Projection device with field splitting element

Described examples include a projection device having a light source. The projection device also has a spatial light modulator arranged to receive light from the light source and provide modulated light. The projection device also has projection optics arranged to receive and project the modulated light. The projection device also has a field splitting element between the spatial light modulator and the projection optics, a first portion of the field splitting element being structured to pass at least a first portion of the modulated light to the projection optics for projection at a first focal length, and a second portion of the field splitting element being structured to pass at least a second portion of the modulated light to the projection optics for projection at a second focal length.

TECHNICAL FIELD

This relates generally to projection devices, and more particularly to modulated projection devices.

BACKGROUND

Modulated projector lights provide great versatility and functionality for automobiles and other applications. For example, imaging systems, such as Light Detection and Ranging (LIDAR) can determine the position of an oncoming vehicle. Rather than shift from high-beam to low-beam to avoid blinding the driver of the oncoming vehicle, the output of the headlights can be modulated to only shine on the lower portion of the oncoming vehicle and avoid blinding the other driver. In addition, images such as warnings can be projected onto the pavement ahead of the vehicle. An example of such a system is shown in U.S. Pat. No. 9,658,474, which is co-owned with this application and is incorporated herein by reference. However, to provide high light throughput, modulated projector systems often use a very wide aperture (f<3). The use of a large aperture causes a narrow depth of focus. Thus, if the projection lenses have a close focus, distant projected images will be out of focus and vice versa.

SUMMARY

In accordance with an example, a projection device has a light source. The projection device also has a spatial light modulator arranged to receive light from the light source and provide modulated light. The projection device also has projection optics arranged to receive and project the modulated light. The projection device also has a field splitting element between the spatial light modulator and the projection optics, a first portion of the field splitting element being structured to pass at least a first portion of the modulated light to the projection optics for projection at a first focal length, and a second portion of the field splitting element being structured to pass at least a second portion of the modulated light to the projection optics for projection at a second focal length.

DETAILED DESCRIPTION

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are not necessarily drawn to scale.

The term “coupled” may include connections made with intervening elements, and additional elements and various connections may exist between any elements that are “coupled.”

In the arrangements, the problem of differing target focal distances is solved by using a field splitting element to provide a projection device with differing focal lengths.

FIG. 1shows a scene100with an automobile102with modulated headlights104. Modulated headlights104project modulated light to two zones: near zone106and far zone108. Among other functions, near zone106projects informational images onto the pavement in front of automobile102. Among other functions, far zone108provides glare-free lighting. In glare-free lighting, the projected light is modulated so that the light does not shine on places with a high probability of blinding a viewer. This allows for the use full intensity light while avoiding blinding drivers in other vehicles. However, the focus of the near zone is less than about 10 meters, but the focus of the far zone is essentially infinite, for example. As a practical example, the focus of the far zone may be set at 25 to 50 meters. Modulated headlights104use a wide aperture (e.g., f<3) to provide high light output. However, a large aperture provides a small depth of focus.

FIG. 2shows an example projection system200using a field splitting element (FSE)202. Projection system200may be used as headlight projection device for an automobile headlight such as modulated headlights104(FIG. 1), among other applications. In projection system200, light source204provides high intensity light. Light source204may be, for example, a light emitting diode (LED), laser diode or a high intensity discharge lamp. Illumination optics206direct the light onto spatial light modulator208. Illumination optics206may include multiple components, such as lenses, light tunnels and other mechanisms to provide homogenous light to spatial light modulator208. In an example, light from light source204may pass at least partially through FSE202on its path to spatial light modulator208.

Spatial light modulator208selectively reflects the light through window210and FSE202. Window210is part of the packaging of spatial light modulator208. For example, spatial light modulator208may be a digital micromirror device (DMD), a liquid crystal display (LCD) or liquid crystal on silicon (LCOS). Window210protects the mirrors of spatial light modulator208while providing optical transparency. In an alternative example, FSE202and window210may be combined into one element. Each pixel of spatial light modulator208selectively reflects light from light source204toward or away from projection optics212to provide the desired projected image. After passing through FSE202, the light is projected by projection optics212. Projection optics212has a fixed focal length or a variable focal length that may be adjusted over a relatively long time, and thus is essentially fixed. As explained further hereinbelow, FSE202, modifies the focal point of an image modulated by spatial light modulator208for different portions of the image projected by projection optics212so that one portion of the image focuses at a greater distance that another portion of the image.

FIG. 3shows an FSE300like FSE202(FIG. 2). In this example, FSE300comprises an optically transparent material with a refractive index n that is greater than one. FSE300has a first portion302with a thickness of d2and a second portion304with a thickness of d. The difference between thicknesses d and d2is thickness d1. The FSEs discussed herein may include a coating to block infrared and ultraviolet radiation to protect the spatial light modulator208(FIG. 2). As further explained hereinbelow, the differing thicknesses of FSE300allow for differing image focal lengths for different portions of the image modulated by a spatial light modulator such as spatial light modulator208.

FIG. 4is a diagram showing differing image focal lengths for differing object points relative to a lens402with a fixed focal length. Object404has a distance to the center of lens402of So1. Object406has a distance to the center of lens402of So2. The distance of image408, which is an image of object404, is Si1. The distance of image410, which is an image of object406, is Si2. The relationship between the object distance, the image distance and the lens focal length is given by Equation (1).

1f=1So+1Si(1)
Where f is the focal length of the lens, Sois the distance of the object to the lens and Siis the focal point for the image of the object. As shown in Equation (1), a larger object distance results in a shorter image distance and vice versa, assuming a fixed focal length for the lens. Using Equation (1), the relationship of object404and image408is given by Equation (2);

1f=1So⁢⁢1+1Si⁢⁢1(2)
and the relationship of object406and image410is given by Equation (3).

Using Equations (4) and (5) to determine the difference between Si1and Si2yields Equation (6).

Δ⁢⁢Si=Si⁢⁢1-Si⁢⁢2=f⁢⁢So⁢⁢1⁢So⁢⁢2-f2⁢So⁢⁢1-f⁢⁢So⁢⁢2⁢So⁢⁢1+f2⁢So⁢⁢2So⁢⁢1⁢So⁢⁢2-f⁢⁢So⁢⁢1-f⁢⁢So⁢⁢2+f2=f2⁢So⁢⁢2-f2⁢So⁢⁢1So⁢⁢1⁢So⁢⁢2-f⁢⁢So⁢⁢1-f⁢⁢So⁢⁢2+f2(6)
The optical path length through a material with a refractive index greater than one is the length of travel through the material multiplied by the refractive index of the material. As shown inFIG. 4, a longer object length provides a shorter image focal point. Referring toFIG. 3, the difference in the optical path length through second portion304(OPL1) and the optical path length through first portion302(OPL2) (i.e., the difference in the image focal point ΔSi) is determined by Equation (7).
ΔSi=OPL1−OPL2=nd−(nd2+d−d2)  (7)
Where n is the refractive index of the material of FSE300. Simplifying Equation (7) yields Equation (8).
ΔSi=d(n−1)+d2(1−n)  (8)
Equating Equation (8) and Equation (6) allows for the determination of the parameters d, d2and n of the FSE to achieve the desired image focal points for images passing through the first portion302(FIG. 3) and second portion304(FIG. 3). This allows for differing image focal points for different portions of the image projected by projection system200(FIG. 2). For example, referring toFIG. 1, this allows for different image focal points for near zone106and far zone108, allowing both zones to have good focus despite a narrow depth of focus provided by projection optics212(FIG. 2).

FIG. 5shows another example FSE500. In examples, FSE500replaces FSE202(FIG. 2). FSE500includes first portion502, transition portion504and second portion506. As with the other described FSEs, the position of the first portion502and the second portion506within a projection system like system200(FIG. 2) is selected based on whether the projection optics212(FIG. 2) inverts the image or not. For example, first portion502is positioned to project the lower portion of the beam (e.g. near zone106(FIG. 1)) and second portion506is positioned to project the upper portion of the beam (e.g. far zone108(FIG. 1)). First portion502and second portion506have a constant thickness, and thus function in a comparable manner to the first portion302and second portion304of FSE300(FIG. 3). Transition portion504has a varying thickness, and thus a varying focal point along its length. This avoids an abrupt change in the image focal position and provides for better focus of images in the transition from a near focal region (first portion502) to a far focal region (second portion506).

FIG. 6shows another example FSE600. FSE600includes a first portion602and a second portion604. In examples, FSE600replaces FSE202(FIG. 2). Rather than first portion602being a uniform thickness as with first portion302of FSE300(FIG. 3) or first portion502of FSE500(FIG. 5), first portion602has a variable thickness that varies linearly from a thickest part at the end of first portion602farthest from second portion604to a thickness equal to the second portion604adjacent to the second portion604. Thus, the focal point of the image projected through first portion602varies linearly from relatively close to relatively far. The purpose of this configuration is shown byFIG. 1.

The image projected through first portion602is projected in near zone106(FIG. 1) in this example. The distance from headlights104to point120(FIG. 1) is relatively small; for example, approximately 2 meters. The distance from headlights104to point122(FIG. 1) is relatively long; for example, 10 meters. The focus issues caused by a projection onto a plane that is not perpendicular to the projector is known as the Scheimpflug principle. A projection system using a field splitting element like FSE600corrects for the Scheimpflug focus distortions and allows for a focused image at all points from point120to point122(FIG. 1).

FIG. 7shows another example FSE700. In examples, FSE700replaces FSE202(FIG. 2). FSE700includes a first portion702and a second portion704having different curvatures in addition to varying thicknesses. First portion702has a tilted, convex profile relative to the incoming light from spatial light modulator208(FIG. 2). Second portion704has a tilted, concaved profile relative to the incoming light from spatial light modulator208(FIG. 2). Because of these profiles, FSE700functions as a lens to tilt the direction of the images passing through FSE700and to modify the focal properties of the light paths along with projection optics212(FIG. 2). Thus, FSE700allows for very precise tailoring of the focal points of the image passing through FSE700.

FIG. 8shows an example process800using the example projection systems described herein. Step802provides light from a light source such as light source204(FIG. 2) to a spatial light modulator such as spatial light modulator208(FIG. 2). Step804provides the light modulated by the spatial light modulator through a field splitting element such as FSE202. Step806provides the light from a first portion of the field splitting element such as first portion302(FIG. 3) to projection optics such as projection optics212(FIG. 2) having a focal length such that a first image that passes through the first portion of the field splitting element is focused at a first distance. Step808provides the light from a second portion of the field splitting element such as second portion304(FIG. 3) to projection optics such as projection optics212(FIG. 2) having a focal length such that a second image that passes through the second portion of the field splitting element is focused at a second distance.