Multi-color wide angle infrared optical systems and methods

Systems and methods for providing a wide angle multi-color infrared (IR) optical system are disclosed. In one embodiment, a system is provided that comprises a negative lens configured to receive an incoming light beam scene over a wide FOV and a beam splitter configured to receive incoming light beam associated with the incoming light beam scene from the negative lens and to provide light beams of a first color to a first channel and to provide light beams of a second color to a second channel. The system further comprises a first positive lens assembly arranged to focus light beams of the first color received from the beam splitter on a first image detector, and a second positive lens assembly arranged to focus light beams of the second color received from the beam splitter on a second image detector.

TECHNICAL FIELD

The present invention relates to optics and, more particularly, to multi-color wide angle infrared (IR) optical systems and methods.

BACKGROUND OF THE INVENTION

Wide angle IR optical systems have a variety of applications, such as in surveillance systems, tracking systems and other wide angle imaging systems.FIG. 1illustrates a prior art wide angle IR optical system10. For example, the illustrated system10can have an optical speed of F/2 meaning that the ratio of the effective focal length (EFL) of the optical system10and the diameter (D) of the entrance pupil is 2. The optical system10can, for example, have a 20°×20° field of view (FOV). The optical system10includes a negative lens assembly14utilized to collect scenes from a wide FOV12and a positive lens assembly20for focusing the incoming beams onto an image detector26. The negative lens assembly14is a first doublet that includes a negative lens16and a positive lens18. The positive lens assembly20is a second doublet formed of a negative lens22and a positive lens24. The positive lens assembly20includes an annular stop21on a front surface of the negative lens22for defining light beam bundles to be focused onto the image detector26.

The performance of the system10is reasonable, if the application is for a single color (i.e., single wavelength bandwidth). However, two important issues arise, when multi-color performance is required. First, there is little room for inserting an optical channel for an additional color. Secondly, even if one or more beamsplitters and the associate optical systems for the additional colors can be inserted, the aberrations (astigmatism, coma and spherical) introduced by the beamsplitters are extremely difficult to compensate. To minimize these aberrations, a convention optical design would require a sophisticated set of focusing lens that includes a compensator, which would substantially complicate the system design and would have many disadvantages, such as increasing weight, package volume, fabrication and alignment costs.

SUMMARY OF THE INVENTION

Systems and methods for providing a wide angle multi-color IR optical system are disclosed. In one aspect of the invention, a system is provided that comprises a negative lens configured to receive an incoming light beam scene over a wide FOV and a beam splitter configured to receive incoming light beam associated with the incoming light beam scene from the negative lens and to provide light beams of a first color to a first channel and to provide light beams of a second color to a second channel. The system further comprises a first positive lens assembly arranged to focus light beams of the first color received from the beam splitter on a first image detector, and a second positive lens assembly arranged to focus light beams of the second color received from the beam splitter on a second image detector.

In yet another aspect of the invention, another wide angle multi-color IR optical system is provided. The system comprises a negative lens configured to receive an incoming light beam scene over a wide field FOV and a beam splitter configured to receive incoming light beam associated with the incoming light beam scene from the negative lens and to provide light beams of a first color to a first channel and to provide light beams of a second color to a second channel. The system further comprises a first positive lens assembly arranged to focus light beams of the first color received from the beam splitter on a first image detector, a second positive lens assembly arranged to focus light beams of the second color received from the beam splitter on a second image detector, a first optical system stop positioned between and spaced apart from the beam splitter and the first positive lens assembly, a second optical system stop positioned between and spaced apart from the second positive lens assembly, a first filter and field flattener positioned between the first positive lens assembly and the first image detector, and a second filter and field flattener positioned between the second positive lens assembly and the second image detector. The first and second filter and field flatteners are configured to further limit the wavelength bandwidth of the first and second colors and to flattening the light beams of the first and second colors.

In yet another aspect of the invention a method of providing a wide angle multi-color IR optical system is provided. The method comprises providing a negative lens configured to receive an incoming light beam scene over a wide FOV, and arranging a beam splitter configured to receive incoming light beam associated with the incoming light beam scene from the negative lens and to provide light beams of a first color to a first channel and to provide light beams of a second color to a second channel. The method further comprises arranging a first and second positive lens assembly associated with respective first and second channels to focus light beams received from the beam splitter on respective image detectors, positioning first and second optical system stops between and spaced apart from the beam splitter and respective first and second positive lens, and positioning first and second filter and field flattener between respective first and second positive lens assemblies and respective image detectors. The first and second filter and field flatteners are configured to further limit the wavelength bandwidth of the first and second colors and to flattening the light beams of the first and second colors.

DETAILED DESCRIPTION OF INVENTION

Systems and methods for providing a wide angle multi-color IR optical system are disclosed. The systems and methods receive an incoming light beam scene over a wide FOV and split the incoming light beam associated with the incoming light beam scene to provide light beams of a first color to a first channel and to provide light beams of a second color to a second channel. The term color is defined as a respective spectral wavelength of light beams.

FIG. 2illustrates a wide angle multi-color IR optical system40in accordance with an aspect of the present invention. This system can have an optical speed of F/2 similar to the system10ofFIG. 1and can have a rectangular FOV of 20°×20°. The exemplary system40is a two channel configuration but could include additional channels based on a desired application and acceptable footprint. The two channels in the exemplary system include a transmission channel64and a reflection channel62. For example, a first channel can be assigned as a short wave infrared (SWIR) bandwidth, and a second channel can be assigned as a mid-wave infrared (MWIR) bandwidth. The system40includes a negative lens44that is employed to collect an incoming light beam scene from a wide FOV42. To minimize aberrations, a set of high order aspheric coefficients has been applied to a first surface43of the negative lens44. The fabrication of set of higher order aspherical coefficients can be accomplished by modern diamond-turned technology.

The collected light beam scene is transmitted to a dichroic coated phase plate46(or beamsplitter). The dichroic coating defines, which of a first wavelength bandwidth (or color) is the transmission channel64and which of a second wavelength bandwidth (or color) is a reflection channel62. A first surface45of the dichroic coated phase plate serves to both correct aberrations and to selectively reflect or transmit the first and second wavelength bandwidth of the light beams associated with the light beam scene. The incoming beams passes through the negative lens44, then either reflect from the first surface45of the phase plate46or transmits through the dichroic coated phase plate46. After either reflecting off or transmitting through the dichroic coated phase plate46, in each channel the beam converges to and is limited by an optical system stop48, then a positive lens assembly50is used to focus the beam, through a filter and field flattener56, onto respective image detectors58. The positive lens assembly50is a second doublet formed of a positive lens52and a negative lens54. Selection of spectral reflection and transmission channels can be achieved by applying an appropriate interference filter coating on the front surface45of the phase plate46.

It is to be appreciated that light beams passing through phase plate46suffer a great deal of image quality degradations. These are the aberrations caused by a converging beam going through a plane-parallel plate with a finite thickness. The aberrations are mainly due to astigmatism plus a small amount of the coma and spherical. To control these aberrations, a set of higher order aspherical coefficients are configured on the front surface45of the phase plate46to nullify the majority of astigmatism, and coma and spherical coefficients. This is similar to the conventional Schmidt corrector plate. In one aspect of the invention, the set of higher order aspherical coefficients configured on the front surface45of the phase plate46are decentered. A set of decentered higher order aspherical coefficients means that the coefficients are not centered with respect to the center of the phase plate46.

To accommodate the phase plate46in the system40, the separation between any two optical elements in the system40has to be of an adequate length to control aberrations, and the incoming cone angle has to be reasonable to achieve a better control in aberrations. With that, the separation between the negative lens44and the positive lens assembly50are greater than that of the conventional system10illustrated inFIG. 1. Since the optical system stops48are spaced apart from the positive lens assemblies50, as opposed to being formed on a front surface of the positive lens assembly20as illustrated inFIG. 1, better control in the beam bundle size can be achieved in the system40and therefore improved aberration control is accomplished.

The positive lens assemblies50are configured to optimize the optical image qualities, and to facilitate control in the Petzval curvature so that the images across the fields are flatter. This is accomplished by controlling the bending of each surface, by applying higher aspherical coefficients on a first surface51of the positive lens52, minimizing the chromatic color with the spectral band by using appropriate material for each lens of the positive lens assemblies50and correct bending on each surface, and optimizing the optical performances in each band by controlling the bending of each surface and the back focal distance. The positive lens assembly50in both the transmission and reflective channels62and64can be made identical, pending on the requirements, thus providing a substantial cost savings.

The last optical element in each channel is the filter and field flattener56. A front surface55of the filter and field flattener56can be coated with a band pass filter coating to further limit the wavelength bandwidth, and a back surface57has a concave surface to flatten the field. Also on top of the back surface57, a set of high order aspheric coefficients has been formed to reduce a bit of distortion and minimize substantially all residual aberrations. The above-described structural configuration of the system40provides for a compact, wide field, and fast 2-color optical system, which is nearly diffraction-limited except a small amount of distortion. The small amount of distortion can be compensated with digital electronics and image processing softwares, if needed.

FIG. 3illustrates another wide angle multi-color optical system70in accordance with an aspect of the present invention. This system70can have an optical speed of F/2 similar to the system10ofFIG. 1and can have a rectangular FOV of 20°×20°. The exemplary system70is also two channel configuration but could include additional channels based on a desired application and acceptable footprint. The two channels in the exemplary system70include a first channel92and a second channel94. For example, one channel can be assigned as a short wave infrared (SWIR) bandwidth, and the other channel as a mid-wave infrared (MWIR) bandwidth. The system includes a negative lens72that is employed to collect an incoming light beam scene from a wide FOV72.

The collected light beam scene is transmitted to a reflecting plate76. The reflecting plate76has a set of decentered high order aspheric coefficients similar to45ofFIG. 2. The light beams reflected from the reflecting pate76are split into two channels by a beam splitter78. The first channel92receives light beams of a first wavelength bandwidth and the second channel94receives light beams of a second wavelength bandwidth. After light beams of respective first and second wavelength bandwidths are split by the beam splitter78, in each channel the beams converge to and are limited by an optical system stop80, then a positive lens assembly82is used to focus the beam, through a filter and field flattener88, onto respective image detectors90. The positive lens assembly82is a second doublet formed of a negative lens84and a positive lens86.

The positive lens assembly82is configured to optimize the optical image qualities, and to facilitate control in the Petzval curvature so that the images across the fields are flatter88, which can be accomplished by the techniques discuss inFIG. 2. The last optical element in each channel is a filter and field flattener. A front surface87of the filter and field flattener88can be coated with a band pass filter to further limit the wavelength bandwidth, and a back surface89has a concave surface to flatten the field. Also on top of the back surface89, a set of high order aspheric coefficients has been formed to reduce a bit of distortion and minimize substantially all residual aberrations.

This system ofFIG. 3can be employed when special performances are required, such as improved straylight rejection. For example, if straylight encounters a refractive optical system (FIG. 1orFIG. 2) it has a significant impact in the background rejection. As the straylight illuminates the negative lens, it can rescatter immediately, then these scattered lights follow the optical train, and create unwanted background all the way to the focal plane directly. However, with a non on-axis reflecting refractor, such as illustrated in the system ofFIG. 3, straylight occurs only at a specific incoming angle, even if the first element has been illuminated with straylight at all angles. Thus, the sunshade length in the system ofFIG. 3can be shorter than the systems illustrated inFIG. 1orFIG. 2.

Additional, the system ofFIG. 3does not require a solar rejection coating to mitigate thermal issues, associated with the Sun and other thermal contributors. An open optical system, such as illustrated inFIG. 1orFIG. 2may require a solar rejection coating on the negative lens to reduce the thermal background. Furthermore, if either/both volume and/or weight are a constraint, the system ofFIG. 3provides for a more compact footprint, because the length of the second color channel along the z-axis is shorter than that in the system ofFIG. 1orFIG. 2.

FIG. 4illustrates a methodology200for providing a wide angle multi-color IR optical system40in accordance with an aspect of the present invention. The methodology begins at100where a negative lens is provided that is configured to receive an incoming light beam scene over a wide FOV. A front surface of the negative lens may be configured with set of higher order aspherical coefficients. At110, a beam splitter is arranged to receive the incoming light beams from the negative lens and to provide light beams of a first color or wavelength bandwidth to a first channel and to provide light beams of a second color or wavelength bandwidth to a second channel. The beam splitter can be a dichroic coated phase plate configured to reflect one of the first and second color, and to pass the other of the first and second color. A front surface of the dichroic coated phase plate may be configured with set of higher order aspherical coefficients. The set of higher order aspherical coefficients may be decentered. Alternatively, the beam splitter can be configured to receive light from a reflecting plate and split the light beams into the first color and the second color. The methodology proceeds to120.

At120, positive lens assemblies associated with respective first and second channels are arranged to focus light beams received from the beam splitter on respective image detectors. A front surface of the positive lens assemblies may be configured with a set of higher order aspherical coefficients. At130, first and second optical system stops are positioned between and spaced apart from the beam splitter and respective first and second positive lens. At140, filter and field flatteners are positioned between respective positive lens assemblies and respective image detectors. The filter and field flatteners can be coated with a band pass filter coating to further limit the wavelength bandwidth, and to have a back concave surface to flatten the field. A front surface of the filter and field flatteners may be configured with set of higher order aspherical coefficients.