Abstract:
An optical system, in particular for endoscopic applications, is disclosed which uses wavelength-compensating optical components, in particular prisms, made of materials with different inter-element coatings and refractive indices to image significantly different wavelength-ranges (VIS and NIR) onto the same image plane of an image acquisition device, such as a CCD sensor.

Description:
BACKGROUND 
     Near-infrared (NIR) imaging using endoscopes has been described in the literature for various clinical applications. Typically, such an imaging modality utilizes a contrast agent (e.g. indocyanine green) that absorbs and/or fluoresces in the 700-900 nm range of the NIR. In some endoscopic imaging systems capable of high resolution simultaneous color and NIR imaging, none of the image sensors (if multiple image sensors are used) or specific pixels of an image sensor (if only a single color image sensor is used) are exclusively dedicated to NIR imaging. One exemplary imaging system, described in the Annex of the present disclosure, utilizes a red, green, blue (RGB) sensor assembly to acquire both color and NIR fluorescence images by employing the red image sensor to, alternately and in rapid succession, acquire both the red light required for the color image and NIR light required for the NIR image. This imaging system is intended to be used in conjunction with image-projecting optical instruments such as endoscopes, microscopes, colposcopes, etc. that have also been optimized for both visible light and NIR imaging applications. Specifically the optical instruments (i.e. endoscopes, microscopes, colposcopes, etc.) and the optical assemblies (optical couplers) that couple these instruments to the sensor assembly of the imaging system are constructed using appropriate visible and NIR transmitting optical materials and antireflection coatings and are optically designed to transmit visible and NIR images for which chromatic and geometric aberrations are minimized.  FIG. 1  depicts a typical configuration of an optical instrument, optical coupler and imaging system such as that being described above. 
     Although the preponderance of optical instruments currently in use are not optimized for both visible (VIS) and NIR light imaging, such instruments may still transmit sufficient NIR light that it may also be desirable to enable the previously described VIS-NIR imaging system for use with these conventional optical instruments. Conventional optical instruments are typically well-corrected for imaging throughout the visible spectrum, but without equivalent correction in the NIR, NIR images acquired with the aforementioned VIS-NIR imaging system through such optical instruments are likely to be of poor quality. Furthermore, although some of the NIR image aberrations introduced by conventional optical instruments may be corrected by applying compensating lens design techniques to the optical couplers, such techniques are typically not powerful enough to correct both the aberrations and the shift in focal plane between the visible and NIR images produced with such instruments. A novel optical coupler capable of correcting for the optical aberrations and for the difference in visible and NIR focal plane locations introduced when using conventional optical instruments is, consequently, highly desirable. 
     SUMMARY OF THE INVENTION 
     The invention described in this disclosure is directed to an optical coupler that corrects for both the optical aberrations and the shift in focal plane between the visible and NIR images that is introduced by conventional optical instruments and enables those instruments to be used with a VIS NIR imaging system of the type described in the Annex to this application. 
     Although well-corrected for visible light imaging and producing substantially coincident focal plane locations for images at wavelengths throughout the visible spectrum (400-700 nm), at NIR wavelengths (700-900 nm) conventional optical instruments will project poorly corrected images at focal plane locations substantially displaced from those for the visible spectrum images. This is particularly problematic for imaging systems using a single color image sensor onto which both visible light and NIR light images are projected. An exemplary single color image sensor is described in the Annex to this application. Conventional optical instruments are not compatible with such imaging systems without some correction to the NIR images that they project. 
     The disclosed system utilizes a combination of correction mechanisms in a novel optical coupler to address the multiple challenges in compensating for the NIR imaging properties of conventional optical instruments without degrading the performance in the visible spectrum. Specifically, this VIS NIR optical coupler splits the optical path into visible and NIR paths within the optical coupler and thereby enables distributed correction for path length differences between the visible and NIR spectrum. The separate visible and NIR optical paths are recombined after compensation for optical path length difference and are then projected by a lens assembly that corrects for aberrations in the NIR without compromising the performance in the visible spectrum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an imaging system operatively connected to an endoscope using an optical coupler according to the invention; 
         FIG. 2  shows an embodiment of a VIS NIR optical coupler with an afocal prism assembly; 
         FIG. 3  shows the afocal prism assembly of  FIG. 2  in more detail; 
         FIG. 4  shows exemplary embodiments of multi-element prisms for use with a sensor assembly incorporating multiple sensors; 
         FIG. 5  shows alternative embodiments of the multi-element prism; 
         FIG. 6  shows ray tracing in an exemplary optical coupler capable of compensating for the difference in visible and NIR focal plane locations; and 
         FIG. 7  shows characteristic design parameters for an exemplary system according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a typical configuration of an optical instrument, optical coupler and imaging system. The components may include an optical instrument such as the endoscope shown or another optical image transmitting instrument such as a microscope, a colposcope, or the like. The optical instrument is connected to the imaging system by an optical coupler that projects an optical image from the optical instrument onto the imaging system&#39;s sensor assembly. The sensor assembly may be a single or multi-sensor (e.g. 3-chip) assembly composed of CCD or CMOS or other solid state image sensors. The sensor assembly converts the optical image into electrical signals which may subsequently be processed and outputted to a display, recording and/or printing device. 
     The conventional optical coupler used with visible light imaging systems typically consists of a multi-element lens assembly with either a fixed or adjustable focus. More sophisticated couplers may incorporate zoom lens designs. As with conventional optical instruments, optical couplers used with visible light imaging systems are also typically well-corrected throughout the visible spectrum and will faithfully project a well-corrected visible light image from an optical instrument onto the imaging system&#39;s sensor assembly. Given their relatively simple design, however, there are insufficient parameters by which the optical properties (i.e. the lens design) of conventional optical couplers can be adjusted to compensate for both the aberrations and the focal plane shifts incurred with NIR images produced by conventional optical instruments without negatively affecting the performance of the coupler in the visible spectrum. 
       FIG. 2  depicts an embodiment of a VIS NIR optical coupler comprising
         an afocal prism assembly that compensates for the optical path length differences between the VIS and NIR focal plane locations produced by the optical instrument, and   imaging optics, that correct for the aberrations in the visible and NIR images produced by the optical instruments and project those corrected images onto the imaging system&#39;s sensor assembly.       

     One embodiment of the afocal prism assembly is depicted in  FIG. 3 . This afocal assembly includes a first lens or lens assembly that imparts some increased optical power to the image forming rays emitted by the optical instrument, a path length compensating multi-element prism to compensate for the optical path length differences between the VIS and NIR focal plane locations produced by the optical instrument, and a lens or lens assembly of the opposite power following the multi-element prism. The multi-element prism is composed of sections of material having different indices of refraction, (Material A and Material B) and having dichroic coatings on the diagonal surface between sections such that one half of the diagonal surface is coated with a short pass coating transmitting visible light and reflecting NIR light and the other half is coated with a long pass coating transmitting NIR light and reflecting visible light. The relationship between the dichroic coatings and the indices of refraction for the prism materials are such that if the index of refraction for Material A is greater than for Material B, light from the optical instrument will encounter a long pass coating (i.e. passes NIR and reflects VIS) at the first dichroic diagonal surface in the multi-element prism and a short pass (i.e. passes VIS and reflects NIR) at the second dichroic diagonal surface. The location of the dichroics is reversed for the opposite relationship between the indices of refraction of Materials A and B. 
     The imaging lens assembly ( FIG. 2 ) accepts the image forming rays from the optical instrument projected through the prism assembly and focuses an optical image corrected for visible and NIR wavelengths onto the imaging system&#39;s sensor assembly. This assembly may be mounted in the optical coupler such that its position along the optical axis can be adjusted (i.e. the image can be focused onto the sensor assembly for a range of object distances). The imaging lens assembly may further be designed for use with a sensor assembly incorporating a multi-channel prism, a number of which are shown in  FIG. 4 . The imaging system disclosed in the Annex incorporates a sensor assembly with a 3 channel (RGB) prism, but this optical coupler may also be used with two or four or more channel sensor assembly. 
     The properties and operation of the afocal prism assembly can then be further described as follows: 
     The light output of optical instruments is typically collimated or nearly collimated and the first lens (or lens assembly) in the afocal prism assembly imparts a negative (or positive) optical power to the light emitted from the optical instrument. The diverging (or converging) light is subsequently transmitted through the path length compensating prism. As can be seen by the ray diagrams in  FIG. 6 , the afocal prism assembly corrects for the difference in the focal plane location of the NIR and visible light images projected by conventional optical instruments. Since the light entering the prism assembly is diverging (or converging), by causing the NIR and visible light to traverse separate optical paths through materials with different refractive indices, the difference in focal plane location can be compensated for before recombining the two optical paths. The second lens (or lens assembly) in the afocal prism assembly subsequently offsets the optical power induced by the first lens (or lens assembly) causing this assembly to be substantially afocal. 
     The properties of the multi-element prism are determined by such factors as the optical power of the light bundle emitted by the first lens assembly, the path length difference between the focal planes of the visible and NIR images projected by the optical instruments, the practical size constraints and ranges of refractive indices of glasses, and the desired effective focal length (or magnification) of the optical coupler. The materials of the imaging optics are consequently selected such that the entire optical system, including the optical instrument, is achromatic for the visible and NIR spectra of interest. Nevertheless, the VIS and NIR image focused onto the imaging system&#39;s sensor assembly will show a slight lateral displacement between the visible and NIR components as a consequence of traversing the multi-element prism. Additionally, there will also be residual magnification differences in the resulting images. Since the visible and NIR images are acquired independently in a VIS NIR imaging system, such as the one described in the Annex, it is possible to compensate for slight lateral displacements or residual magnification differences between the visible and NIR image components by means of image processing software. These means of registration correction and image scaling in software are commonly understood and practiced by those skilled in the art and require no further explanation here. 
     Alternative embodiments of the multi-element prism are shown in  FIGS. 5   a  and  5   b . In these embodiments, the lateral displacement introduced by the prism assembly in the first embodiment is better corrected by utilizing a more sophisticated design. The lateral displacement between the visible and NIR images is minimized by providing sections within ( FIG. 5   a ), or in addition to the multi-element prism ( FIG. 5   b ) that better compensate for any lateral shifts that are induced as the image rays traverse the prism assembly. Again, any residual lateral displacements or magnification differences between the visible and NIR image components may be further corrected by means of image processing software. 
       FIG. 6  provides a specific example of an optical coupler that can be used to compensate for the difference in visible ( FIG. 6   b ) and NIR ( FIG. 6   c ) focal plane locations introduced when using conventional optical instruments. The optical coupler has a total of 34 optical surfaces with characteristic physical properties (radius, thickness, material, diameter) listed in  FIG. 7   a  for visible light and in  FIG. 7   b  for NIR light. The optical surfaces are numbered from left to right in  FIG. 7 . The block on the right represents the multi-channel prism (see  FIG. 4 ) to which the CCDs are attached. However, the reference numbers are omitted from  FIG. 6  so as not to obscure the drawing. Performance metrics shown for this sample design are provided in  FIGS. 6   d - e.    
     Under certain circumstances there may be difference in magnification between the two VIS and NIR images formed on the detector. This difference in magnification could be addressed by processing the NIR signal separately and matching (e.g., electronically using known edge detection and resizing algorithms) the size of the NIR image to that of the VIS image. 
     Most remaining optical aberrations not related to the difference in magnification/focal point between the VIS and NIR images, commonly called Seidel aberrations, such as coma, astigmatism, spherical aberration, etc., can be reduced to an acceptable amount using the degrees of freedom in the lens assembly. 
     The Annex to this disclosure, which includes 9 sheets of drawings, forms an integral part of the disclosure, and its content is incorporated herein in its entirety as if set forth herein. 
     While the invention has been disclosed in connection with the preferred embodiments shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art.