27
and
28
by condensing reflectors
31
and
32
. The beams are thence reflected by surfaces
27
and
28
into a condensing lens system
33
.
Due to the proximity of the reflection surfaces to each other and to the object point of the condensing lens system
33
, the respective illumination beams are said to be made incident on lens system
33
in a state of superposition. However, due to the fact that the points of reflection of the two beams from the reflection surfaces are separated from one another, the state of superposition is not perfect, and the etendue of the superposed beams is increased.
Moreover, the need to locate the reflecting surfaces close to the object point of the condensing lens severely restricts the design freedom of such a system.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a dual lamp illumination system which overcomes the disadvantages of the prior art.
It is another object of the invention to provide a dual lamp illumination system which does not increase the etendue of the illumination beam.
It is another object of the invention to provide a projection display system incorporating such a dual lamp illumination system.
In accordance with the invention, there is provided a dual lamp illumination system in which two lamps having collimating reflectors are arranged so that their beams overlap to form a first composite beam along at least a portion of an optical path. A pair of plane mirrors are then moved into the path of the composite beam in order to reflect part (eg., one half) of the composite beam out of the beam path. The mirror surfaces are angled so that a new composite collimated beam is formed.
The light rays in the portion of the composite beam which are not reflected into the path of the new composite beam then cycle between the lamp reflectors until they pass into the portion of the composite beam which is in the path of the plane mirror surfaces, whence they are reflected out into the new composite beam.
Such a dual lamp illumination system has the advantage that light output is almost doubled, while the collimation of the original combined beams is substantially maintained. Thus, the lumen output of a projection display system employing such an illumination system can be substantially increased over one with a single lamp, without the need for costly collimating lenses.
In the case in which one half of the first composite beam is reflected, there is substantially no or very little increase in the cross-sectional area of the new composite beam. Since the collimation is also substantially maintained, there is also substantially no or very little increase in the etendue of the beam.
In accordance with a preferred embodiment of the invention, further increases in lumen output are obtainable by moving the mirrors to reflect slightly more than one half of the first composite beam. For example, moving the mirrors further into the beam by an amount of up to 4 times (eg., ½ to 2 times) the diameter of the light source result in an increase in lumen output of as much as 20 to 30 percent. However, the cross-sectional area of the beam increases, thus decreasing etendue. Adjusting the position of the mirrors within the above range can enable the realization of an optimum trade-off between increased lumen output and reduced etendue for a particular projection system.
In accordance with another feature of the invention, the plane mirror surfaces can be rotatable about an axis corresponding to the proximal edges (or common edge) of the plane mirror surfaces, enabling directing or
steering
of the new composite beam in any desired direction within the angle of rotation, without divergence and with only a moderate increase in cross-sectional area, and thus a moderate increase in etendue.
As an alternative to operating the lamps in the illumination system of the invention at or near full power in order to obtain increased lumen output of the system, the lamps could be operated at reduced power in order to increase the operating life of the lamps. Such reduced-power operation also enables continued operation in the event of the failure of one of the lamps, by simply operating the remaining lamp at full power. If desired, the plane mirror surfaces can be moved further into the beam path to reflect the entire beam from the operating lamps.
Preferably, the lamps are of substantially identical type and size, and are arranged facing each other. Alternatively, the optical paths of the lamps can be folded using one or more mirrors. Preferably, the plane mirror surfaces employed to reflect the lamps' composite beam are formed on two adjacent sides of a cube or half cube.
In accordance with the broad aspects of the invention, the dual lamp illumination system comprises:
a first lamp and a second lamp, each lamp comprising a light source and a collimating reflector for forming a collimated illumination beam, the lamps being positioned with respect to one another so that the beam of the first lamp is directed into the reflector of the second lamp, and the beam of the second lamp is directed into the reflector of the first lamp, in a manner so that the beams at least partially overlap to form a first composite beam;
first and second plane mirror surfaces oriented adjacent to one another and positioned so as to each reflect a portion of the first composite beam in a different but common direction along a common axis, to form a second composite beam.
In accordance with another aspect of the invention, there is provided a projection display system comprising at least one light modulating display panel, an illumination system for illuminating the display panel, and a projection lens for projecting the modulated light onto a display surface, characterized in that the illumination system comprises the dual lamp illumination system of this invention.
REFERENCES:
patent: 1728558 (1929-09-01), Gilson
patent: 2497676 (1950-02-01), Las
Faller F. Brice
Philips Electronics North America Corp.
Sember Thomas M.
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