Abstract:
An imaging system includes a wavelength dependent aperture stop, which transmits light with different ranges of wavelengths through apertures of different diameters. Thus, different colored light will have different F-stops, which can be selected based on the power transfer and image quality requirements for the different colored light. For example, a smaller F-stop may be used with a weaker light source to produce a higher throughput for a specific range of wavelengths. Accordingly, the optical system&#39;s design and optimization is wavelength and F-stop dependent.

Description:
FIELD OF THE INVENTION 
   The present invention is related to imagining optics and in particular to an optical system with an aperture stop that transmits light through apertures having different diameters based on the wavelength of the light. 
   BACKGROUND 
   In imaging system design, such as for a projection system, it is necessary to correct for aberrations to improve image quality while maximizing the light throughput of the system to increase brightness. Conventionally, in a chromatic coaxial optical system, i.e., where all wavelengths of light are transmitted along a common optical system, the imaging optics are designed and optimized based on common optical elements for all wavelengths. In other words, conventional optical systems are designed with all wavelengths of light passing through the same lenses and aperture stops. 
   Difficulty arises in the design and optimization of optical systems because different wavelengths of light have different optical path lengths through the optical system based on their wavelength. Thus, an optical system transfer function could be different for any specific wavelength causing the system modulation transfer functions (MTF) to be wavelength dependent as well. The optimization of the MTF for one color in an optical system generally results in the degradation of the MTF for other colors, same as the optimization of one field point generally results in the degradation of the MTF for the other field points. Accordingly, compromise is generally necessary in the design and optimization of an imaging system, among different wavelengths as well as different field points. 
   The design of optical systems is further complicated where independent color light sources, such as light emitting diodes (LEDs) are used. The throughput of an optical system is defined by the extent of the aperture stop without any vignetting, i.e., no ray in the optical system is cut off along the optical train. Because some color light sources may be brighter than others, and because the throughput of the system is conventionally designed to be equal for all wavelengths, weaker light sources stay relatively weak at the output of the optical system. Accordingly, the optical design in imaging systems using multiple light sources is typically limited to a choice of optical elements and transmitting apertures that will provide adequate throughput for all wavelengths. 
   SUMMARY 
   An imaging system, in accordance with an embodiment of the present invention, includes a wavelength dependent aperture stop that transmits light with different wavelengths through apertures of different diameters. Accordingly, different colored light will have different F-stops, which can be selected based on, e.g., the power transfer requirements for the different wavelengths. For example, a smaller F-stop may be used with a weaker light source to produce a higher throughput for a specific range of wavelengths, while a larger F-stop is used with a different range of wavelengths. 
   In accordance with one embodiment, an apparatus includes a light source that produces light that impinges on an object to be imaged with multiple wavelengths along an optical axis and a wavelength dependent aperture stop positioned along the optical axis in the path of the light. The wavelength dependent aperture stop has a first aperture of a first diameter through which a first range of wavelengths is transmitted and a second aperture of a second diameter through which a second range of wavelengths is transmitted. The second diameter is larger than the first diameter. The apparatus further includes at least one imaging lens positioned along the optical axis in the path of the light produced by the light source to produce an image, where the wavelength dependent aperture stop is positioned between the object and the image produced by the imaging lens. The second range of wavelengths may be a subset of the first range of wavelengths. In some embodiments, the wavelength dependent aperture stop may have additional apertures, such as a third aperture of a third diameter through which a third range of wavelengths is transmitted, where the third diameter is greater than the second diameter. 
   In another embodiment, a method includes producing polychromatic light along an optical axis, transmitting a first range of wavelengths from the polychromatic light through a first aperture in an aperture stop and transmitting a second range of wavelengths from the polychromatic light through a second aperture that is in the aperture stop. The first and second apertures are concentric. The first aperture has a first diameter and the second aperture has a second diameter that is greater than the first diameter. The method further includes projecting the light that is transmitted through the first aperture and the second aperture in the aperture stop onto an image plane. In one aspect, the method further includes transmitting a third range of wavelengths from the polychromatic light through a third aperture that is in the aperture stop and that is concentric with the first aperture, where the third aperture has a third diameter that is greater than the second diameter. 
   Another embodiment is an imaging apparatus that includes a light source that produces polychromatic light along an optical axis and an aperture stop and at least one optical lens positioned along the optical axis. The aperture stop has different diameter apertures through which different ranges of wavelengths are transmitted and the at least one optical lens defines a focal length for the imaging apparatus, such that there is a first F-stop for a first range of wavelengths and a second F-stop for a second range of wavelengths. The second F-stop is smaller than the first F-stop. Additional F-stops may be used, such as a third F-stop for a third range of wavelengths, where the third F-stop is smaller than the second F-stop. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an optical system with a wavelength dependent aperture stop. 
       FIGS. 2A and 2B  illustrate a plan view and a cross-sectional view, respectively, of a wavelength dependent aperture stop, in accordance with an embodiment of the present invention. 
       FIGS. 3A and 3B  are cross-sectional views of other embodiments of a wavelength dependent aperture stop. 
       FIG. 4  graphically illustrates the operation of a wavelength dependent aperture stop. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates an optical system  100 , such as a projection system, that includes a wavelength dependent aperture stop  110 , in accordance with an embodiment of the present invention. Optical system  100  includes a light source  102  that produces polychromatic light along optical axis  103 . The light source  102  may be a number of light emitting diodes (LEDs), as illustrated in  FIG. 1 , which emit, e.g., red, green and blue light, which are combined with appropriate optics (not shown). Alternatively, a single LED that emits white light, e.g., through phosphor conversion, may be used. Other light sources may be used with the present invention if desired. As illustrated in  FIG. 1 , the light from the light source  102  is transmitted through (or reflected from) an object  101 , such as a micro-display, where the projection system optics consisting of lenses  104 , aperture  110  and lenses  106 , image  101   image  of the object  101  on the screen or viewing plane  108 . Aperture  110  is positioned at the aperture stop of the optical system consisting of projection optics  104  and  106 . 
   It should be understood that the optical system  100  shown in  FIG. 1  is one example of an optical system with which the wavelength dependent aperture stop  110  may be used. Except for the wavelength dependent aperture stop  110 , the general operation of an optical system, such as that shown in  FIG. 1 , is readily understood in the art. If desired, the wavelength dependent aperture stop  110  may be used with other optical systems as would be understood by those skilled in the art in light of the present disclosure. 
   The use of the wavelength dependent aperture stop  110  enables the diameter of the aperture through which light is transmitted to vary as a function of wavelength. Thus, with the focal length defined by the lens systems and the use of different diameter apertures for different wavelengths, the optical system  100  has different F-stops for different wavelengths of light. By way of example, a smaller F-stop may be used for the wavelengths of light for which a higher throughput is desired, e.g., where the wavelengths are produced by a weak light source or where additional brightness is desired for the wavelengths. Consequently, larger numerical aperture rays pass through the optical system, increasing the etendue of the system. As is known in the art, etendue is an optical extent of the light passing through an optical system (proportional to the product of the image area and the numerical aperture) and for an etendue limiting optical system, the f# (inversely proportional to the numerical aperture stop) is an indication of the limiting etendue, or the amount of light that can be handled by an optical system; a larger etendue generally corresponds to a brighter optical system. 
   Thus, according to an embodiment of the present invention, the F-stop, i.e., throughput, for different wavelengths of light, or the etendue of the optical system can be optimized for different wavelengths, e.g., based on the different power transfer requirements. Moreover, based on the numerical aperture for each wavelength, the new color dependent error functions can be used to optimize the optical system in terms of image quality and aberrations. 
     FIGS. 2A and 2B  illustrate a wavelength dependent aperture stop  110 , in accordance with an embodiment of the present invention, in a plan view and a cross-sectional view (along lines A-A of  FIG. 2A ), respectively. Aperture stop  110  includes a transparent plate  112  that is covered with two ring shaped transmission filters  114  and  116  and an opaque film  118 , where the filters  114  and  116 , e.g., thin film coatings that are appropriately deposited on plate  112 . The inner diameter of transmission filter  114  defines a circular aperture  115  through which all wavelengths of light may pass. The inner diameter of transmission filter  116  defines a circular aperture  117  and the inner diameter of opaque film  118  defines a circular aperture  119 . The circular apertures  115 ,  117 , and  119  are configured to be concentric. In operation, aperture  115  (in  FIGS. 2A and 2B ) permits all desired wavelengths of light to pass through, while ring shaped transmission filters  114  and  116  permit only specific ranges of wavelengths to pass through and opaque member  118  does not permit any light to pass through. 
   It should be understood that other wavelength dependent aperture constructions may be used if desired. By way of example, other embodiments may have multiple wavelength dependent apertures where the filters are more narrow band filters for specific wavelength ranges of the light. In another embodiment, the aperture boundaries could present a more gradual variation of the filter transmission over the wavelength, e.g., using a varying transmission filter, rather than an abrupt boundary between different wavelength transmission functions. For example, as shown in  FIG. 3A , an aperture stop  200  may include a varying transmission filter  202  that provides a gradually diminishing aperture diameter that is dependent on the wavelength of the light and an opaque member  204  that passes no light. 
   Moreover, the wavelength dependent aperture stop may be produced using alternative configurations. For example,  FIG. 3B  illustrates, in cross-sectional view, an aperture stop  210  that is formed with overlapping transmission filters  214 ,  216  and an opaque member  218 , thereby obviating the need for a transparent plate  102  and providing at least one aperture opening with a transmission function that is a product of two or more overlapping color filters. The transmission filters  214 ,  216 , and opaque member may partially or fully overlap. In one embodiment, transmission filters  214 ,  216  and opaque member  218  have concentric circular apertures  215 ,  217 , and  219 , respectively. With such as configuration, transmission filters  214  and  216  will have ring shapes resulting in the plan view shown in  FIG. 2A . Other possible configurations to produce wavelength dependent aperture stop will be clear to those skilled in the art in light of the present disclosure. 
     FIG. 4  graphically illustrates the operation of wavelength dependent aperture stop  110 , in which transmission filter  114  blocks blue light and transmit both green and red light, while transmission filter  116  blocks red and blue light and pass only green light, and opaque element  118  blocks the red, green and blue light. Accordingly, green light will pass through aperture  115  and transmission filters  114  and  116  and, therefore, has an F-stop defined by the diameter of aperture  119  in the opaque film  118 . Red light will only pass through aperture  115  and only transmission filter  114  and, therefore, has an F-stop defined by the diameter of aperture  117  in transmission filter  116 . Blue light will pass through only aperture  115  and, therefore, has an F-stop defined by diameter of aperture  115 . In such an embodiment, green light has a smaller F-stop, and therefore greater throughput, than red or blue light, while red light has a smaller F-stop than blue light. 
   The particular design of aperture stop  110  may depend on the requirement of the optical system, e.g., the power transfer requirements of the light sources and/or the required chromatic final image quality. Thus, transmission filters  114  and  116  may transmit any desired range of wavelengths. Moreover, if desired, fewer or additional transmission filters may be used with the wavelength dependent aperture stop. For example, only a single transmission filter may be used, e.g., transmission filter  114 , with the opaque element. In such a configuration, the single transmission filter may transmit one range of wavelengths, e.g., green light or green and red light, while the remaining wavelengths of light are transmitted only through the unfiltered aperture, i.e., aperture  115 . In another embodiment, additional transmission filters may be used for smaller ranges of wavelengths, e.g., additional transmission filters may be used to provide different sized apertures for different colors, such as blue, cyan, green, amber and red. 
   Although the present invention is illustrated in connection with specific embodiments for instructional purposes, the present invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.