Compact, wide-field-of-view imaging optical system

An imaging optical system includes a first imaging structure having a first optical axis and a first field of view, wherein the first imaging structure forms an image on a common focal plane, and a second imaging structure having a second optical axis parallel to the first optical axis and a second field of view different from the first field of view, wherein the second imaging structure forms an image on the common focal plane. The imaging structures preferably contain identical lens modules, most preferably identical Petzval lenses, and achromatic or apochromatic prisms of different spatial orientations. A planar sensor structure lies in the common focal plane, wherein the first optical axis and the second optical axis pass through the planar sensor structure.

This invention relates to a wide-field-of-view optical system and, more particularly, to such an optical system having a short physical length and a small size.

BACKGROUND OF THE INVENTION

Wide-field-of-view (WFOV) imaging optical systems are used in a number of applications. For example, surveillance systems, helmet-mounted sensors, missile warning sensors for aircraft, and space and aircraft acquisition sensors all may use WFOV optical systems. In this context, a WFOV optical system typically seeks a field of view of at least 60 degrees in at least one dimension.

Most WFOV imaging optical systems are of the inverse telephoto or fisheye type. The inverse telephoto optical system has a front (that is, nearest the scene) lens or lens group with a negative optical power, in order to view a wide field of view, and a rear (that is, nearest the eye or the sensor) lens or lens group with a positive optical power to focus the light rays onto the focal plane. This distribution of optical powers is necessary to minimize the angle of incidence of the chief ray, thereby minimizing the aberration. The associated effect, however, is that the incoming beam first diverges when it passes through the negative-optical-power front lens or lens group. The additional lenses required to focus the light beam to the focal plane result in an overall length of the optical system that is typically 3–8 times the effective focal length. It is therefore difficult to design a compact, light-weight WFOV optical system. Additionally, the numerical aperture of the inverse telephoto lens is typically relatively small, resulting in a low signal-to-noise ratio and a required long exposure time to produce a usable image.

There is a need in many applications for a WFOV imaging optical system that is more compact and lighter in weight than the inverse telephoto lens. The present invention fulfills this need, and further provides related advantages.

SUMMARY OF THE INVENTION

The present invention provides a wide-field-of-view (WFOV) imaging optical system that is compact and light in weight, and is typically one-half or less of the length of a WFOV inverse telephoto lens system having a comparable field of view. The WFOV imaging optical system may also have a large numerical aperture, resulting in good light gathering capability, a high signal-to-noise ratio, and good light sensitivity.

In accordance with the invention, an imaging optical system comprises a first imaging structure having a first field of view and a second imaging structure parallel to the first imaging structure and having a second field of view different from the first field of view. The first imaging structure includes a first lens module having a first-lens-module input end, a first optical axis, and a first focal plane, and an achromatic or apochromatic first prism positioned adjacent to the first-lens-module input end and having a first prism outer face remote from the first-lens-module input end. The first optical axis passes through the first prism, and the first prism has a first normal orientation of the first prism outer face. The second imaging structure includes a second lens module having a second-lens-module input end, a second optical axis parallel to the first optical axis, and a second focal plane. The first focal plane and the second focal plane are coplanar in a common focal plane. The second imaging structure further includes an achromatic or apochromatic second prism positioned adjacent to the second-lens-module input end and having a second prism outer face remote from the second-lens-module input end. The second optical axis passes through the second prism, and the second prism has a second normal orientation of the second prism outer face different from the first normal orientation of the first prism outer face. It is preferred that the first lens module and the second lens module are identical. Preferably, there is an opaque light baffle between the two imaging structures to isolate them and prevent spillover of light from one to the other.

A planar sensor structure lies in the common focal plane. The first optical axis and the second optical axis pass through the planar sensor structure. Most preferably, the planar sensor structure is exactly one focal plane array that senses the images of both of the imaging structures, although more than one focal plane array may be used in some applications. The planar sensor structure may be selected as sensitive to any light wavelength, including ultraviolet, visible, and infrared, and subranges thereof.

The lens modules may be of any operable type. Preferably, the first lens module comprises a first Petzval lens, and the second lens module comprises a second Petzval lens. Most preferably, the two Petzval lenses are identical.

Desirably, the first normal orientation and the second normal orientation both lie in a first-second optical axis plane that also contains the first optical axis and the second optical axis. A field of view of the imaging optical system in the first-second optical axis plane is a sum of the first field of view in the first-second optical axis plane plus the second field of view in the first-second optical axis plane.

The above-described approach increases the field of view in the first-second optical axis plane. To increase the field of view in a plane perpendicular to the first-second optical axis plane, and thereby achieve an increased field of view over all angles, the imaging optical system may further include a third imaging structure having a third optical axis parallel to the first optical axis and a third field of view. The third imaging structure forms an image on the common focal plane. The imaging optical system has a fourth imaging structure having a fourth optical axis parallel to the third optical axis and a fourth field of view different from the third field of view. The fourth imaging structure forms an image on the common focal plane. The third optical axis and the fourth optical axis lie in a third-fourth optical axis plane that is perpendicular to the first-second optical axis plane. The result is increased fields of view about two mutually perpendicular axes that are each perpendicular to the optical axes. Where there are more than two imaging structures, there is preferably an opaque light baffle isolating each of the imaging structures from the other imaging structures.

In another embodiment, an imaging optical system comprises a first imaging structure having a first optical axis and a first field of view, wherein the first imaging structure forms an image on a common focal plane, and a second imaging structure having a second optical axis parallel to the first optical axis and a second field of view different from the first field of view. The second imaging structure also forms an image on the common focal plane. A planar sensor structure lies in the common focal plane, and the first optical axis and the second optical axis pass through the planar sensor structure. Compatible features discussed elsewhere herein may be used with this embodiment as well, and there may be a third and fourth imaging structure oriented to increase the field of view in the third-fourth optical axis plane perpendicular to the first-second optical axis plane.

The present approach provides a WFOV imaging optical system that is more compact than conventional WFOV optical systems such as the inverse telephoto optical system. The length of the present WFOV imaging optical system is typically one-half or less of the length of the inverse telephoto optical system having the same field of view. The present approach also has a large numerical aperture, leading to a high signal-to-noise ratio for the imaging optical system.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1depicts an imaging optical system20. The imaging optical system20includes a first imaging structure22having a first field of view24, and a second imaging structure26parallel to the first imaging structure22and having a second field of view28different from the first field of view24.

The first imaging structure22includes a first lens module30having a first-lens-module input end32closest to the scene, a first optical axis34, and a first focal plane36. The depicted preferred embodiment of the first lens module30includes a first lens38, a second lens40, and a third lens42, although other specific lens arrangements may be used. The first lens38is between the input end32and an aperture stop44, and the second lens40and third lens42are between the aperture stop44and the first focal plane36. The lenses of the first lens module30are selected to image light rays entering the input end32from a scene onto the first focal plane36. The first lens module30has a viewing field52(in the absence of a prism).

The first lens module30is preferably a Petzval lens. In the Petzval lens, the first lens38(or grouping of lenses on one side of the aperture stop44, here the side closest to the scene) has a positive optical power, and the second lens40and third lens42(the lenses on the other side of the aperture stop44, here the side closest to the first focal plane36) together have a positive optical power. By contrast and will be discussed in greater detail subsequently in relation toFIG. 5, in the inverse telephoto lens the lens group closest to the scene has a negative optical power.

The first imaging structure22further includes an achromatic or apochromatic first prism46positioned adjacent to the first-lens-module input end32and having a first prism outer face48remote from the first-lens-module input end32. An achromatic prism has no primary angular spread. An apochromatic prism has no primary angular spread and no secondary angular spread. See, for example, U.S. Pat. No. 5,625,499, whose disclosure is incorporated by reference. The first optical axis34passes through the first prism46. The first prism46may be described as having a first normal orientation50of the first prism outer face48.

The second imaging structure26includes a second lens module60having a second-lens-module input end62closest to the scene, a second optical axis64, and a second focal plane66. The depicted preferred embodiment of the second lens module60includes a first lens68, a second lens70, and a third lens72, although other specific lens arrangements may be used. The first lens68is between the input end62and an aperture stop74, and the second lens70and third lens72are between the aperture stop74and the second focal plane66. The lenses of the second lens module60are selected to image light rays entering the input end62from a scene onto the second focal plane66. The second lens module60has a viewing field82(in the absence of a prism).

The second lens module60is preferably a Petzval lens. In the Petzval lens, the first lens68(or grouping of lenses on one side of the aperture stop74) has a positive optical power, and the second lens70and third lens72(the lenses on the other side of the aperture stop74) together have a positive optical power.

Preferably, the first lens module30and the second lens module60are identical and, more preferably, are identical Petzval lenses.

The second imaging structure26further includes an achromatic or apochromatic second prism76positioned adjacent to the second-lens-module input end62and having a second prism outer face78remote from the second-lens-module input end62. The second optical axis64passes through the second prism76. The second prism76may be described as having a second normal orientation80of the second prism outer face78. The second normal orientation80of the second prism outer face78is and must be different from the first normal orientation50of the first prism outer face48.

That is, although the first lens module30and the second lens module60may be identical in structure, the first imaging structure22and the second imaging structure26may not be identical both in structure and in spatial orientation, because the second normal orientation80of the second prism outer face78is and must be different from the first normal orientation50of the first prism outer face48. The directions of the normal orientations50and80, as well as the index of refraction of the prism, determine the angular positions of the respective fields of view24and28of the respective imaging structures22and26. The first imaging structure22and the second imaging structure26are therefore necessarily not identical to each other in structure and spatial orientation in order to achieve an enhanced total field of view of the imaging optical structure20, because of the different orientations of the first prism46and the second prism76. The first imaging structure22and the second imaging structure26may be identical in physical structure, but they cannot be identical in spatial orientation. If the first imaging structure22and the second imaging structure26were identical to each other in respect to the spatial orientations of the prisms46and76, the present approach would not function in its intended manner to achieve an increased field of view of the imaging optical system20with a reduced length and size.

The presence of the prisms46and76makes it possible to have the short, compact WFOV system of the present approach. In an alternative approach that is not within the scope of the present approach, the lens modules30and60do not have their optical axes34and64parallel to each other, and instead are tilted outward with respect to the normal to the planar sensor structure90. Consequently, there is an angular separation between the “best” image planes of the two lens modules30and60, relative to the planar sensor structure90. The result is a significant defocussing of the image.

To prevent the formation of a “ghost” image spillover between the lens modules30and60in the present approach, a baffle84in the form of an opaque wall is optionally but preferable positioned between the two lense modules30and60over at least a portion of their lengths adjacent to the lens planar sensor structure90. Where there are more than two imaging structures, each imaging structure is preferably isolated from the other imaging structures by an opaque light baffle.

It is preferred that an angle A between the first optical axis34and the first normal orientation50be +θ, and an angle B between the second optical axis64and the second normal orientation80be −θ. That is, the magnitudes of these angles are the same, but they are of different sign.

A planar sensor structure90is positioned to receive light at the first focal plane36and at the second focal plane66. The first optical axis34and the second optical axis64pass through the planar sensor structure90. The planar sensor structure90is desirably one or more focal plane arrays that receive the light and convert it to electrical or other analyzable signals. Focal plane arrays are known for the various wavelength ranges of light, including ultraviolet, visible, and infrared ranges, and subranges thereof. Preferably, the first focal plane36and the second focal plane66lie in and are coplanar in a common focal plane92. In this preferred embodiment, the planar sensor structure90lies in a single plane in the common focal plane92. Most preferably, the planar sensor structure90is a single (i.e., exactly one) focal plane array94, although more than one focal plane array may be used where appropriate.

As used herein, a field of view is defined as the maximum angular viewing range, for the imaging optical system20taken as a whole or for any of the individual imaging structures, such as the imaging structures22,26. The first field of view24is the maximum angular viewing range of the first imaging structure22, including both the first lens module30and the first prism46. The second field of view28is the maximum angular viewing range of the second imaging structure26, including both the second lens module60and the second prism76.

In the preferred embodiment ofFIG. 1, the first normal orientation50and the second normal orientation80lie in a first-second optical axis plane96, which inFIG. 1is the plane of the illustration. The first-second optical axis plane also contains the first optical axis34and the second optical axis66. In the embodiment, a total field of view98of the imaging optical system20in the first-second optical axis plane96is a sum of a numerical value of the first field of view24in the first-second optical axis plane96plus the numerical value of the second field of view28in the first-second optical axis plane96.

The imaging optical system20ofFIG. 1achieves an increased field of view98in the first-second optical axis plane96, which is the plane of the illustration inFIG. 1. In many applications, it is desirable that the field of view be increased in two mutually perpendicular planes, so that the field of view is increased in both the azimuthal and the elevational angular orientations. The present approach allows such a general increasing of the fields of view, as described next.

FIGS. 2 and 3are views taken parallel to the optical axes34and64, and additionally illustrating the presence of a third imaging structure100having a third optical axis102parallel to the first optical axis34and to the second optical axis64. The third imaging structure has a third field of view. The third imaging structure100desirably forms its image on the common focal plane92(which is not visible inFIGS. 2 and 3). There is also a fourth imaging structure104having a fourth optical axis106parallel to the third optical axis102(and thence parallel to the first optical axis34and to the second optical axis64). The fourth imaging structure has a fourth field of view different from the third field of view. The fourth imaging structure104forms its image on the common focal plane92. The third optical axis102and the fourth optical axis106lie in a third-fourth optical axis plane108that is perpendicular to the first-second optical axis plane96.

The third imaging structure100and the fourth imaging structure104desirably each have the same structure (but not the same spatial orientation), each comprising a lens module and a prism, as the first imaging structure22and the second imaging structure26. The discussion of the first imaging structure22and the second imaging structure26is incorporated herein as modified to apply to the third imaging structure100and the fourth imaging structure104. The third normal orientation and the fourth normal orientation are different from each other and from the first normal orientation50and the second normal orientation80. It is preferred that the angle between the third optical axis102and the third normal orientation be +φ, and the angle between the fourth optical axis106and the fourth normal orientation be −φ measured in the third-fourth axis optical plane108. That is, the magnitudes of these angles are the same, but they are of different sign. The values of θ and φ may be the same or different. Stated alternatively, in the preferred embodiment ofFIG. 2, a view in the third-fourth axis optical plane108would be like that ofFIG. 1, except that the third imaging structure100and the fourth imaging structure104would be laterally separated, and the magnitudes of the angles A and B may be different from the magnitudes of angles A and B inFIG. 1. In the embodiment ofFIG. 3, imaging structures all meet at a center line.

FIG. 4depicts an integrated imaging device109using the present approach. The 2×2 imaging optical system20, including the first imaging structure22, the second imaging structure26, the third imaging structure100, the fourth imaging structure104, and the planar sensor structure90, are contained in a housing110supported on a front side of a substrate112. Digital electronics114and analog electronics116, as needed, are supported on an oppositely disposed back side of the substrate112. Output signals from the planar sensor structure90are provided to the electronics114and116for processing.

In an embodiment of the integrated imaging device109designed by the inventors, the imaging optical system20has an 60 degree×80 degree field of view98. The length LOSof the imaging optical system20is 0.53 inches, the total length LTis 1.2 inches, and the substrate112is square in plan view with a total width WTin each direction of 2 inches. The length LTis about ⅓ of the length required for the same integrated imaging system using a conventional single inverse telephoto or fisheye lens to achieve the same field of view. The imaging structures may be designed using commercially available ray-tracing software such as the Code VRor Zemax software.

FIG. 5illustrates the ray paths for an inverse telephoto or fisheye lens structure120that is not within the scope of the present invention, and over which the present invention is an improvement. A front lens group122has a negative optical power, requiring greater positive optical power in a back lens group124. The present approach avoids this arrangement of negative optical power/positive optical power through the use of the achromatic or apochromatic prisms, so that the imaging structures employ positive optical power/positive optical power (although weak negative power lenses may be present to aid in shaping the focal surface to planar).