Patent Application
20070279595
The disclosure may be more completely understood in consideration of the following detailed description of various exemplary embodiments in connection with the accompanying drawings, in which:
Like numerals in different figures refer to similar elements. While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
The present invention is applicable to illumination systems for projection displays, and is believed to be particularly useful for rear projection displays such as televisions and monitors, and also for front projection systems.
Upon exiting the integrator 106, the light 104 is directed towards the projection core 110, which includes an x-cube color combiner 112, imaging devices 114a-114c and respective polarizing beamsplitters (PBSs) 116a-116c. One example of a suitable projection core is the Vikuiti™ Optical Core available from 3M Company, St. Paul, Minn. The projection core may include various polarization control elements, such as quarter-wave retarders and the like, not shown in
Each imaging device 114a-114c and its associated PBS 116a-116c is used to form an image in a respective color band, which is combined in the color combiner 112 with the colored images produced by the other imaging devices 114a-114c to form a full color image. The full color image is projected by a projection lens unit 119 to a projection screen 123.
The light 104 is split into different color components, associated with different color channels, for separately illuminating the different imaging devices 114a. For example, the light 104 is split into first and second separated beams 118a, 118b at a first dichroic separator 120. The first separated beam 118a is directed via a second folding mirror 122 to imaging device 114c, while the second separated beam 118b is split by a second dichroic separator 124 into third and fourth separated beams 126a and 126b which are directed respectively to imaging devices 114a and 114b. A third folding mirror 128 may be used to direct the second separated light beam 118b towards the projection core 110.
In other embodiments, the light beam 106 may be incident on the first dichroic separator 120 from a different direction, with the first separated beam 118a being reflected by the first dichroic separator 120 and the second separated beam 118b being transmitted through the first dichroic separator 120.
An image of the output of the integrator 106 is relayed to the imaging devices 114a-114c using an image relay system that includes a number of lenses. In this exemplary embodiment, the image relay system includes a negative first lens unit 130 positioned close to the output of the integrator 106, a positive second lens unit 132, and two third lens units (TLUs) 134a and 134b. The first lens unit 130 and second lens unit 132 are common to all colors, since they are positioned before the first dichroic separator 120. One of the two TLUs 134b is used by one color of light, and the other TLU 134a is used by light of two colors. The two TLUs 134a, 134b are spaced apart from the second lens unit 132 by the same optical path length, and the two TLUs 134a, 134b may have the same focal length. In some conventional types of illumination systems, different color channels require lenses of different optical lengths, which complicates the assembly. In contrast to these conventional systems, when the TLUs 134a, 134b have the same focal length, the number of different types of lenses required to assemble the illumination system is reduced. Also, the optical path length for illumination light may be substantially the same from the first dichroic separator 120 to each of the imager devices 114a-114c. This ensures that all image devices 114a-114c are illuminated using substantially identical illumination beams, so that the color and intensity properties of the resulting full color image are uniform.
The different lens units 130, 132, 134a, 134b may be formed using one or more lens elements. In the exemplary embodiment illustrated in
Typically, an illumination system that illuminates three imager devices with light of three different colors requires a telecentric light arrangement at the imager devices to provide uniform contrast across the field. This means that identical cones of light illuminate different zones of the imager devices. Although the illumination light 118a, 126a, 126b is incident on the imaging devices as telecentric light, the light is not telecentric at all points within the image relay system. A telecentric system is one where the aperture stop is located at the front focus, resulting in the chief rays being parallel to the optical axis in image space, i.e. the exit pupil is at the infinity. Consequently, in a telecentric light beam, the angular distribution of light at one point of an imager device is the same as the angular distribution of light at another location of the imager device. Where a light beam is non-telecentric, the angular distributions of the light associated with the two points of the imager are different. Thus, in the illumination system illustrated in
Since the light 104 in the space of the first dichroic separator 120 is non-telecentric, various portions of the light beam 104 coming onto different points of the imager devices 114a-114c are incident at the first dichroic separator 120 with different angular distributions. The dichroic separator 120 is typically formed of a multilayer dielectric coating whose optical properties are dependent on the angle of incidence. Consequently, the spectrum of light passed by the first dichroic separator 120 is not uniform across the imagers 114a-114c. This phenomenon is often referred to as color shift. A first trim filter 136 may be disposed in the first separated beam 118a to uniformize the spectrum of light incident at the third imager device 114c. A second trim filter 138 may also be used to trim the spectrum of light whose wavelength band is adjacent the wavelength band of the light in the first separated beam 118a. For example, where the light in the fourth separated beam 126b is green and the light in the first separated beam 118a is red, then a second trim filter 138 may be disposed in the fourth separated beam 126b. In some exemplary embodiments, the trim filters 136 and 138 are tilted with respect to the incident light beams. This eliminates the reflection of some light back to the imager devices, 114a-114c, which can otherwise result in ghosting effects. In some exemplary embodiments the trim filters 136 and 138 may be tilted at an angle of about 12° relative to an axis of the incident light.
The first dichroic separator 120 is oriented so that the axis of light 104 incident on the first dichroic separator 120 is less than 45°. Instead, the light is incident on the first dichroic separator at an angle of less than 40°, and may be less than 35° or even 30°. This permits the illumination path lengths to the imager devices 114a-114c to be the same when using an x-cube color combiner for combining the image light beams. The actual angle of incidence on the first dichroic separator 120 is a design choice that is affected, at least in part, by the size of the system components, particularly the diameter of the third lens unit 134a and the width of the light beam 118a. In some embodiments the angle of incidence of the light 118b on the third folding mirror 128 is substantially the same as the angle of incidence of the light 104 on the first dichroic separator 120, and so the light reflected by the third folding mirror 128 is substantially parallel to the light 104 incident on the first dichroic separator.
The angle of incidence on the first dichroic separator 120 being less than 45° and the ability to maintain the same optical path lengths for all three color channels can be understood further with reference to
From a consideration of the geometry shown in
a+(b−d)/(sin γ)=b+c (1)
and
a−(b−d)/(tan γ)=c−d (2)
From (1) and (2) it can be shown that:
b=d×(1+cos(γ)+sin(γ))/(1+cos(γ)−sin(γ)) (3)
and
c=a−b+(b−d)/sin(γ) (4)
Thus, the designer may select a desired value of “γ” and then calculate “b” using (3). The value of “a” can be selected within a range of distances in which the beam 118b is not vignetted by the lens units 132 and 134a. For the smallest system footprint, the value of “a” is selected as the smallest value within the non-vignetting range, from which the value of “c” can then be calculated.
In addition, aperture stops 140 and 142 may be positioned on the first and second separated beams 118a and 118b. The actual position of the aperture stops is dependent on the optical design of the image relay system. Furthermore, in some embodiments, a pre-polarizer 144 may be used to pre-polarize the light incident at the PBSs 116a-116c. In the illustrated embodiment, a pre-polarizer 144 is positioned closely following the exit of the integrator 106, where the light beam 104 has a small cross-section and thus the pre-polarizer 144 can also be small. Thus, the costs of the pre-polarizer may also be reduced. It will be appreciated, however, that the pre-polarizer may be positioned elsewhere. The pre-polarizer may be any suitable type of polarizer, for example a wire grid polarizer, a multilayer film polarizer or a PBS.
In some embodiments of projector system, the illumination light 104 may be unpolarized, but the PBSs 116a-116c direct light in only one polarization state to the imager devices 114a-114c, and so 50% of the light 104 would otherwise be unused. In the projection system 200 schematically illustrated in
There are two important points related to the relative orientation of the integrator and the polarization converter unit:
An advantage of the arrangement illustrated in
Two exemplary optical systems are presented. In the first optical system, there is no pre-polarizer or polarization converter, and the second lens unit is a single, plastic aspheric lens. In the second optical system, the second lens unit comprises two spherical lenses. The co-ordinates are relative to the y-z axis shown in
The conic constant of the aspheric lens used in the second lens unit 132 is −0.6646. The angle of incidence of axial light on the first and second folding mirrors 121 and 122, and on the second dichroic separator 124, is 45°. The angle of incidence on the first dichroic separator and the third folding mirror 128 is 30°. In Example 1, the pre-polarizer and field stops are omitted.
The co-ordinates for mirrors show the geometrical center of the mirror surface. Each mirrors is offset −6.5 mm down in the plane of mirror. The trim filters are set so that the light is incident at an angle of 12°. The co-ordinates for the first dichroic separator 120 are for the geometrical center of the surface. The first dichroic separator 120 is offset +2.0 mm up in the plane of the reflecting layer. The co-ordinates for the second dichroic separator 124 are for the geometrical center of the surface. The second dichroic separator 124 is offset +3.0 mm up in the plane of the reflecting layer. The angle of incidence on the first dichroic separator and the third folding mirror 128 is 32°.
In the above description, the term angle of incidence, when used to describe the incidence of light having an angular distribution on a surface, refers to the angle that the axial ray makes relative to the normal to the surface.
The present disclosure should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present disclosure may be applicable will be readily apparent to those of skill in the art to which the present disclosure is directed upon review of the present specification. The claims are intended to cover such modifications and devices.
