Patent Application
20130050489
The invention generally relates to an optical device configured to refractively shift light propagating along a field of view axis of a field of view about a vehicle to an image sensing device axis of an image sensing device, and more particularly relates to an optical device wherein the field of view axis is directed through a vehicle window characterized as having a substantial rake angle.
Imaging systems are frequently used in motorized vehicles to provide views of the area around the vehicle. In the case of forward looking imaging systems, images provided by the imaging system are often used for collision avoidance applications (e.g. lane tracking systems, adaptive cruise control systems, etc.). The proper functioning of these systems depends in part on the quality of the images produced by the imaging system. Typically, these imaging systems are located adjacent to a vehicle window. In the forward looking imaging system, it is typically located between the vehicle's windshield and the rear view minor to maintain a view of the road ahead which is similar to the driver's view.
Some imaging systems make use of an optically opaque boot or enclosure to completely enclose the optical path of the imaging system to the vehicle window to optically isolate it from the rest of the vehicle passenger compartment to reduce glare reflections. This boot or enclosure is commonly known as a glare shield. The size of the glare shield is determined primarily by the proximity of an image sensing device to the vehicle window and the rake angle of the vehicle window. There may be a large difference in window rake angles, particularly windshield rake angles between a large sports utility vehicle (SUV) or truck (typically 33 degrees to 65 degrees from horizontal) and a performance oriented car (typically around 15 degrees to 25 degrees from horizontal). The low windshield rake angles used for some automotive vehicle platforms may create a situation in which the glare shield of the imaging system will become prohibitively large. This may make locating the imaging system between the windshield and the rear view mirror problematic. A large glare shield in that location may also significantly reduce the driver's view through the windshield.
With vehicles becoming more aerodynamically shaped to present the lowest wind resistance in order to increase vehicle fuel efficiency, windshield rake angles will likely continue to decrease in future vehicle designs. For example, internal combustion/electric hybrid vehicles and electrical vehicles that are designed for reduced energy consumption typically have a smaller windshield rake angle than their internal combustion engine vehicle predecessors. As more aerodynamic designs are adapted to improve fuel consumption, the low windshield rake angles used for current sports car designs may become commonplace for family vehicles.
It has been suggested that interposing a refractive block having an index of refraction that is greater than the index of refraction of air (about 1.00), may reduce the optical path length of the light between the windshield and the image sensing device lens. This is optically the same as bringing the image sensing device lens closer to the windshield surface.
A single prismatic-shaped refractive block shape has been considered. The natural wedge shape created by the windshield rake angle creates a volumetric area that a prismatic-shaped refractive block member can be placed within. The single prismatic-shaped refractive block can substitute for the air space normally between the windshield and the image sensing device lens and reduce the optical path length.
The refractive block shown in U.S. Pat. No. 7,095,567 shows a technique using a single refractive block with a light-entrance surface that is mounted to a windshield or other refractive boundary on a vehicle and a light-exit surface. The refractive block is configured to refract an optical path of light corresponding to an imaged area and direct the light to an image sensing device.
A disadvantage of the single prismatic-shape refractive block is potential optical distortion due to the non-symmetrical light bending or refraction. This is because the varying refraction (or deviation) of light ray angles from the field of view caused by a prismatic shape varies with the incoming light ray's incident angle. The varying incoming light rays of the image sensing device's field of view enter the refractive block surface at various incident angles. Due to the variation in thickness across the refractive block, the refraction introduces a deviation of the varying exit angles of the light rays and does so at incrementally varying amounts across the field of view. This variation in the refracted angle deviation can cause optical distortion of the total field of view.
For example, the single prismatic-shape refractive block may be regarded as essentially a one dimensional optical surface. In the horizontal (or azimuth) direction, the single prismatic-shape refractive block is optically equivalent to parallel planes of glass and hence causes no effective optical deviation or distortion in this direction. However, in the vertical (or elevation) direction, the single prismatic-shape refractive block's prismatic shape geometry may cause varying amount of refracted light deviation and potentially introduces distortion in the vertical plane.
Additionally, variation in windshield rake angles may result in non-standard versions of the imaging system. The size and shape of the refractive block may need to be tailored for different vehicle platforms, causing design changes to the imaging system particularly the mounting/packaging subsystem. This variation may cause high recurring design and module costs to implement the imaging system in different vehicle platforms.
It is therefore appropriate to consider novel techniques which maintain the quality of the imaging system's image, reduce the physical size of the imaging system glare shield, and create a common modular reusable design applicable to several vehicle platforms.
In accordance with one embodiment of this invention, an optical device is provided. The optical device is configured to refractively shift light propagating along a field of view axis of a field of view about a vehicle to an image sensing device axis of an image sensing device. The field of view axis is directed through a vehicle window characterized as having a substantial rake angle. The optical device includes a first refractive element formed of material having a first index of refraction. The first refractive element defines a first surface configured to be optically coupled with the vehicle window. The first refractive element defines a second surface oriented at a first prism angle to the first surface, whereby light impinging on the first surface along the field of view axis passes through the first refractive element and exits the second surface along an intermediate axis. The optical device further includes a second refractive element formed of material having a second index of refraction. The second refractive element defines a third surface separated from the second surface by a refractive boundary layer. The second refractive element defines a fourth surface oriented to the third surface at a second prism angle, whereby light propagating through the refractive boundary layer and impinging on the third surface along the intermediate axis passes through the second refractive element and exits the fourth surface along the image sensing device axis, whereby light from the field of view is directed to the image sensing device.
In another embodiment of the present invention, an imaging system is provided. The imaging system is configured to refractively shift light propagating along a field of view axis of a field of view about a vehicle to an image sensing device axis of an image sensing device. The field of view axis is directed through a vehicle window characterized as having a substantial rake angle. The imaging system includes a first refractive element formed of material having a first index of refraction. The first refractive element defines a first surface configured to be optically coupled with the vehicle window. The first refractive element defines a second surface oriented at a first prism angle to the first surface, whereby light impinging on the first surface along the field of view axis passes through the first refractive element and exits the second surface along an intermediate axis. The imaging system also includes a second refractive element formed of material having a second index of refraction. The second refractive element defines a third surface separated from the second surface by a refractive boundary layer. The second refractive element defines a fourth surface oriented to the third surface at a second prism angle, whereby light impinging on the third surface along the intermediate axis passes through the second refractive element and exits the fourth surface along the image sensing device axis. The image sensing device is in optical communication with the second refractive element, whereby light from the field of view is directed into the image sensing device.
Further features and advantages of the invention will appear more clearly on a reading of the following detailed description of the preferred embodiment of the invention, which is given by way of non-limiting example only and with reference to the accompanying drawings.
The present invention will now be described, by way of example with reference to the accompanying drawings, in which:
The optical device 30 may further include a second refractive element 50 formed of optically transparent material having a second index of refraction. The second refractive element 50 defines a third surface 52 separated from the second surface 44 by a refractive boundary layer 54. The refractive boundary layer 54 provides a refractive boundary for the first refractive element 40 and the second refractive element 50. The refractive boundary layer 54 may define a region having a third index of refraction within the optical device 30 without adding significant mass to the optical device 30. The second refractive element 50 defines a fourth surface 56 oriented to the third surface 52 at a second prism angle 58. Light propagating through the refractive boundary layer 54 and impinging on the third surface 52 along the intermediate axis 48 passes through the second refractive element 50 and exits the fourth surface 56 along the image sensing device axis 34. Light from the field of view is directed to the image sensing device 36. The second prism angle 58 is selected so that light impinging on the third surface 52 is not reflected within the second refractive element 50.
It has been observed that limiting or eliminating internal reflection improves the quality of an image of the field of view directed through the optical device 30. The quality of the image may be further improved by controlling light at a peripheral surface of the first refractive element 40 and the second refractive element 50, which include all surfaces of the first refractive element 40 and the second refractive element 50 other than the first surface 42, the second surface 44, the third surface 52, or the fourth surface 56. These peripheral surfaces may be treated or formed to intentionally minimize the transmission of light while simultaneously minimizing the reflection of light within the first refractive element 40 or the second refractive element 50. In a non-limiting example, these peripheral surfaces may be intentionally roughened or otherwise textured in order to minimize light transmission and/or the reflection of light within the block from these surfaces. Further, these peripheral surfaces may be coated with optically opaque materials to achieve the desired results.
Further, it may be advantageous to form antireflective coatings or films on the first surface 42, the second surface 44, the third surface 52, or the fourth surface 56 to reduce the amount of internal reflection. One non-limiting example may include a single film of magnesium fluoride (MgF2). The thickness of such a film may be tuned to achieve the most desirable results to minimize surface reflections for the wavelength or wavelengths of interest. It will be instantly recognized by a person skilled in the art that more complicated antireflective structures are also useable in the embodiments described in the present application.
The image sensing device axis 34 may not be parallel to the field of view axis 32.
Continuing to refer to
The refractive boundary layer 54 may be preferentially filled by air or other optically transparent gas to reduce the mass of the optical device 30. The refractive boundary layer 54 may alternately be filled by an optically transparent gel or liquid. The gel or liquid may define a third index of refraction within the optical device 30. The gel or liquid may be chosen to have a different index of refraction than the first refractive device 40. The gel or liquid may minimize the collection of contaminants on the second surface 44 and the third surface 52. The boundary layer may alternately be filled by an optical or photonic metamaterial. Optical metamaterials have a negative refractive index based on micro-structural properties, which may provide a shorter optical path through the optical device 30.
Due to the orientation of the first surface 42 relative to the second surface 44 of the first refractive element 40 at a first prism angle 46, the first refractive element 40 may be characterized as a wedge prism shape. Therefore, the first refractive element 40 may have a non-uniform thickness. It has been observed that this non-uniform thickness may produce optical distortion of the image. Without subscribing to any particular theory, it is believed that this is because the refraction (or deviation) of the field angles caused by the wedge prismatic shape varies with the incoming light ray's incident angle. The incoming light rays coming from the intended field of view enter the first surface 42 at various incident angles. Due to the variation in thickness across the first refractive element 40, the refraction introduces additional deviation of the varying exit field angles of the light rays and does so at incrementally varying amounts across the field of view. This variation in the refracted angle deviation may cause a minor amount of optical distortion of the total field of view.
The optical distortion may be determined by calculating the greatest angular deviation for any light ray in the intended field of view entering the optical device 30 as a percentage of the intended total field of view. For example, if the largest angular deviation of any light ray in the field of view of the optical device 30 is 2 degrees and the field of view of the optical device 30 is 20 degrees, then the optical distortion is 10%.
The first refractive element 40 and the second refractive element 50 may define complementary shapes selected to minimize optical distortion. The shape of the second refractive element 50 may be selected to have a complementary wedge prism shape. The complementary wedge prism shape is selected to correct or offset the optical distortion created by the first refractive element 40. The complementary wedge prism shape is dependent upon the vehicle window rake angle 74, the angular relationship of the image sensing device axis 34 to the fourth surface 56, and the first index of refraction and the second index of refraction.
Referring again to
Referring now to
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The optical device 630 further includes a second refractive element 650 formed of optically transparent material having a second index of refraction. The second refractive element 650 defines a third surface 652 separated from the second surface 644 by a refractive boundary layer 654. The refractive boundary layer 654 provides a refractive boundary for the first refractive element 640 and the second refractive element 650. The refractive boundary layer 654 defines a region having a third index of refraction within the optical device 630 without adding significant mass to the optical device 630 The second refractive element 650 defines a fourth surface 656. The third surface 652 may be parallel to the fourth surface 656. Light propagating through the refractive boundary layer 654 and impinging on the third surface 652 along the intermediate axis 48 passes through the second refractive element 650 and exits the fourth surface 656 along the image sensing device axis 34. Light from the field of view is directed to the image sensing device 36.
Since the first surface 642 and the second surface 644 of the first refractive element 640 are parallel, the first refractive element 640 has a uniform thickness. Therefore, the first refractive element 640 may not produce optical distortion due to a variation in the angle of refraction. Likewise, since the third surface 652 and the fourth surface 656 of the second refractive element 650 are parallel, the second refractive element 650 may not produce a significant amount of optical distortion due to a variation in the angle of refraction. The first surface 642, the second surface 644, the third surface 652, and the fourth surface 656 may all be planar surfaces. The first refractive element 640 and the second refractive element 650 may be characterized as a parallel plate shape. The first refractive element 640 and the second refractive element 650 may have a different thickness.
The use of a first refractive element 640 and a second refractive element 650 with parallel plate shape shapes allows compression of an optical path length since the index of refraction of the first refractive element 640 and the second refractive element 650 (typically 1.45 to 1.55) is greater than air (1.00). The first refractive element 640 and the second refractive element 650 may have the same index of refraction or a different index of refraction (e.g. to achieve an achromatic or color correction by using complementary indexes of refraction similar to an achromatic lens design).
Referring once again to
The imaging system 28 includes a first refractive element 40 formed of optically transparent material having a first index of refraction. The first refractive element 40 defines a first surface 42 configured to be optically coupled with the vehicle window 38. The first refractive element 40 defines a second surface 44 oriented at a first prism angle 46 to the first surface 42. Light impinging on the first surface 42 along the field of view axis 32 passes through the first refractive element 40 and exits the second surface 44 along an intermediate axis 48.
The imaging system 28 further includes a second refractive element 50 formed of optically transparent material having a second index of refraction. The second refractive element 50 defines a third surface 52 separated from the second surface 44 by a refractive boundary layer 54. The second refractive element 50 defines a fourth surface 56 oriented to the third surface 52 at a second prism angle 58. Light impinging on the third surface 52 along the intermediate axis 48 passes through the second refractive element 50 and exits the fourth surface 56 along the image sensing device axis 34.
The image sensing device 36 is in optical communication with the second refractive element 50. Light defining an image from the field of view is directed into the image sensing device 36.
Continuing to refer to
Referring yet again to
By incorporating an optical device 30 with a plurality of refractive elements, an imaging system 28 design may be established that can be adapted to a variety of vehicle platforms using common components (i.e. the image sensing device 36, lenses, and electronics assemblies) for the remainder of the imaging system 28. Since the vehicle window rake angle 74 does not specifically alter the end refraction result, the specific shapes of the refractive elements may be common as well.
While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow. Moreover, the use of the terms first, second, etc. does not denote any order of importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items.
