# Patent

## Title
TIR light valve

## Abstract

A light valve that switches light does so by enabling and disabling total internal reflection (TIR) on a surface of the light valve. The disabling of the TIR is accomplished by putting another optical element in contact with the surface and then diffusing or changing the direction of the light. These valves may be used in an array for a projection system or one at a time. The mechanical mechanism to move the optical element is a simple in that it only moves the optical element a small distance to change the valve from an on position to an off position.

## Background
FIELD OF THE INVENTION
This invention relates generally to light switching means, and more particularly, is a means of switching of light by enabling and disabling total internal reflection, TIR.
BACKGROUND OF THE INVENTION
Many fields require the switching of light to accomplish their task. One major application for the switching of light is in the field of computer data projection and television projection systems. Currently these products use either LCDs or MEMS mirror arrays to accomplish the task of switching light.
Another major industry that uses light switching technology is the communications market. In the communications field, switches are used to control light transmission to and from fiber optic cables.
Light valves are being used in more and more TVs and projection display systems. In TV applications the projector is often used in a rear projection configuration. For computer monitors using projection display, the front projection mode is more commonly used.
The MEMS mirror array type of light valve is disclosed in U.S. Pat. Nos. 4,566,935; 4,596,992; 4,615,595; 4,662,746; 4,710,732; 4,956,619; and 5,028,939; all by inventor Larry Hornbeck of Texas, and assigned to Texas Instruments (TI) of Texas. The TI patents are the foundation of the technology that is used by most manufacturers of TVs and computer projection displays. The TI technology uses an array of MEMS mirrors that change their incidence angle to the light path to switch the light from an off position to an on position. When the mirror is in the on position, the mirror reflects the light through the optical path. When the mirror is in the off position, the light is reflected to a path that falls outside the projection optics. This in effect turns the light valve to an off state.
There are many deficiencies with this technology. One is that the light transmission is less than 70%. To allow for the change of angular orientation of the mirrors, there must be a substantial space between adjacent mirrors. The required gap causes a lot of light to be wasted. Further, the reflected light is absorbed into the light valve. The absorbed energy makes cooling switching devices that use this technology a challenge.
Further, the high amount of absorption limits the amount of power that can be pushed through the light valve. This limitation either eliminates this type of device from being used in high power applications, or causes the necessity of a complex cooling solution.
Another shortcoming of devices using the TI technology is that the MEMS structure to create mirrors that can rotate is a complex one to manufacture.
Still another shortcoming in this technology is that the angle of deflection of the light is not precise. In some applications this lack of control over the angle to which the light is transmitted causes a further reduction of transmission efficiency.
Another popular technology for use in projection applications is LCD technology. However, LCDs are not efficient for the transmission of light. LCDs are slow in response time and do not work well at elevated temperatures. Because of their thermal limitations, the size of the LCDs must be much greater in size than competing technology devices.
Accordingly, it is an object of the present invention to provide a light valve with greatly improved efficiency.
It is another object of the present invention to provide a less complex light valve structure thereby making possible lower cost switching systems.
It is a further object to provide a light valve that reflects almost 100% of the light received, thereby enabling systems to run at extremely high powers while requiring less elaborate cooling systems than are required by current art systems.
It is a still further object of the invention to provide a light valve that can switch faster. This is because there is only a small movement in the MEMs elements during the switching.
It is yet another object of the present invention to provide a higher contrast ratio of the on state to the off state of the light valve.
It is a further object of the invention to provide a light valve that enables light to be accurately switched to two paths.
SUMMARY OF THE INVENTION
The present invention is a light valve for use in projectors and telecommunication switching equipment. The light valve switches light from a first controlled optical path to either a diffused path or to a second controlled optical path. The diffused path effectively eliminates any light from continuing through the original first controlled optical path. The light switch directs light to the second controlled optical path in a controlled manner with high efficiency. The light that travels through the first (not switched) controlled path does so with high efficiency. The efficiency of the light transmission is obtained by internal reflection at the interface between two elements with different optical indexes of refraction.
An advantage of the light valve structure of the present invention is that the light valve reflects almost 100% of the light received, thereby enabling systems utilizing the switch to run at extremely high powers while requiring less elaborate cooling systems than are required by current art systems.
Another advantage of the present invention is that it provides a less complex light valve structure thereby making possible lower cost switching systems.
Still another advantage of the present invention is that the light valve can switch faster. This is because only a small movement in the MEMs elements is required.
These and other objectives and advantages of the present invention will become apparent to those skilled in the art in view of the description of the best presently known mode of carrying out the invention as described herein and as illustrated in the drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a perspective view of the light valve of the present invention.
FIG. 2 shows a front view of a ray trace of light traveling through the light valve with the valve in the on position.
FIG. 3 shows a front view of a ray trace of light traveling through the light valve with the valve in the off position.
FIG. 4 shows a right side view of a ray trace of the light traveling through the light valve with the valve in the off position.
FIG. 5 shows a right side view of a ray trace of the light traveling through a modified second surface of the light switch.
FIG. 6 shows a front view of a ray trace of the light traveling through another modified second surface.
FIG. 7 shows a front view of a ray trace of light traveling through another modified second surface.
FIG. 8 is a perspective view of a three-by-three array of light valves.
FIG. 9 is a perspective view of the actuating mechanism for the light valve.
FIG. 10 is a perspective view of the base of the actuating mechanism.



DETAILED DESCRIPTION OF THE INVENTION
Referring first to FIG. 1, the light valve 10 of the present invention comprises a prism 11. The prism 11 has three optical surfaces; the hypotenuse surface 12, the input optical surface 13, and the output optical surface 14. The input surface 13 is where light rays (not shown in FIG. 1) enter the light valve 10. A switch element 15 is located in close proximity to the hypotenuse surface 12 of the prism 11.
Light is projected onto the input optical surface 13 of the prism 11. The light source can be chosen from many systems of lenses, reflectors, and/or lamps. The light source can be one of many types, including arc lights and LEDs. The collection of the light from the light source is accomplished with a lens or reflector or any combination thereof. One skilled in the art of light sources and the collection of light can easily construct many systems to efficiently collect and direct light toward the light valve.
In the case of a telecommunications system, the light source may be a fiber optic cable or a laser. The light might be parallel, converging, or divergent. (The orientation of the light rays is more a requirement of the light source collection optics and the projection optics in the case of a projector system. In the case of a telecommunications system, light orientation would be more of a function of the associated devices and not the light valve.)
In the case of a projection system, a color wheel might be deployed between the light source and the light valve. The color wheel rotates faster than a human eye can detect. The light valve is synchronized with the colors of the wheel so that a color projection can be created. A single color filter may also be used in the case where there is one light valve for each color. Projection systems typically have three valves, one for each of the primary colors.
Referring now to FIG. 2, a front view of the light valve 10 of the present invention, exterior light rays 21 are shown as they are directed to the prism 11. The light 21 hits the input surface 13 at an angle normal to the surface 13. The incidence angle of the light 21 could be other than normal, but for purposes of this disclosure, a normal contact angle will be described. The rays 21 pass through the prism's input surface 13. The interior rays 22 maintain the same normal relationship to the input surface 13 of the prism 11 as the exterior light rays 21. (If the angle of incidence of the exterior light rays 21 were other than normal, the light 21 would be refracted and the interior angle would not be the same as the exterior angle.) The interior rays 22 hit the hypotenuse 12 side of the prism 11. The difference of the angle that the interior rays 22 hit the hypotenuse 12 from normal is the same angle as the input surface relative to the hypotenuse 12. The prism 11 used in this instance is a 45° prism, so the interior light 22 hits the hypotenuse 12 at 45° from normal. While in the preferred embodiment, the angle of the prism 11 is chosen to be 45°, other prism angles could also be used.
When the index of refraction of the prism material is much greater than that of the exterior region, light reflects off the hypotenuse 12. In the case of the light switch 10 of the present invention, the exterior region is chosen to be air or a vacuum in order to provide a low index of refraction. It should be noted that other exterior materials could be used that have an optical index less than that of the prism. The equation that determines the angle of internal reflection, which is defined herein as total internal reflection (TIR), is determined by:

TIR angle=arcsine(index of refraction exterior/index of refraction prism).

The internal reflections 23 off the hypotenuse 12 reflect at the same angle that they hit the surface. The light passes through the output optical surface 14. The output light rays 24 are then directed into the rest of the optical system. In the case of a projection system, the remainder of the system would include lenses and a screen. In the case of a telecommunications system, the system would most likely include a fiber optic cable or a detector.
The use of a prism is a common means to bend light at right angles and is used in thousands of different types of equipment and products. There are many different types of angled surfaces that can create internal reflections on at least one of the surfaces. People knowledgeable in the art of optics could conceive of thousands of different ways to create a total internally reflecting (TIR) surface. Using a prism is the most common method.
The switch element 15 is located below the hypotenuse 12. The switch element 15 is shown to be positioned close to the hypotenuse surface 12. The gap 16 between the switch element 15 and the hypotenuse surface 12 needs to be only approximately the length of the maximum wavelength of the system in which the valve 10 is being used. In the case of a blue light system, the gap 16 would be on the order of 500 nanometers. For a white light system, the gap 16 would be around 700 nanometers. For tolerance reasons, the gap 16 might actually be nominally spaced at 1500 nanameters. The system need only have an extremely small gap 16 for the light 22 to TIR off the surface of the prism 12. FIG. 2 is not to scale.
When the gap 16 is made much smaller than the minimum operating wavelength, (as shown in FIGS. 3 and 4), the light 22 no longer reflects off of the prism's internal hypotenuse surface 30. Instead the light 22 passes through the first surface 31 of the switch element 15. If the index of refraction is the same for both the prism 11 and the switch element 15, the light continues in the same direction as interior rays 22. If the indexes of refraction are different, the light 22 refracts off the first surface 31 in a non-parallel direction.
To ensure the gap 16 between the switch element 15 and the hypotenuse surface 12 is sufficiently narrow, a thin layer of a transparent elastic material may be coated onto either the hypotenuse surface 12 or the first surface 31 of the switch element 15.
Referring to FIG. 4, a right side view, the switched light contacts a serrated second surface 34 of the switch element 15. The serrations of the second surface 34 are at an angle to the incoming light. The light reflects off these serrated surfaces 34 and is directed towards the front and/or the back of the valve 10 as off light 36. By being reflected to the front and/or back, the light no longer travels through the output optical surface 14 of the prism 11, and therefore the switch 15 of the valve 10 is in the off position. The angles of the serrations on the second surface 34 of the switch element 15 need only be large enough to prevent the light from passing through the output section 14 of the prism 11. The angles of the serrations can be very shallow.
FIG. 5 shows a serrated second surface 34 where very shallow angles are used to direct the light to an off center location on the prism 11. This allows the light to be switched to a different path than when the light reflects off of the hypotenuse of the prism. This would be useful in a telecommunications application.
Another conformation that switches the light to a different direction is shown in FIG. 6, a front side view. The second surface 34 has angled surfaces in a direction orthogonal to those of the second surface 34 displayed in FIG. 5.
FIG. 7 shows a similarly angled serrated second surface 34 where the surface refracts the light rather than producing total internal reflection. The configuration illustrated in FIG. 7 directs the light to an alternate direction.
In addition to the conformations described above, there are at least three alternate methods that can be employed to stop the light from total internal reflecting off the second surface 34 of the switch element 15. The first of the alternate methods is to absorb the light in the switch element 15. This method would not work well in anything but applications involving low power levels. The second alternate method would be to diffuse the light as it propagates through the switch element 15. By diffusing the light, only a very small portion would find its way to the exit surface 14 of the prism 11 and then through the rest of the optical system. The third alternate method is to build the second surface 34 with a diffuse topography. With a diffuse topography, the second surface 34 would allow only a small portion of the light to be transmitted through the output side 14 of the prism 11.
FIG. 8 shows an array of nine switch elements under one prism. In a projection system application, there might be over a million of these switches in an area of around 8 mm×10 mm. The individual switches are extremely small. The light valves 10 of the present invention are around 30 microns square.
As mentioned above, the diffusion elements need to move only a small distance to turn on or off the light internal reflection. One such structure to achieve this effect is shown in FIG. 9. The prism and switch elements are not shown in this figure for clarity. A first conductive layer 40 is shown on top. The first conductive layer 40 supports and locates the switch element 15 (not shown in FIG. 9). At least one spring element 42 is located on the first conductive layer 40. In the preferred embodiment, two springs 42 are utilized. A first end of the spring element 42 is attached to the first conductive layer 40, and the second end of the spring element 42 is attached to a base 44. The springs 42 serve two purposes—to locate the first conductive layer 40 in the horizontal plane, and to provide an upward force to keep the switch 15 in contact with the prism 11.
FIG. 10 is a view of the base 44 with the first conductive layer 40 and the springs 42 removed. Base posts 47 serve as the attachment points for the springs 42. The base posts 47 and the springs 42 are mechanically and electrically connected to the first conductive layer 40. The electrical connection allows a charge to be placed on these elements. The base posts 47 and the springs 42 are mechanically connected to the first conductive layer 40 so they mechanically align the switch element 15 with the prism 11 and keep the switch element 15 in contact with the prism 11.
The base posts 47 are surrounded by an insulating layer 48. Under the insulating layer 48 is a second conductive layer 50. The second conductive layer 50 is not electrically connected to the first conductive layer 40; however, the second conductive layer 50 is mechanically joined to the first conductive layer 40.
By applying either opposite charges or by applying no charge to the two conductive layers 40, 50, a force is created to draw the conductive layers 40, 50 together. When opposite charges are applied to the conductive layers 40, 50, the switch element 15 is moved away from the prism 11, which causes the light valve 10 to be in the on state.
The above disclosure is not intended as limiting. Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the restrictions of the appended claims.

## Claims

1. A light valve comprising:
a means for bending light, said means for bending light having an index of refraction much greater than an index of refraction of a surrounding area, so that light projected onto a first surface of said means for bending light is reflected off a second surface of said means for bending light,
a switching element comprising a reflecting surface, and
a means for moving said reflecting surface of said switching element relative to said second surface of said means for bending light; wherein
when said switching means is in an on position, light passing through said first surface of said means for bending light reflects off of said second surface of said means for bending light and passes through an output surface of said means for bending light in a direction not substantially parallel to a direction of said light passing through said first surface, and
when said switching means is in an off position, said reflecting surface is moved to a position in close proximity to said second surface of said means for bending light, so that light passing through said first surface of said means for bending light is not reflected off said second surface, but rather continues on to a surface of said switching means and is redirected through said switching means in a direction not substantially parallel to a direction of said light passing through said first surface of said means for bending light.

2. The light valve of claim 1 wherein:
said switching means is moved between said on position and said off position by an electrostatic force.

3. The light valve of claim 1 wherein:
light is reflected off a serrated surface of said switch element when said switch element is in said off position.

4. The light valve of claim 1 wherein:
light is absorbed by said switch element when said switch element is in said off position.

5. The light valve of claim 1 wherein:
light is diffused by said switch element when said switch element is in said off position.

6. The light valve of claim 1 wherein:
said switch element has a diffuse topography.

7. The light valve of claim 1 wherein:
said means for bending light is a prism.

8. A light valve comprising:
an array of functionally equivalent light valves, each said light valve comprising:
a means for bending light, said means for bending light having an index of refraction much greater than an index of refraction of a surrounding area, so that light projected onto a first surface of said means for bending light is reflected off a second surface of said means for bending light,
a switching element comprising a reflecting surface, and
a means for moving said reflecting surface of said switching element relative to said second surface of said means for bending light; wherein
when said switching means is in an on position, light passing through said first surface of said means for bending light reflects off of said second surface of said means for bending light and passes through an output surface of said means for bending light, and
when said switching means is in an off position, said reflecting surface is moved to a position in close proximity to said second surface of said means for bending light, so that light passing through said first surface of said means for bending light is not reflected off said second surface, but rather continues on to a surface of said switching means and is redirected through said switching means in a direction not substantially parallel to a direction of said light passing through said first surface of said means for bending light.

9. The light valve of claim 8 wherein:
at least one of said switching means is moved between said on position and said off position by an electrostatic force.

10. The light valve of claim 8 wherein:
light is reflected off a serrated surface of each said switch element when said switch array is in said off position.

11. The light valve of claim 8 wherein:
light is absorbed by said switch elements comprising said array when said switch array is in said off position.

12. The light valve of claim 8 wherein:
light is diffused by each said switch element when said switch array is in said off position.

13. The light valve of claim 8 wherein:
at least one of said switch elements has a diffuse topography.

14. The light valve of claim 8 wherein:
said means for bending light is a prism.