Color mixing rod integrator in a laser-based projector
A system for providing illumination includes a color mixing rod integrator for receiving and homogenizing laser light or output to a spatial light modulator. The rod integrator recombines the light of the primary colors, thus obviating dichroic recombination optics. Entrance and exit faces of the integrator have reflective layers where the length of the integrator is decreased and/or the f-number in the system is increased.
1. A color mixing rod integrator for a plurality of beams of laser light of a plurality of primary display colors, comprising:
a rod-shaped body having an interior that passes laser light therethrough and an interior surface that reflects light incident thereon;
an entrance face covered with a first reflective coating to reflect light incident thereon back into the interior of the rod-shaped body and positioned on a first end of the rod-shaped body having a plurality of holes therein such that each hole of said plurality of holes allows light of at least one beam of said plurality of beams to pass therethrough into the interior of the rod-shaped body;
an exit face positioned on a second end of the rod-shaped body opposite to and parallel to the entrance face, said exit face being configured to allow a part of light incident thereon to pass therethrough; and
at least one lens for diverging each of said beams toward said exit face through said interior,
wherein part of the laser light entering said color mixing rod integrator via said plurality of holes is forced to pass through the interior thereof several times before passing through said exit face and then exiting through said exit face as exit light, and wherein the exit light is homogenized by the color mixing rod integrator.
2. The color mixing rod integrator of claim 1 , wherein said at least one lens includes at least one positive lens positioned at said plurality of holes such that divergence of a beam at end of a first pass through the color mixing rod integrator is a divergence selected from the group consisting of a spot across entirety of the exit face and a spot of a predetermined diameter on the exit face.
3. The color mixing rod integrator of claim 2 , wherein:
said exit face is covered with a partially reflective coating that allows a part of light incident thereon to pass therethrough and reflects a part of light incident thereon back into the rod-shaped body toward a source of said light; and
said first reflective coating on said entrance face has a reflectivity which is higher that a reflectivity of the partially reflective coating.
4. The color mixing rod integrator of claim 3 , wherein:
the first reflective coating is selected from the group consisting of silver and a dielectric stack; and
the plurality of holes each has a diameter that exceeds the diameter of the laser light that can be focused into a first hole of the plurality of holes by at least a factor of two.
5. The color mixing rod integrator of claim 4 , wherein:
said laser light is focused to enter each of the plurality of holes with a half angle θ 1/2 <20 degrees in air;
each of said plurality of holes has a diameter ≧2d where d is determined by the equation
d
=
1.22
λ
2
tan
(
θ
1
/
2
)
;
to minimize diffraction effects; and
said second reflective coating is dimensioned to have a transmission coefficient T e that is lower than 50% and a reflection coefficient R e =1−T e such that part of the light is transmitted and part is reflected back into the color mixing rod integrator thereby,
wherein the reflected light travels back to the entrance face and is reflected for another pass through the color mixing rod integrator with a net effect that light exiting the color mixing rod integrator at the exit face is composed of light that has passed through the color mixing rod integrator at least twice.
6. The color mixing rod integrator of claim 5 , wherein said half angle θ ½ <15 degrees in air.
7. A method for integrating a plurality of laser beams of light each beam comprising one of a plurality of primary display colors, the method comprising the acts of:
providing a color mixing rod integrator having a rod-shaped body that includes an entrance face positioned on a first end and an exit face positioned on a second end opposite to and parallel to the entrance face and having an interior that passes the plurality of laser beams of light therethrough and an interior surface that reflects light incident thereon back into the interior of the rod-shaped body;
covering the entrance face with a first reflective coating having a plurality of holes therein to allow light of the plurality of laser beams to pass therethrough into the interior of the rod-shaped body, said first reflective coating being configured to reflect light incident thereon back into the interior of the rod-shaped body; and
diverging each of the plurality of laser beams toward said exit face through said interior,
wherein part of the light of said laser beams entering said rod-shaped body via the plurality of holes is forced to pass through the interior a plurality of times before passing through said exit face and then exiting through said exit face as exit light, and wherein the exit light is homogenized by the color mixing rod integrator.
8. The method of claim 7 , further comprising the act of providing a partially reflective coating covering the exit face that reflects a part of the light incident thereon back into the interior of the color mixing rod integrator and allows a remaining part of the light incident thereon to pass therethrough.
9. The method of claim 8 , wherein each hole of the plurality of holes has a diameter d that exceeds the diameter of the beams of laser light that pass through the plurality of holes by at least a factor of two.
10. The method of claim 9 , wherein said diverging act further comprises the act of providing a plurality of optical elements positioned at the plurality of holes such that divergence of a beam at end of a first pass through the color mixing rod integrator is a divergence selected from the group consisting of a spot across entirety of the exit face and a spot of a predetermined diameter on the exit face.
11. The method of claim 10 , further comprising the acts of:
focusing each of said beams of light to enter a hole of said plurality of holes with a half angle θ 1/2 <20 degrees in air;
sizing each said hole of said plurality of holes to have a diameter ≧2d where d is determined by the equation
d
=
1.22
λ
2
tan
(
θ
1
/
2
)
;
to minimize diffraction effects; and
dimensioning said second reflective coating to have a transmission coefficient T e that is substantially lower than 50% and a reflection coefficient R e =1−T e such that part of the light is transmitted and part is reflected back into the interior of the color mixing rod integrator thereby,
wherein the reflected light travels back to the entrance face for being substantially reflected for another pass through the color mixing rod integrator with a net effect that light exiting the color mixing rod integrator at the exit face is composed of light that has passed through the color mixing rod integrator at least twice.
12. The method of claim 11 , wherein said half angle θ 1/2 <15 degrees in air.
13. A projection engine comprising:
a color mixing rod integrator including:
a rod-shaped body having an interior that passes a plurality of beams of laser light of a plurality of primary display colors therethrough and an interior surface that reflects light incident thereon back into the interior of the rod-shaped body),
an entrance face positioned on a first end of the rod-shaped body covered with a first reflective coating having a plurality of holes therein to allow the plurality of beams of light to pass therethrough into the interior of the rod-shaped body, said first reflective coating to reflect light incident thereon back into the interior of the rod-shaped body,
at least one lens for diverging each beam of said plurality of beams toward said exit face through said interior, said at least one lens being positioned in proximity to said plurality of holes, and
an exit face positioned on a second end of the rod-shaped body opposite to and parallel to the entrance face,
wherein part of the beams of laser light entering said color mixing rod integrator via said plurality of holes at a given half angle is forced to pass through the interior several times before passing through said exit face and then exiting through said exit face as exit light, and wherein the exit light is homogenized by the color mixing rod integrator,
a display panel configured in proximity to said exit face of said color mixing rod integrator such that illumination of said display panel is by the homogenized light exiting through said exit face.
14. The engine of claim 13 , further comprising a second reflective coating that is a partially reflective coating, covering the exit face
wherein a part of the plurality of beams of laser light is reflected back toward sources of the plurality of beams of laser light by the partially reflective coating and a remaining part of the plurality of beams of laser light exits through the exit face.
15. The projection engine of claim 14 , wherein said at least one lens includes at least one positive lens positioned at said plurality of holes such that the divergence of a beam at end of a first pass through the color mixing rod integrator is a divergence selected from the group consisting of across entirety of the exit face and a spot of a predetermined diameter on the exit face.
16. The projection engine of claim 15 , wherein said first reflective coating on said entrance face is a reflective coating.
17. The projection engine of claim 16 , wherein: the reflective coating is a material selected from the group consisting of silver and a dielectric stack;
the plurality of holes each has a diameter such that each of the plurality of beams of laser light can be focused into one of said plurality of holes and can pass therethrough.
18. The projection engine of claim 17 , wherein:
each said beam of laser light is focused to enter one of said plurality of holes with a half angle θ 1/2 <20 degrees in air;
each of said plurality of holes has a diameter ≦2d where d is determined by the equation
d
=
1.22
λ
2
tan
(
θ
1
/
2
)
;
to minimize diffraction effects; and
said second reflective coating is dimensioned to have a transmission coefficient T e that is lower than 50% and a reflection coefficient R e =1−T e such that part of the light is transmitted and part is reflected back into the color mixing rod integrator thereby,
wherein the reflected light travels back to the entrance face and is reflected for another pass through the color mixing rod integrator having a net effect that light exiting the color mixing rod integrator at the exit face is composed of light that has passed through the color mixing rod integrator at least twice.
19. The projection engine of claim 18 , wherein said half angle θ 1/2 <15 degrees in air.
20. The projection engine of claim 18 , further comprising a relay optics in proximity to said exit face configured to accomplish illumination of said display panel.