Abstract:
A method includes configuring an imaging lens section to be free of structure with optically refractive power and to have a lens with an optically diffractive characteristic, and passing radiation from a scene through the imaging lens section, the imaging lens section causing the radiation to form an image at an image plane. An apparatus includes an imaging lens section which is responsive to radiation from a scene for causing the radiation to form an image at an image plane, the imaging lens section being free of structure with optically refractive power and including a lens which has an optically diffractive characteristic.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    This invention relates in general to optical systems and, more particularly, to optical systems which form an image in response to incident radiation.  
         BACKGROUND OF THE INVENTION  
         [0002]    There are a variety of optical systems which can form an image in response to incident radiation. Some of these optical systems are specifically configured to image infrared radiation. In recent years, there has been a decrease in the cost of optical systems which image infrared radiation. Nevertheless, the current cost of infrared imaging optical assemblies is still too high to permit wide use of these assemblies in high volume, low cost markets such as the automotive industry, where competitive price pressures are very strong.  
           [0003]    Some of the techniques which have been used in recent years to reduce the cost of infrared imaging lens assemblies have included replacement of some (but not all) refractive lenses with diffractive lenses, in order to eliminate costly refractive elements. Further, proper material selection for some elements, such as use of an appropriate infrared glass, has permitted the formation of lenses using some high volume manufacturing processes, such as molding or casting, thereby reducing fabrication costs. The result has been infrared imaging lens assemblies which contain a combination of refractive optics and diffractive optics. While systems of this type have been generally adequate for their intended purposes, they have not been satisfactory in all respects.  
         SUMMARY OF THE INVENTION  
         [0004]    From the foregoing, it may be appreciated that a need has arisen for a method and apparatus which can image radiation, and which can be easily manufactured at low cost and in large volumes. One form of the invention involves an apparatus having an imaging lens section which is responsive to radiation from a scene for causing the radiation to form an image at an image plane, the imaging lens section being free of structure with optically refractive power and including a lens which has an optically diffractive characteristic.  
           [0005]    Another form of the invention involves a method which includes configuring an imaging lens section to be free of structure with optically refractive power and to have a lens with an optically diffractive characteristic, and passing radiation from a scene through the imaging lens section, the imaging lens section causing the radiation to form an image at an image plane.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawings, in which:  
         [0007]    [0007]FIG. 1 is a diagram of a lens assembly which images infrared radiation using only diffractive optics, and which embodies aspects of the present invention; and  
         [0008]    [0008]FIG. 2 is a graph showing a nominal modulation transfer function for the lens assembly of FIG. 1 as a function of fractional bandwidth.  
     
    
     DETAILED DESCRIPTION  
       [0009]    [0009]FIG. 1 is a diagrammatic view of a lens assembly  10  which embodies aspects of the present invention. As discussed below, the lens assembly  10  does not have any structure which is capable of refracting radiation, but instead uses only diffractive structure to effect imaging of radiation.  
         [0010]    The lens assembly  10  receives infrared radiation emitted by a scene which is shown diagrammatically at  12 , and influences this radiation in a manner so that it forms at an image  14  at an image plane. The disclosed embodiment is configured to effect imaging of far infrared radiation having wavelengths in a waveband of 8 to 14 microns. However, the present invention is not limited to this particular waveband, and could alternatively be used to effect imaging of near infrared radiation having wavelengths in a waveband of approximately 3 to 5 microns, or narrowband radiation in some other portion of the optical spectrum, including but not limited to visible radiation.  
         [0011]    The lens assembly  10  includes two lenses in the form of lens elements  16  and  17 . In the disclosed embodiment, the lens elements  16  and  17  are each made from silicon. However, they could alternatively be made of any other suitable material, including but not limited to an infrared polymer, or a combination of silicon and an infrared polymer. As discussed above, the disclosed embodiment is configured to effect imaging of radiation in the far infrared waveband, but could be adapted for use in other wavebands. It will be recognized that the particular material used for each lens element will depend on the particular waveband within which that element is being used.  
         [0012]    The side of each lens element  16  or  17  nearest the scene  12  is referred to herein as the first or front surface thereof, and the opposite side of each lens element  16  or  17  is referred to herein as the second or rear surface thereof. The lens element  16  has a diffractive surface  21  on the rear side thereof, and the lens element  17  has a diffractive surface  22  on the rear side thereof.  
         [0013]    As explained above, the lens elements  16  and  17  in the disclosed embodiment are made from silicon. The diffractive surface  21  or  22  on the rear side of each lens element  16  or  17  is formed by etching the material of the lens element, or alternatively by embossing the material of the lens element. Etching and embossing techniques suitable for forming the diffractive surfaces  21  and  22  are known in the art, and are therefore not described here in detail. The formation of diffractive surfaces through the use of etching or embossing techniques permits each of the lens elements  16  and  17  to be accurately and efficiently manufactured at low cost and in large volumes.  
         [0014]    A diamond-like carbon (DLC) coating  41  is provided on the front side of the lens element  16 . Suitable DLC coating materials are well known in the art. In the disclosed embodiment, the DLC coating  41  is a multi-layer coating of a type known in the art, and is therefore not described here in detail. The DLC coating  41  is a hard coating that protects the lens element  16  from scratching or other damage due to the external environment. By providing the coating  41  on the lens element  16 , it is not necessary for the lens assembly  10  to have a separate protective non-imaging window element disposed between the scene  12  and the lens element  16 , thereby reducing the overall cost of the lens assembly  10 .  
         [0015]    A bandpass filter coating  43  is provided on the front surface of the lens element  17 . The bandpass filter coating  43  serves as a narrow pass filter which rejects radiation other than radiation in the specific wavebend of interest, which in the disclosed embodiment is 8 to 14 microns. The bandpass filter coating  43  actually includes a number of separate layers, but they are not separately illustrated because the structure of the filter coating  43  is technology known in the art.  
         [0016]    Anti-reflective (AR) coatings  46  and  47  of a known type are provided on each of the rear surfaces  21  and  22  of the lens elements  16  and  17 , which are the diffractive surfaces. The AR coatings  46  and  47  help to reduce the loss of energy which would otherwise occur as a result of undesirable reflections if these surfaces were left uncoated. In particular, the AR coatings reduce the Fresnel reflection losses and raise the transmittance of the lens elements  16  and  17 . In the disclosed embodiment, the AR coatings  46  and  47  are each a single-layer coating of a known type, but it would alternatively be possible to use a multi-layer AR coating.  
         [0017]    Some specific characteristics of the lens assembly  10  are set forth in TABLE 1. In TABLE 1, the length dimension refers to the distance from the DLC coating  41  to the image  14 . The fractional bandwidth of operation within the wavelength range of operation is defined by the formula:  
         (λ1−λ2)/((λ1+λ2)/2).  
         [0018]    For example, where the wavelength range of operation is from 8 microns to 14 microns, λ1 in this formula would be 14 microns, and λ2 would be 8 microns.  
                             TABLE 1                       CHARACTERISTICS                                    Field of View   25 degrees           Effective Focal Length   23 mm           F/Number   F/1           Total Number of Elements   2           Flat Surfaces   4           Diffractive Surfaces   2           Aspheric Surfaces   0           Substrate Material   Silicon           Length   1.75 inches           Fractional Bandwidth   0.2 microns           Wavelength Range of Operation   8-14 microns                      
 
         [0019]    Some basic parameters of the lens elements  16  and  17  are set forth in TABLE 2, where R1 refers to the first or front surface encountered by radiation reaching a lens, and R2 means the second or rear surface encountered by the radiation.  
                                             TABLE 2                           Parameters                    Index               Diffractive               at   Surface       Diffractive   Parameters       Lens   Material   10 μm   Type   Radii   Surface   (R2)               16   Si   3.42   R1 = Sphere/   R1 = Infinity   R1 No   C1 = 0.057982                   Flat           C2 = −0.151640                   R2 = Diffractive   R2 = Infinity   R2 Yes   C3 = −0.097673                               C4 = −0.50003                               C5 = −0.116280       17   Si   3.42   R1 = Sphere/   R1 = Infinity   R1 No   C1 = 0.561890                   Flat           C2 = −0.003646                   R2 = Diffractive   R2 = Infinity   R2 Yes   C3 = 0.019104                               C4 = −0.042072                               C5 = 0.031824                  
 
         [0020]    Exact lens parameters for the lens elements  16  and  17  of the disclosed embodiments are set forth in TABLE 3, including radii, centered thickness, air gaps, aspheric coefficients and diffractive surface parameters. The information in TABLE 3 is set forth in a format suitable as input for an optical design software program, such as the program which is commercially available under the trademark CodeV® from Optical Research Associates of Pasadena, Calif.  
                                                                                                                                                                                                                                                                                                                                                 TABLE 3                           CODE V &gt; lis       23 mm/10 mm format f/1 All-Si                    RDY   THI   RMD       GLA           &gt;OBJ:   INFINITY   INFINITY           1:   INFINITY   0.000000           2:   INFINITY   0.000000           3:   INFINITY   0.500000           4:   INFINITY   0.060000       ‘si’           5:   INFINITY   0.005000                HOE:                               HV1:   REA   HV2:   REA   HOR:   −1           HX1:     0.000000E+00   HY1:     0.000000E+00   HZ1:     0.713927E+18           CX1:   100    CY1:   100   CZ1:   100           HX2:     0.000000E+00   HY2:     0.000000E+00   HZ2:     0.713927E+18           CX2:   100    CY2:   100   CZ2:   100           HWL:   10200.00   HTO:   SPH   HCT:   R           HCO/HCC           C1:      5.7982E−02   C2:    −1.5164E−01   C3:    −9.7673E−02           C1    0    C2:    0   C3:    0           C4:    −5.0003E−02   C5    −1.1628E−01           C4    0    C5:    0                STO:   INFINITY   0.878923                       7:   INFINITY   0.010000       ‘si’           8:   INFINITY   0.814195                HOE:                               HV1:   REA   HV2:   REA   HOR:   −1           HX1:     0.000000E+00   HY1:     0.000000E+00   HZ1:   0.713927E+18           CX1:   100    CY1:   100   CZ1:   100           HX2:     0.000000E+00   HY2:     0.000000E+00   HZ2:   0.713927E+18           CX2:   100    CY2:   100   CZ2:   100           HWL:   10200.00   HTO:   SPH   HCT:   R           HCO/HCC           C1:      5.6189E−01   C2:    −3.6460E−03   C3:    1.9104E−02           C1:    0    C2:    0   C3:    0           C4:    −4.2072E−02   C5:      3.1824E−02   C3:    0           C4:    0    C5:    0                 9:   INFINITY   0.000000           10:   INFINITY   0.000000           IMG:   INFINITY   0.000000            SPECIFICATION DATA                FNO   1.00000                   DIM   IN           WL   11000.00   10000.00   9000.00           REF   2           WTW   1   2   1           INI   rbc           XRI   0.00000   0.00000   0.00000           YRI   0.00000   0.20000   −0.20000           WTF   1.00000   1.00000   1.00000           VUX   0.00000   0.00000   0.00000           VLX   0.00000   0.00000   0.00000           VUY   0.00000   0.00000   0.00000           VLY   0.00000   0.00000   0.00000            PRIVATE CATALOG            PWL   12000.00   10000.00   8000.00   5000.00   4000.00   3000.00       ‘si’   3.417223   3.417740   3.418400   3.422100   3.425390   3.432390            REFRACTIVE INDICES                GLASS CODE   11000.00   10000.00   9000.00           ‘Si’   3.417467   3.417740   3.418049            No solves defined in system       No pickups defined in system       This is a decentered system. If elements with power are       decentered or tilted, the first order properties are probably       inadequate in describing the system characteristics.            INFINITE CONJUGATES                EEL   0.9055           BFL   0.0000           FFL   0.9988           FNO   1.0000           IMG DIS   0.0000           OAL   2.7701            PARAXIAL IMAGE                HT   0.2044           ANG   12.7230            ENTRANCE PUPIL                DIA   0.9055           THI   1.0246            EXIT PUPIL                DIA   31.8389           THI   −31.8389            CODE V &gt; out t                  
 
         [0021]    In the embodiment of FIG. 1, the diffractive surface  46  of lens  16  has as its primary purpose the correction of pupil aberrations, one example of which is spherical aberrations. The diffractive surface  47  on lens  17  has as its primary function the focusing of infrared energy so that the energy forms an image  14  at the image plane, and has as its secondary function the correction of field aberrations. Alternatively, however, it would be possible for the diffractive structure to collectively perform a larger or smaller number of functions, and for the functions to be allocated differently among one or more diffractive surfaces. The configuration of FIG. 1 provides a highly corrected and good quality image with a very high modulation transfer function (MTF) for a particular wavelength, where the MTF will decrease as the fractional bandwidth increases.  
         [0022]    In this regard, FIG. 2 is a graph showing a nominal modulation transfer function (MTF) for the lens assembly of FIG. 1, as a function of fractional bandwidth. In general, the wider the bandwidth of the radiation imaged by the lens assembly  10 , for example as determined by the bandwidth of the bandpass filter coating  43 , the lower the MTF, which is a measure of the contrast of the lens assembly.  
         [0023]    As discussed above, the lens elements  16  and  17  of the disclosed embodiment are made of silicon, but could alternatively be made of an infrared polymer of a type known in the art. The polymer lens elements could have AR coatings of the type discussed above. However, polymer lens elements have relatively low reflectance and relatively high transmittance even without AR coatings, and the AR coatings could therefore be optionally omitted. Polymer lens elements could optionally be made relatively thin, for example on the order of approximately 0.002 inch. In that event, a non-imaging window could be provided between the scene and the lens elements, in order to provide protection for the lens elements. The window could, for example, be silicon or germanium, with a DLC coating on the front or outer side and an AR coating on the rear or inner side. A further window could be provided on the opposite side of the lens elements, for example in the region of the image plane, and could have the bandpass filter coating thereon. Alternatively, the AR coating could be omitted and the bandpass filter coating could be provided on the rear or inner side of the outer window.  
         [0024]    The invention provides a number of advantages. One such advantage is that, through the careful selection and combination of lens materials, spectral band, diffractive surfaces and performance requirements, an imaging lens assembly is provided which can produce an image using only diffractive optical elements, and without using any refractive optical surfaces with power. As a related advantage, the use of only diffractive surfaces which are approximately flat permits the diffractive surfaces to be fabricated using traditional, high volume, low cost processes, such as etching or embossing. Consequently, the imaging lens assembly can be manufactured at a very low cost. In fact, by using very inexpensive materials and processes, suitable performance can be achieved while reducing the manufacturing cost by a factor of ten times or more in relation to pre-existing lens systems.  
         [0025]    The invention is advantageous when used to implement an imaging lens assembly intended for use in imaging infrared radiation. As a result, an imaging lens assembly which embodies the invention can be very advantageous in markets where high volume and low cost are important due to competitive pricing pressures, one example of which is an infrared imaging system intended for nighttime use in a vehicle. The invention is also advantageous for other military and commercial uses where a reasonable level of performance is needed at a relatively low cost, including surveillance applications.  
         [0026]    Although one embodiment has been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.