Wide field of view optical module for linear sensor

A sensing module includes a high speed, linear image sensor and an optical unit facing the sensor. The unit includes an optical element having a curved surface and a covering on an outward surface of the optical element. The covering has a slit formed therein. The optical unit images a wide field of view onto a single pixel of the linear sensor, wherein light impinging normal to the slit, at any location along the slit, is imaged on a central pixel of the linear sensor while light impinging at a non-normal angle to the slit, at any location along the slit, is imaged on a non-central pixel of the linear sensor.

FIELD OF THE INVENTION

The present invention relates to optical lenses generally and to optical lenses for linear imaging in particular.

BACKGROUND OF THE INVENTION

As can be seen inFIG. 1A, to which reference is now made, a two-dimensional image sensor10captures a two-dimensional scene12. Linear image sensors, on the other hand, provide a single line of sensors. They require significantly less bandwidth and computation than 2D image sensors. However, as shown inFIG. 1Bto which reference is now made, a linear sensor16only views a portion (e.g. one line) of scene12.

To utilize any kind of image sensor, whether 2D sensor10or linear sensor16, focusing units14formed of lenses of various kinds, are usually placed in front of the sensors to focus light onto the sensors.

Reference is now made toFIG. 2, which illustrates a prior art measuring system used in a motion tracking system discussed in U.S. Pat. No. 6,324,296, entitled “BI-DISTRIBUTED-PROCESSING MOTION TRACKING SYSTEM FOR TRACKING INDIVIDUALLY MODULATED LIGHT”. The system of U.S. Pat. No. 6,324,296 has a linear sensor16and a cylindrical lens20. As can be seen, linear sensor16is placed perpendicularly to cylindrical lens20. Cylindrical lens20gathers light from a point P1and provides it to sensor16. The sensor ofFIG. 2is fixed in space and views a scene in which objects to be tracked move around.

SUMMARY OF THE PRESENT INVENTION

There is provided, in accordance with a preferred embodiment of the present invention, a sensing module including a high speed, linear image sensor and an optical unit facing the sensor. The unit includes an optical element having a curved surface and a covering on an outward surface of the optical element. The covering has a slit formed therein. The optical unit images a wide field of view onto a single pixel of the linear sensor, wherein light impinging normal to the slit, at any location along the slit, is imaged on a central pixel of the linear sensor while light impinging at a non-normal angle to the slit, at any location along the slit, is imaged on a non-central pixel of the linear sensor.

Further, in accordance with a preferred embodiment of the present invention, the optical element is a single monolithic lens.

Moreover, in accordance with a preferred embodiment of the present invention, the outward surface is flat, the curved surface is an inward surface of the optical element facing the sensor, and a magnification of the optical element is varied to map vertical lines to a single pixel of the linear sensor with minimal distortion.

Further, in accordance with a preferred embodiment of the present invention, the outward surface is flat and the optical element is a freeform lens. The lens includes an aspheric optical surface with variable optical power located on an inward surface of the lens facing the sensor. The surface focuses light incident on the slit at each vertical elevation angle across a linear sensor. The optical power of the lens varies to maintain a constant magnification.

Alternatively, in accordance with a preferred embodiment of the present invention, the outward surface is the curved surface and the optical element is rotationally invariant around a longitudinal axis of the linear sensor.

Further, in accordance with a preferred embodiment of the present invention, the optical element is a domed lens and the covering covers the outer surface of the domed lens.

Alternatively, in accordance with a preferred embodiment of the present invention, the optical element is an aspheric toroidal lens mounted in an opaque dome and the covering is on the lens.

Finally, in accordance with a preferred embodiment of the present invention, the optical element is an oval-shaped dome.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Applicant has realized that, for tracking applications where the tracking unit moves around with the object whose position is being tracked, whether as part of a virtual reality system or on a moving device (a mobile phone, a drone, a car, etc.), it is necessary to compress a two-dimensional scene with as few ill effects, such as distortion or rotation errors, as possible, in order to provide accurate and fast information about the angular location of the object being tracked.

Applicant has also realized that placing a slit onto an outer surface of an optical element of some kind, may provide the desired image compression with relatively few side effects.

A Domed Slit

Reference is now made toFIG. 3, which illustrates a sensing module100of the present invention. Sensing module100comprises a high speed, linear image sensor102and an optical element104comprising a curved, narrow slit110positioned over, or on, the curved outer surface of an optically clear, dome substrate120.

Linear sensor102may be oriented perpendicularly to the direction of slit110and one point, such as the midpoint, of linear sensor102may be located at or near a center O of the radius of curvature of slit110. Linear sensor102may be any suitable high speed, linear sensor, such as the ELIS-1024, commercially available from Dynamax Imaging of Canandaigua N.Y.

In a preferred embodiment, clear dome substrate120may be a glass or plastic half ball lens. Alternatively, substrate120may be hollowed out. In an alternative embodiment (not shown), optical substrate120may be a solid or hollowed out half cylinder with the slit oriented at the center of the curved section of the cylinder.

Slit110may be formed according to any suitable means. For the embodiment where substrate120is a dome, slit110may be formed by printing a covering for the curved surface of dome substrate120. The covering may have two opaque regions130separated by a clear region forming slit110for light to pass through.

It will be appreciated that light can enter slit110at any azimuth angle α, parallel to sensor102, and at any collection angle β along slit110, perpendicular to sensor102. From the principles of geometrical optics, it is evident that light entering slit110from a particular direction will illuminate a specific region of the sensor defined by azimuth angle α and a radius R of dome120. For example, light coming from an angle normal to the outer surface of dome120will illuminate a region A of sensor102. Similarly, light from any collection angle β (all of which are normal to the outer surface of dome120) may reach sensor102at the same region A. However, light coming from non-normal angles α will reach other regions, such as regions B and C, of sensor102.

In practice, the geometry of dome120may effectively integrate all of the light along each arc at angle α into a single data point along sensor102. Applicant has realized that the angle of the optical collection is largely independent of the distance to the light source so optical element104may integrate light from light sources of any size room and may also track lights or the sun outdoors.

The angular resolution may be determined by the width of slit110and radius R between slit110and sensor102, with larger slits collecting more light but providing lower resolution. The overall detection angle will be limited by radius R of dome120and of the size of sensor102.

It may be appreciated that the width of slit110or the optical transmission, defined by the amount of optical transparency, may be varied along the length of the slit to normalize, otherwise compensate for, or enhance, any geometrical effects that may reduce the strength of the incident signal as the collection angle β increases.

A Toroidal Lens with Dome Slit

Reference is now made toFIG. 4, which shows an alternative, improved embodiment of the present invention in a perspective schematic view. In this embodiment, the sensing module is sensing module200, comprising linear image sensor102and an optical element204comprising an aspheric toroidal lens210. Lens210may be mounted in an opaque dome212with a slit214defining the aperture of lens210. Lens element210may have a curved inner surface216which may act to focus the light arriving across slit214to a single point on sensor102. This may combine the resolution of a narrow slit, such as slit110ofFIG. 3, with the light collection efficiency of a wider slit. As in the previous embodiment, sensor102may be placed at or near center O of the radius of curvature of the slit aperture214of toroidal lens210and may be oriented generally perpendicular to slit214.

It will be appreciated that the focusing power of inner surface216may be adjusted to ensure that each pixel of sensor102may respond similarly for all angles impinging upon it.

It will be appreciated that optical elements104and204may optically compress a two-dimensional scene to reduce the data bandwidth. It will also be appreciated that use of linear sensor102may reduce the computational requirements compared to conventional 2D or 3D image processing. Sensing modules100and200may be useful for identifying, locating and tracking bright light sources as well as high contrast vertical features in a scene and may be useful in the tracking system discussed in the application entitled, “Method and Apparatus for Indoor Positioning”, being filed on the same day herewith, assigned to the common assignee of the present invention, and incorporated herein by reference.

In an alternative embodiment, the dome may be aspherical or oval-shaped, having one radius of curvature along the direction of the slit and a second radius of curvature along the axis of sensor102. This may allow the slit to be located closer to sensor102, reducing the overall height of the module, while still utilizing the entire length of the sensor and achieving sensing angles extending greater than 90 degrees.

It will be appreciated that in the embodiments ofFIGS. 3 and 4, the position of light on sensor102from a particular azimuth angle α is generally invariant to the rotation of sensor module100or200around a main axis103of sensor102. Thus, a user may rotate sensor module100or200and the light from a particular light source, like a light bulb, will always impinge on the same section of sensor102. This invariance may be achieved since sensor102is located at the center O of the radius of curvature.

Improved Optical Element

Applicant has realized that, in order to identify and track vertical features in a scene, it is desirable to image all the points of a vertical feature onto a single pixel of the linear sensor. This is challenging to achieve in practice with low cost imaging optics, particularly when imaging over a very large field of view. Cylindrical lenses, which are commonly employed with linear sensors, suffer from image distortion and their focus is poorly corrected at large angles. The curved slits in the embodiments ofFIGS. 3 and 4also introduce distortion of vertical features as a result of their curvature.

However, a straight slit may provide a relatively, distortion free mapping. It will be appreciated that the optical resolution is directly proportional to the slit width and such a straight slit may not have the rotational invariance of the embodiments ofFIGS. 3 and 4.

Reference is now made toFIG. 5, which illustrates a sensing module300comprising high speed linear sensor102and a single, free form optical element or lens304that is generally optimized to produce a largely distortion free and relatively sharply focused mapping of vertical features over large fields of view onto the individual pixels of linear sensor102. As in the previous embodiments, optical element304may be positioned in front of horizontal linear sensor102.

Lens304may comprise a relatively flat outward surface222and an aspheric optical inward surface230, designed to face linear sensor102. Inward surface230may have a positive radius of curvature along a horizontal axis at a central point P2and a negative radius of curvature along a vertical axis at point P1designed to be placed opposite linear sensor102. Outward surface222may be partially covered to create a largely rectangular vertical slit220, which may form an optical aperture.

As can be seen, light may pass through slit220and into lens304until it reaches inward surface230, which may focus the light sharply onto linear sensor102.

It will be appreciated that the optical power may be decreased when moving out from the center of the lens while the thickness may be increased to adjust the focus while maintaining a constant magnification. As shown inFIG. 5, a ray272of light incident on lens330from a central vertical line270and reaching sensor102from a direction normal to slit220, at an angle β1, may pass through a small thickness (T1) and a small radius of curvature (R1) of lens330to be sharply focused on sensor102. On the other hand, a ray274of light incident on slit220and reaching sensor at a larger incident angle, labeled β2, will encounter a cross section of lens330with a greater lens thickness (T2) and a larger radius of curvature (R2) to also be sharply focused on the same pixel A of sensor102. It will be appreciated that the parameters R and T may be adjusted so that light exiting the lens330at point P2, which is at a greater distance from the sensor than point P1, is also brought into sharp focus at point A.

Thus, the optical power of aspheric optical surface230may be set at each point so that the magnification of the azimuthal angle α may remain constant irrespective of the collection angle β (i.e. a point along slit220) of incidence.

Reference is now made toFIG. 6A, which illustrates light incident on sensor102from 2 different azimuth angles α (from central vertical line270and from a non-central vertical line280). Note that light ray272impinges on point A of sensor102. However, like the embodiments ofFIGS. 3 and 4, light ray282from non-central vertical line280impinges on point C of sensor102.

Reference is now made toFIG. 6B, which shows superposition of light incident on sensor102at 2 different azimuth angles α (from vertical lines270and280and at 2 different points β (β1and β2) along slit220. Note that features located along a constant horizontal azimuthal angle α, such as features from light rays282and284and those from light rays272and274, may map to the same pixel (C and A, respectively), regardless of the collection angle β, of linear sensor102.

An example of an aspheric surface Z(x,y) meeting the above requirements may be given by the following equation:
Z(x,y)=a(y)X2+b(y)
Where:
P=abs(y);
a(y)=0.001*(−8.736P3+47.52P2+0.254P−347.2);
b(y)=0.001*(−18.27P3+94.21P2−19.56P+2263);  Equation 1

−1.76<X<1.76; and

Such a freeform lens may readily be fabricated by single point diamond turning or by injection molding.

Additional Embodiments

It is appreciated that any of the optical elements described hereinabove may be used for linear imaging in conjunction with a 2D sensor, such as when the sensor is operated in a rolling shutter or windowing mode.

Moreover, a measurement unit may have multiple types of optical elements together in a single unit. This may provide a measurement unit with both rotation invariance as well as distortion-free mapping.