Optics: image projectors – Reflector
Reexamination Certificate
2002-10-23
2004-03-16
Dowling, William C. (Department: 2851)
Optics: image projectors
Reflector
C353S034000, C348S771000
Reexamination Certificate
active
06705734
ABSTRACT:
The present invention relates to an optic system of illumination for a videoprojector based on DMD (Digital Micromirror Device) technology.
Videoprojection systems based on DMD (Digital Micromirror Device) technology are increasingly spreading, above all for the excellent image quality they are able to obtain, in particular for the brightness and resolution of the image itself, and also for their smaller projectors sizes compared e.g. to the devices utilizing kinescopes. A DMD device consists essentially of a set of aluminum square mirrors, with a side of micrometric size, e.g. 16 &mgr;m, each one associated to an element of the image to be projected. i.e. to a pixel. Said mirrors can have a small angle rotation around a diagonal, such as ±10 degrees, where rotation in either direction is produced by two electrodes located under the mirror in opposite positions with respect to the rotating axis. Therefore, the light hits the mirror with an angle of about 20 degrees with respect to the perpendicular to the mirror plane when the latter is in its “rest” condition, i.e. not attracted by any of the two electrodes. If the mirror is rotated in one direction, the reflected ray undergoes a deflection that does not affect in the projection lens and therefore, it is not sent to the screen. Therefore, the corresponding pixel is “off”. If rotation occurs in the opposite direction the pixel is “on”, since the reflected light affects on the projection lens and is sent to the screen.
To each pixel of the image is associated a cell of a static memory of the type SRAM (Static Random Access Memory), containing the information for directing the electrodes that cause mirror rotation. Even if the reflected light always has the same intensity, changing the time during which a pixel remains “on”, is obtained the effect of a luminosity change due to the integrating action produced by the human eye. A videoprojector may comprise only one DMD device, in which case its mirrors are illuminated sequentially by the three primary colours, i.e. red, green and blue, which are obtained sending the light of the lighting lamp to a revolving wheel, called colour wheel, divided in at least 3 segments, each one consisting of a dichroic filter, i.e. selective with respect to the wavelength, related to one of the 3 primary colours. Wheel rotation causes the light beam sent to the DMD device to take all three different colours sequentially. In the event, vice-versa, of a videoprojector with three DMD devices, the light of the lighting lamp is split in the three colours by a prism, and each colour is sent to a different DMD device.
In a DMD videoprojector, the choice of the optic system of illumination has taken on particular importance, since both the dimensions and utilization procedures of the videoprojector itself depend on it.
A first known illumination system is illustrated in
FIG. 1
by means of a basic diagram. This basic diagram is plotted assuming that a videoprojector
21
is placed in horizontal position for frontal projection to a vertical screen
22
, and therefore, said diagram corresponds to a plan view of said videoprojector
21
: this premise applies to all subsequent figures, unless otherwise specified. Moreover, for clarity's sake, the blocks indicated with the same reference number in the various figures have the same function. With reference number
1
is indicates a lighting lamp with a parabolic reflector, number
2
indicates an aspheric condenser focusing the light at the input of an integrating rod
4
consisting of an optic glass parallelepiped, whose function is to obtain a uniform light beam from the lighting lamp
1
. The integrating rod
4
is preceded by a colour wheel
3
, which, as said above, allows the reproduction of the colours through its dichroic filters in those videoprojectors using only one DMD device, as in the example of FIG.
1
. In some instances, the distance from the lighting lamp
1
to the colour wheel
3
is closed by a collector, not shown in the figure, whose purpose is to hinder that reflected rays are spread in the surrounding space illuminating the environment The output light from the integrating rod
4
is collected by a lens system, in the specific instance three converging lenses, known as relay lens, and indicated collectively with the reference number
5
. Said lenses
5
, along with a mirror
6
and a prism
7
convey the light emitted by the lighting lamp
1
towards an image microforming device. i.e. a DMD device, indicated with number
9
, on which is formed an focused image which is enlarged with respect to the one at the output of the integrating rod
4
. This illumination diagram, in which the focusing occurs on the image microforming device
9
, is known as a critical or Abbe's illumination. The optical path from the lighting lamp and the image microforming device
9
undergoes two deflections: a first deflection due to the reflecting surface of the mirror
6
; and a second deflection due to the prism
7
. Said prism
7
conveys the light beam towards the image microforming device
9
with an angle of about 20 degrees, as requested by the manufacturer's specifications for the image microforming device
9
. The prism
7
is a common prism, such as TIR (Total Internal Reflection), i.e. operating with full reflection, for the presence of an air layer of about 10 &mgr;m separating it from a second prism indicated with reference number
8
. Said prism
8
deflects the light beam coming from the micromirrors on the surface of the image microforming device
9
towards a projection lens indicated with the reference number
10
, which projects the image on a vertical screen
22
.
A dotted line in
FIG. 1
also indicates the optical path of the light beam emitted by the lighting lamp
1
. A first segment AB, directed along the illumination axis of the lighting lamp
1
, departs from a point A in line with said illumination lamp
1
to reach a point B in line with the mirror surface
6
. Said first segment AB lies in a first plane indicated with P
1
in
FIG. 1
a
, where a basic perspective view of the optical path is reported within the videoprojector
21
.
Then the light beam is deflected upwards by the mirror
6
, as it can be clearly noticed in
FIG. 1
a
, and reaches a point C pertaining to a second plane P
2
located on the prism
7
, wherefrom it is reflected to a point D pertaining to the surface of the image microforming device
9
. As mentioned above, the image is formed by the image microforming device
9
modulating the light beam. Finally, said modulated light beam reaches a point E directly outside of the projection lens
10
, i.e. identifying a projection segment DE, which is part of the projection axis. The extension of the segment DE reaches the screen
22
.
It should be noticed that the mirror
6
deflects the optical path upwards along the segment BC, i.e. it is inclined, appearing in the plan view like a rectangle instead of a segment. The above deflection is quite a significant one to prevent that any large sized components, such as the prisms
7
and
8
and the image microforming device
9
, bearing an associated rather voluminous piloting card not shown in
FIG. 1
, may interfere with the optical path of the light beam along the segment AB and/or the integrating rod
4
. This illumination system may also be used in a mirror back projection configuration. i.e. the configuration where the image is projected upwards, since the illumination axis of the lighting lamp
1
is horizontal and substantially perpendicular to the projection axis and therefore, placing the videoprojector
21
upright, so that the projection lens
10
sending the image upwards, not change the position of the lighting lamp
1
, which, in the position illustrated in
FIG. 1
, is in its optimal condition for heat dissipation, warranting a long service life of the videoprojector
21
.
However, the illumination system for a videoprojector according to
FIG. 1
has the drawback of excessive overall dimensions, particularly for its height, which is d
Dowling William C.
Levine & Mandelbaum
SIM2 Multimedia S.p.A.
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