Abstract:
An imaging system for use in a vehicle headlamp control system includes an opening, an image sensor, a red lens blocking red complement light between the opening and the image sensor, and a red complement lens blocking red light between the opening and the image sensor. Each lens focuses light onto a different subwindow of the image sensor. The imaging system allows processing and control logic to detect the presence of headlamps on oncoming vehicles and tail lights on vehicles approached from the rear for the purpose of controlling headlamps. A light sampling lens may be used to redirect light rays from an arc spanning above the vehicle to in front of the vehicle into substantially horizontal rays. The light sampling lens is imaged by the image sensor to produce an indication of light intensity at various elevations. The processing and control logic uses the light intensity to determine whether headlamps should be turned on or off. A shutter may be used to protect elements of the imaging system from excessive light exposure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/677,906, entitled “IMAGING SYSTEM FOR VEHICLE HEADLAMP CONTROL,” filed on Oct. 3, 2000, now U.S. Pat. No. 6,291,812, which is a divisional of U.S. patent application Ser. No. 09/093,993, entitled “IMAGING SYSTEM FOR VEHICLE HEADLAMP CONTROL,” filed on Jun. 9, 1998, now U.S. Pat. No. 6,130,421. The entire disclosures of both the above applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to imaging systems for use in a control system such as a vehicle headlamp control. 
     Headlamps illuminate a region in front of a vehicle allowing a driver to view the region when ambient light is insufficient. Headlamps also allow the vehicle to be seen by pedestrians and drivers of other vehicles. High beam headlamps provide even greater illumination and have a greater coverage region. However, high beam headlamps may blind drivers in oncoming vehicles and drivers in vehicles traveling in the same direction within the high beam coverage region. Traditionally, a driver has had to manually control turning headlamps on and off and switching between high beam and low beams. 
     One difficulty with manual control is that the driver may forget to turn headlamps on at dusk making the vehicle difficult to see. Another difficulty is that the driver may neglect to dim high beam headlamps for oncoming traffic or when approaching another vehicle from behind. 
     Previous attempts to automatically control the operation of vehicle headlamps have used sensors which provide a single output signal or a very small number of output signals to the associated control system. For example, a single output sensor has been used to sense ambient light for determining when to turn headlamps on or off. Also, a single output sensor has been used for determining when to dim automotive headlamps. Whereas a headlamp on/off control using a single sensor input has achieved limited success in automotive applications, a single sensor headlamp dimmer control is not currently offered because of its many shortcomings. 
     Array imaging sensors and various scanning techniques have been proposed, but even with the reduced costs made possible by today&#39;s electronics, these sensors and techniques have not produced satisfactory headlamp dimming and on/off control functions. Such sensing systems typically have hundreds of rows and columns of pixel sensors generating hundreds of thousands or even millions of pixels. At a typical video rate of 30 frames per second, this requires conversion and data processing rates in the millions of operations per second. 
     Headlamp on/off control can be based on ambient light levels. Headlamp dimmer control can be based on recognizing the headlamps from oncoming vehicles and the tail lamps of vehicles approached from behind. Since the resolution required to detect ambient light levels and to detect headlamps and tail lights is less than required for traditional images, a smaller imaging array, and hence, slower processing electronics, may be used. 
     In order to distinguish red tail lamps from other lights, the imaging system must produce readings in at least two different color bands. The first of two methods usually used to sense colors with an image sensor has been to cover one-third of the pixel sensing sights in the imager with a red or red complement filter, one-third of the pixels with a blue or blue complement filter, and one-third of the pixels with a green or green complement filter. This is often done, for example, by placing alternating red, green, and blue stripes over columns of pixels. Each pixel site registers one color and interpolation is used to supply the two missing colors at each pixel sight. 
     When coupled with a low resolution imager, this technique for sensing color creates a problem. Due to the optics used, the projected image of a headlamp or tail light viewed by the imaging sensing array is very small, probably smaller than the resolving power of the lens. This projected image will be referred to as a dot. When pixel spacing is significantly smaller than the dot size projected by the lens, a portion of a dot of a particular color may not always strike a sensor sight of that color. As the pixel size or area of optical coverage per pixel is increased due to a corresponding reduction in the number of pixels, the voids between the like colored pixel sights become larger unless a complicated interdigitated pixel pattern is used. Even if readout of a particular color is not completely lost by having the entire dot image projected on a pixel of another color or colors, the readout will be coarse depending on what portion of the dot strikes a pixel. Since distinguishing a color is usually a matter of determining balance between two or more color components and not just determining the presence or absence of a particular color component, when the small spot of light in the projected image of a headlamp or tail light falls more on one pixel of one color than another, the measured balance is altered accordingly. 
     A further disadvantage with this method results from dyes used to implement the color filters. The dyes are normally organic and are subject to degradation from thermal and light exposure. Since the dye sits directly over individual pixel sites, the energy from a strong light source, such as the sun, is focused by the lens system directly onto the dye. 
     A still further problem with this method is that having the color filter dye applied to and precisely registered with the pixel sensor sight on the image sensor is expensive. The cost of adding color filters directly on the pixel sensor may be as expensive as the silicon image sensing chip itself. 
     A second method for imaging color splits light from the image into red, green, and blue components which are projected onto separate image sensors, each of which measures its respective color filtered image. This requires a complicated optical arrangement and three separate image sensors. The color separation technique often utilizes mirrors which selectively reflect one color and transmit the complementary color. These optical arrangements normally require widely separated non-planar image sensor sights making it difficult, if not impractical, to place the three sensors on a common silicon substrate or even in a common package. This technique presents a three-fold problem. A single sensor array cannot be used, a single silicon chip cannot be used, and a single package cannot be used. 
     What is needed is a cost effective imaging system to be used in, for example, a headlamp control system. To limit costs and complexity in the optics, the sensor array, the processor, and processor interface, a minimal number of pixels, preferably in a range which would be considered too small for satisfactory pictorial image presentation, should be used. The imaging system should not use spectral filtering that would place dyes or color-selecting materials in the focal point of the lens system. The imaging system should supply signals appropriate for determining headlamp dimming control, headlamp on/off control, or both. The imaging system should also be protected against excessive light or heat damage. 
     SUMMARY OF THE INVENTION 
     A further object of the present invention is to produce different color components of a scene using an optical system that does not place filters in the focal plane of the optical system. 
     In carrying out the above objects and other objects and features of the present invention, an imaging system is provided for use in a vehicle headlamp control system. The imaging system includes a housing defining an opening, the opening generally towards a scene, an image sensor within the housing opposite from the opening, a first lens to focus light from the scene onto a first portion of the image sensor, and a second lens to focus light from the scene onto a second portion of the image sensor, the second portion of the image sensor separate from the first portion. 
     In one embodiment, the first lens focuses light at a first wavelength onto the image sensor and the second lens focuses light at a second wavelength onto the image sensor. In a refinement, the focal length of the first lens at the first wavelength is substantially the same as the focal length of the second lens at the second wavelength. In a preferred embodiment, the first lens attenuates light substantially cyan in color and the second lens attenuates light substantially red in color. 
     In another embodiment, the image sensor has a low resolution. 
     In yet another embodiment, a baffle extends from an area between the first lens and the second lens towards the image sensor. The baffle reduces light passing through the first lens from striking the second portion of the image sensor and reduces light passing through the second lens from striking the first portion of the image sensor. 
     In a further embodiment, the imaging system includes a shutter for reducing the intensity of light entering the opening. In a preferred embodiment, the shutter is an electrochromic window. 
     In a still further embodiment, a maximum focal length is the largest of the focal length of the first lens and the focal length of the second lens. The housing defines the opening at least two times the maximum focal length away from the first lens and the second lens. In yet a further embodiment, a first portion of the housing defining the opening is positioned to block light which would otherwise travel through the first lens and impinge as stray light on the second portion of the image sensor and a second portion of the housing defining the opening is positioned to block light which would otherwise travel through the second lens and impinge as stray light on the first portion of the image sensor. 
     An imaging system is also provided that includes a housing defining an opening generally towards a scene in front of a vehicle, an image sensor located within the housing, and a light sampling lens positioned near the opening. The light sampling lens gathers light rays from a region defined by a vertical arc extending from substantially above the opening to substantially in front of the opening, and redirects the gathered light rays towards the image sensor. The lens may gather light rays from a narrow horizontal arc in front of the opening. 
     In one embodiment, the light sampling lens is further operative to gather light rays from elevationally separate regions and to redirect the gathered light rays from each elevationally separate region to a different set of pixel sensors in the image sensor, allowing the image sensor to detect the light level at different angular elevations. The elevationally separate regions may be regions separated by 10° of elevation. 
     In another embodiment, the system includes a first subwindow of pixel sensors, a second subwindow of pixel sensors, a red lens within the housing between the light sampling lens and the image sensor for projecting substantially red components of the redirected light rays onto the first subwindow, and a red complement lens within the housing between the light sampling lens and the image sensor, the red complement lens for projecting substantially red complement components of the redirected light rays onto the second subwindow. 
     A system for controlling at least one headlamp includes a headlamp controller operative to turn the headlamps on and off based on a received on/off control signal, an image sensor comprised of an array of pixel sensors, a lens system operative to gather light rays from a region defined by a vertical arc extending from substantially above the vehicle to substantially in front of the vehicle and to redirect the gathered light rays towards the image sensor, and a processing and control system operative to read light levels from pixel sensors and to determine the on/off control signal based on comparing the light levels to a threshold. 
     In one embodiment, the processing and control system can determine the threshold based on color components projected onto the first and second subwindows. Alternatively, the processing and control system can determine whether the region defined by the vertical arc images a blue sky or a cloudy sky and to use a lower threshold for the blue sky than for the cloudy sky. 
     In another embodiment, the processing and control system can determine the on/off control signal based on comparing the light levels to a hysteretic threshold. 
     In yet another embodiment, the processing and control system can determine the on/off control signal based on a time delay from a previous change in the on/off control signal. 
     The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a headlamp control system that may use an imaging system according to the present invention; 
     FIG. 2 is a schematic diagram of an image sensor according to the present invention; 
     FIG. 3 is an optical system according to the present invention; 
     FIG. 4 is an enlarged portion of the optical system shown in FIG. 3; 
     FIG. 5 is an alternative embodiment of an imaging system including a baffle according to the present invention; 
     FIG. 6 is a schematic diagram illustrating the operation of two lenses for an embodiment of the present invention; 
     FIG. 7 is a lens for use in an embodiment of the present invention for headlamp on/off control; and 
     FIG. 8 is an illustrative optical system incorporating the lens of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a block diagram of a system incorporating the present invention is shown. Headlamp control system  20  is used in a vehicle to control one or more of headlamp  22 . Control operations may include automatically turning on and off headlamp  22  and automatically switching between the high beam and low beam for headlamp  22 . 
     Scene  24  is generally in front of a vehicle. Light rays  26  from scene  24  enter imaging system  28  by first passing through optical system  30 . Focused rays  32  from optical system  30  strike image sensor  34  in the focal plane of optical system  30 . Processing and control system  36  receives image sensor output  38  and produces image sensor control  40 . Processing and control system  36  also generates automatic headlamp control signal  42  which is received by headlamp controller  44 . 
     Processing and control system  36  may perform continuous cycles to check for the presence of headlamps and tail lights in scene  24 . During each cycle, two images are acquired from image sensor  34 . As will be described in more detail below, one image has predominantly red components and one image has predominantly red complement components. Bright spots in the red image may indicate the presence of tail lights in scene  24 . Bright spots in both the red and red complement images may indicate the presence of headlamps in scene  24 . Counters may be used to indicate the number of successive frames for which a bright spot has been detected in approximately the same location. Once the count reaches a threshold value, the bright spot is assumed to be from another vehicle and an appropriate action, such as dimming headlamp  22 , is taken. The above description is a simplification of the embodiments described in U.S. Pat. No. 5,837,994 entitled “CONTROL SYSTEM TO AUTOMATICALLY DIM VEHICLE HEAD LAMPS,” issued Nov. 17, 1998. 
     Headlamp controller  44  generates headlamp controller signal  46 , which is received by headlamp  22  causing headlamp  22  to turn on or off or to switch between a high beam and low beam. Headlamp  22  may produce headlamp illumination  48 , illuminating a portion of scene  24 . Headlamp controller  44  may also receive manual on/off signal  50  from manual on/off control  52  and manual dimmer signal  54  from manual dimmer control  56 . Manual on/off control  52  and manual dimmer control  56  allow the driver to manually control the operation of headlamps  22 . In an alternative embodiment, one or both of headlamp on/off signal  50  and manual dimmer signal  54  may be used by processing and control system  36  to determine the state of headlamp  22 . 
     In an alternative embodiment, shutter  58  is placed before imaging system  28 . Shutter  58  then receives light rays  26  from scene  24  and outputs attenuated light rays  60  to optical system  30 . Shutter  58  reduces or eliminates the amount of light reaching image sensor  34  when light from scene  24  is excessive such as, for example, at dawn or dusk when the sun is near the horizon. Shutter  58  may be implemented using a mechanical means such as blinds, an iris, or the like, under the control of processing and control system  36  as provided by shutter control signal  62 . Alternatively, shutter  58  may be a photosensitive glass or plastic. In a further alternative, shutter  58  may be an electrochromic window as described in U.S. Pat. No. 4,902,108 titled “SINGLE-COMPARTMENT, SELF-ERASING, SOLUTION-PHASE ELECTROCHROMIC DEVICES, SOLUTIONS FOR USE THEREIN, AND USES THEREOF” to H. J. Byker which is hereby incorporated by reference. 
     Image sensor  34  should include a minimum number of sensing elements to reduce processing requirements and decrease cost. To efficiently use image sensor  34  with a relatively few number of pixel sensors, the projected image of a distant tail light or headlamp in scene  24  should be comparable in size or smaller than that of a single pixel in image sensor  34 . The relative intensities of color components calculated from processing the image data from such a projected image should be generally independent of the specific position of the projected image on the array. Therefore, it is desirable to simultaneously project differently filtered images of scene  24  on spatially separate frames preferably within the same pixel array or alternately in separate pixel arrays. The one or more pixel arrays are preferably on the same substrate and in the same package. 
     A preferred arrangement is to project the separate frames on a common array large enough to include the frames in separate subwindows, and to use common control logic which provides a means to simultaneously expose and process the multiple frames. A control of this type is described in U.S. Pat. No. 5,990,469, entitled “CONTROL CIRCUIT FOR IMAGE ARRAY SENSORS,” issued on Nov. 23, 1999, which is hereby incorporated by reference. Descriptions of the image array and lens systems are provided with regards to FIGS. 2 through 8 below. 
     In a preferred embodiment, when a small area light source is detected, the frame is analyzed to determine the single or the small group of adjoining pixels having illumination levels substantially higher than the background level of the surrounding pixels. The light reading is integrated or summed over this group of pixels with an optional subtraction of the average background level. This process is repeated for the frame corresponding to each color component. In this manner, readings are relatively independent of whether the illumination is contained on one pixel sensor or the illumination strikes a pixel boundary and casts portions of light on two or more adjoining pixel sensors. This technique increases the tolerance for a small registration error between the subwindows for different color components when the ratiometric comparison of the various color components of a given small area light source is made. 
     Referring now to FIG. 2, a schematic diagram representing an image sensor according to the present invention is shown. Image sensor  34  includes an array of pixel sensors, one of which is indicated by  70  and arranged in rows and columns. In an exemplary embodiment, image sensor  34  includes 80 rows by 64 columns of pixel sensors, most of which are not shown for clarity. Image sensor  34  includes top border  72 , bottom border  74 , left border  76 , and right border  78  defining a region covered by pixel sensors  70 . The use of directionality such as, for example, top, bottom, left, and right is provided for ease of explanation, and is not meant to limit the present invention to a particular orientation. 
     Image sensor  34  is divided into several subwindows. In one embodiment, two subwindows are used to image scene  24  into two color components. Upper subwindow  94  is bounded by lines  78 ,  80 ,  82 , and  84 , and contains pixel sensors  70  struck by an image projected through a lens which is dyed to pass red light. Lower subwindow  96  is bounded by lines  78 ,  86 ,  82 , and  88 , and includes pixel sensors  70  onto which an image is projected through a lens which is dyed to pass cyan or red complement light. 
     The lenses provide a field of view of scene  24  such as, for example, 22° wide by 9° high. A space between line  80  and top edge  72  and between lines  84  and  90  allows for an elevational adjustment to correct for misalignment of imaging system  28  in the vehicle. To accomplish the adjustment, upper subwindow  94  boundaries, represented by line  80  and line  84  respectively, are moved up or down within the range between top edge  72  and line  90 . Similarly, lines  86  and  88  represent boundaries for lower subwindow  96  that may be moved between bottom edge  74  and line  92 . In the exemplary embodiment, an elevational adjustment through a range of about 4.8° is allowed. Subwindows  94  and  96  are normally moved upward or downward together but the origin of one relative to the other is also adjustable to compensate for variations in the registration of one subwindow with regards to the other. 
     Pixel sensors  70  that lie within the region bordered by lines  90  and  92  may receive light from both the red and red complement lenses. Therefore, this region is not normally used as part of the active imaging area. Pixel sensors  70  from this region may be removed to make room for other circuits, but because of the relatively small percentage of area lost and the flexibility to use the entire 64×80 pixel array in other applications, leaving pixel sensors  70  in the region bordered by lines  90  and  92  may be of greater benefit. Also, it is not convenient to interrupt the signal paths along the columns in the array. In the exemplary embodiment, less than 8.5% of pixel sensors  70  falls between lines  90  and  92 . An embodiment limiting the width required between lines  90  and  92  is described with regards to FIG. 5 below. The red and red complement lenses are described with regards to FIGS. 3 through 6 and FIG. 8 below. 
     In an embodiment of the present invention, pixel sensors  70  lying between left edge  76  and line  82  are used for headlamp on/off control. This use is described with regards to FIG. 8 below. 
     In another embodiment of the present invention, image sensor  34  is divided into more than two subwindows for imaging scene  24  into a plurality of color components. For example, upper subwindow  94  and lower subwindow  96  may each be split into two subwindows, creating four subwindows. The multiple subwindows may be arranged in a two-by-two grid or a one-by-four grid. Spacing between subwindows allows for vertical and horizontal adjustment. 
     Pixel sensors  70  in image sensor  34  may be charge-coupled devices, photodiodes, or the like. In a preferred embodiment, pixel sensors  70  are CMOS active pixel sensors. An APS image sensor is described in U.S. Pat. No. 6,008,486 entitled “WIDE DYNAMIC RANGE OPTICAL SENSOR,” issued Dec. 28, 1999, which is hereby incorporated by reference. 
     Referring now to FIG. 3, an illustrative embodiment of the present invention is shown. Imaging system  28  includes housing  100  with opening  102  opening towards scene  24 . Image sensor  34  is located within housing  100  opposite of opening  102 . Support member  104  is located within housing  100  and holds red lens  106  and red complement lens  108  between image sensor  34  and opening  102 . The support member  104  includes a first aperture for the first lens and a second aperture for the second lens. Support  104  also prevents light coming through opening  102  from striking image sensor  34  unless the light passes through red lens  106  or red complement lens  108 . The range of pixel sensors  70  used to form top subwindow  94 , namely top edge  72  and line  90 , as well as to form bottom subwindow  96 , namely bottom edge  74  and line  92 , is indicated on image sensor  34 . 
     Preferably, opening  102  is located several focal lengths of lenses  106 ,  108  in front of lenses  106 ,  108 . Opening  102  is characterized to minimize the distance between the borders of two images separately projected onto image sensor  34 , reducing the amount of optical crosstalk between upper subwindow  94  and lower subwindow  96 . This is accomplished by using one border of opening  102  positioned to block light which would otherwise travel through lens  108  and impinge as stray light on upper subwindow  94 . Likewise, another border of opening  102  is positioned to block light which would otherwise travel through lens  106  and impinge as stray light on lower subwindow  96 . The use of opening  102  to limit optical crosstalk is described with regards to FIG. 4 below. A further improvement is to incorporate a baffle positioned between the lens systems  106 ,  108  and extending towards image sensor  34  to further reduce the distance required between upper subwindow  94  and lower subwindow  96  to adequately minimize optical crosstalk. The use of a baffle is described with regards to FIG. 5 below. As a further extension, a light collecting optical system is placed in a portion of opening  102  so that a usable image is projected into a third region of image sensor  34  while maintaining adequate optical separation between the three images. The light collecting optical system and its application is described in FIGS. 7 and 8 below. Red lens  106  and red complement lens  108  are shown conceptually. An embodiment of the shape and further operation of red lens  106  and red complement lens  108  are described with regards to FIG. 6 below. 
     In an embodiment of the present invention, optical system  30  includes more than two lens systems  106 ,  108  to project a plurality of color filtered images of scene  24  onto image sensor  34 . For example, four lenses can be arranged in a two-by-two array of lenses. Three of the lenses may pass light in a different color band, such as red, green, and blue, for true color imaging. The fourth lens may pass substantially unfiltered light for low light level imaging. 
     Referring now to FIGS. 3 and 4, the operation of image system  28  will now be described. Low point  110  represents a distant point in scene  24  which is projected as point  112  onto image sensor  34 . Low point  110  is at the lower extent of the field of view and projects onto point  112  at the upper extent of lower subwindow  96  as indicated by line  92  of the unobstructed portion of the image projected by a red complement lens  108 . Since low point  110  is a distance of typically 50 to 200 meters away for headlamps of oncoming vehicles and tail lights of rearwardly approached vehicles when most headlamp controller actions are initiated, light rays  26  indicated by lower light ray  114 , upper light ray  116 , and central light ray  118  are nearly parallel prior to striking red complement lens  108 . Red complement lens  108  focuses lower ray  114 , upper ray  116 , and central ray  118  into point  112  on image sensor  34 . Lower aperture edge  120  of opening  102  is positioned so that lower ray  114  just clears lower aperture edge  120  and the lower edge of red complement lens  108  indicated by  122 . With this arrangement, opening  102  is just large enough not to block light from low point  110  which would otherwise fall on red complement lens  108  to be focused on point  112 . 
     Ray  124  is the most upwardly directed ray which will clear lower aperture edge  120  and pass through red complement lens  108 . Compared to ray  114 , ray  124  traverses a path which is angled upward by an increasing amount so that it is higher by one lens diameter than ray  114  when it enters red complement lens  108  at the top of lens  108  indicated by  126 . This angular deviation of ray  124  from parallel rays  114 ,  116 , and  118  is approximately preserved as ray  124  leaves red complement lens  108 . Ray  124  strikes image sensor  34  at lower boundary  90  of upper side window  94  at a point indicated by  128 . 
     In one embodiment, red lens  106  and red complement lens  108  have an F number of 4, are nominally 1 millimeter in diameter, and have a focal length, dimension A, of 4 millimeters. Opening  102  is 6 focal lengths from red lens  106  and red complement lens  108 . Dimension B for housing  100  is about 28 millimeters. 
     One of the advantages of miniaturization is that opening  102  can be spaced a reasonably large number of focal lengths from red lens  106  and red complement lens  108  without incurring an excessively large structure. The farther opening  102  is from lenses  106  and  108 , the more distance between lines  90  and  92  can be reduced so that the choice of spacing from opening  102  to lenses  106  and  108  is a practical matter of balancing size against lost sensing area. 
     For the illustrative embodiment described above, ray  124  travels one-sixth as far from red complement lens  108  to image sensor  34  as from opening  102  to red complement lens  108 . Therefore, ray  124  strikes image sensor  34  at a point which is approximately one-sixth the diameter of red complement lens  108  above point  112 . 
     High point  130  is at the upper extent of the field of view of scene  24 . The projection of high point  130  through red complement lens  108  strikes image sensor  34  at a point lower than the region covered by lower subwindow  96 . These rays are not depicted since the projected image is not within either subwindow  94  or  96 . 
     Since high point  130  is also distant from opening  102 , upper ray  132 , lower ray  134 , and middle ray  136  are substantially parallel prior to striking red lens  106 . Red lens  106  focuses rays  132 ,  134 , and  136  onto point  128  on image sensor  134  at the lower boundary of upper subwindow  94  as marked by line  90 . As with ray  124  described above, ray  138  is the most downwardly directed ray which can pass upper opening edge  140  and still be focused by red lens  106 , striking image sensor  34  at point  112 . Thus, while the stray light from red complement lens  108  diminishes to substantially zero in going from line  92  to line  90 , the stray light from red lens  106  diminishes to substantially zero in going from line  90  to line  92 . 
     Referring now to FIG. 5, an alternative embodiment of the present invention is shown. FIG. 5 shows the same area of imaging system  28  as seen in FIG.  4 . The embodiment depicted in FIG. 5 is the same as depicted in FIG. 4 with the exception of the addition of baffle  142 . Baffle  142  decreases the region of image sensor  34  onto which light from both red lens  106  and red complement lens  108  can strike. 
     As a simplified generalization, for a lens at infinity focus and aperture of diameter d, a stop or baffle which is n focal lengths in front of the lens can be positioned to block rays which would strike the focal plane at a distance of more than din away from the portion of the image which is unaffected by the stop. 
     Baffle  142  extends substantially perpendicular to support  104  towards image sensor  34 . Ideally, baffle  142  would extend until nearly touching image sensor  34 . However, image sensor  34  may include sensor package cover glass  144  which may limit the extension of baffle  142 . 
     Baffle  142  blocks ray  124  from striking image sensor  34 . With baffle  142  in place, ray  146  represents the lowest ray which will clear lower opening edge  120 , pass through red complement lens  108 , and strike image sensor  34  at point  148 . Point  148  is about two-thirds of the distance from line  92  to line  90 . 
     Ray  150  is the most upwardly directed ray which could be focused through red complement lens  108  and onto image sensor  34  in the absence of lower opening edge  120 . Ray  150  strikes image sensor  34  at a point indicated by  152  well into the area reserved for the image from red lens  106 . 
     There is little room for good optical treatment of baffle  142  and rays such as  124  which strike baffle  142  at a shallow angle will reflect significantly even from the most blackened surfaces. Opening  102  in front of lenses  106  and  108  performs much better than baffle  142  in the exemplary embodiment shown, but the combination of opening  102  and baffle  142  gives the best performance in minimizing the distance separating upper subwindow  94  and lower subwindow  96  to prevent a significant amount of light which enters one of lens  106  or  108  from falling onto the subwindow projected by the other lens. Note that, instead of spacing subwindows  94  and  96  by the distance between lines  90  and  92 , this distance could be reduced by applying a baffle similar to baffle  142  but thinner, by the reduction of subwindow spacing, and by recentering lenses  106  and  108  and resizing opening  102 . 
     Referring now to FIG. 6, an exemplary embodiment of an aspherical lens pair for use in the present invention is shown. The drawing is provided to illustrate operation of the lenses and not to represent the precise shape or positioning of the lenses. 
     Red lens  106  has front surface  200  facing away from image sensor  34  and back surface  202  facing towards image sensor  34 . At its farthest point, front surface  204  is located dimension C of 4.25 millimeters from image sensor  34 . Front surface  200  is an ellipsoid described by Equation 1:       Z   =         c                   r   2         1   +       1   -       (     1   +   k     )          c   2          r   2               +       ∑     n   =   2     10            C     2      n            r     2      n                                    
     where Z is the value of the height of the lens surface along the optical axis as a function of the radial distance r from the optical axis, c is the curvature, k is the conic constant, and the coefficients C 2n  are the even order polynomial coefficients. For front surface  200 , c equals 0.7194 and k equals −0.4529. Rear surface  202  is spherical with a radius of 4.05 millimeters. The diameter of red complement lens  108 , shown as dimension D, is 1.2 millimeters. Red complement lens  108  has a thickness, shown as dimension E, of 0.2 millimeters at its center. The focal length of red lens  106  is frequency dependent and is 4.25 millimeters for a wavelength of 680 nanometers. 
     Red complement lens  108  has front surface  204  facing away from image sensor  34  and rear surface  206  facing towards image sensor  34 . At its farthest point, front surface  200  is located dimension C of 4.25 millimeters from image sensor  34 . Front surface  204  is also an ellipsoid described by Equation 1 with curvature c equal to 0.7059 and conic constant k equal to −0.4444. Rear surface  206  is spherical with a radius of 4.05 millimeters. The diameter of red complement lens  108 , shown as dimension F, is 1.2 millimeters. Red complement lens  108  has a thickness, shown as dimension E, of 0.2 millimeters at its center. The focal length of red complement lens  108  is frequency dependent and is 4.25 millimeters for a wavelength of 420 nanometers. 
     Referring again to FIG. 6, the effects of frequency dependent focal lengths in lenses  106  and  108  are described. Due to the different aspherical front surfaces of red lens  106  and red complement lens  108 , red light rays  210  and blue light rays  212  are focused differently through each lens. The focal point for red light rays  210  passing through red lens  106  is at the surface of image sensor  34  whereas blue light rays  212  passing through red lens  106  focus a distance in front of image sensor  34 . Likewise, blue light rays  212  passing through red complement lens  108  focus onto the surface of image sensor  34  and red light rays  210  passing through red complement lens  108  focus a distance behind the surface of image sensor  34 . 
     In a preferred embodiment, red lens  106  is manufactured from a polymer which includes a dye for reducing the magnitude of red complement light transmitted through red lens  106 . Red complement lens  108  is manufactured from a polymer which includes a dye for reducing the magnitude of red light transmitted through red complement lens  108 . As an alternative, at least one surface of red lens  106  and red complement lens  108  may be coated to achieve red filtering and red complement filtering, respectively. A further alternative is to use separate filters between scene  24  and image sensor  34 . In particular, filters may be attached to support  104  either directly in front of or in back of lenses  106  and  108 . 
     In an embodiment of the present invention, more than two lenses  106 ,  108  are used. Each lens may be dyed or tinted to emit a different color frequency. Preferably, each lens is shaped such that the focal length of any lens  106 ,  108  at the pass frequency of that lens is the same as the focal length of any other lens  106 ,  108  at the pass frequency of the other lens. 
     Referring now to FIG. 7, a lens for use in an embodiment of the present invention for headlamp on/off control is shown. Light sampling lens  250  collects light from a range of directions, shown as rays  251  through  260 , from the horizontally forward direction to the vertically upward direction. The inclinations of rays  251  through  260  are spaced in approximately 10° increments. Lens  250  redirects incoming rays  251  through  260  to outgoing rays  261  through  270  along approximately horizontal paths. 
     Approximately vertical ray  251  is refracted to ray  271  at front surface  272  of lens  250 . Ray  271  is internally reflected to ray  273  at surface  274  and ray  273  is refracted to ray  261 . Surface  275  is approximately parallel to ray  271  or is at an angle with surface  274  slightly larger than the angle which would place surface  275  parallel to ray  271 . If surface  275  is at an angle with surface  274  less than the angle which would place surface  275  parallel to ray  271 , ray  271  would be blocked when ray  251  entered at a higher point on surface  272 , thereby casting an objectionable shadow on surface  274  close to the intersection of ray  271  with surface  275 . Lens  250  bends incoming rays  252  through  255  in a similar manner to produce outgoing rays  262  through  265 . Surface  274  forms the lower side and surface  275  forms the upper side of a triangular feature with a vertex pointing generally away from front surface  272 . 
     Ray  256  is refracted at surface  280  to ray  281  and ray  281  is refracted to ray  266  at back surface  282 . Similarly, ray  257  is refracted by surface  283  to become ray  284 , which is refracted by back surface  282  to become ray  267 . Surface  285  is approximately parallel to ray  281  and surface  286  is oriented to approximately bisect the angle between ray  256  and ray  284 . Lens  250  refracts incoming rays  258  through  260  in a similar manner to produce outgoing rays  268  to  270 . Surface  280  forms the lower side and surface  285  forms the upper side of a triangular feature with a vertex pointing generally away from back surface  282 . 
     In a preferred embodiment of lens  250 , outgoing rays  261  through  270  are angled progressively from slightly downward for ray  261  to slightly upward for ray  270 . 
     In one embodiment, lens  250  is formed from acrylic with a cross section as shown in FIG.  7  throughout. This embodiment will collect light in a vertically oriented  900  fan with a relatively small angle in the horizontal direction. In an alternative embodiment, increased horizontal coverage is obtained by modifying front surface  272  and back surface  282 . Surface  272  can be formed with a concave cylindrical shape, with the axis of the cylinder parallel to the length of lens  250 . Surface  282  can be formed with a negative cylindrical shape, the axis of the cylinder again parallel to the length of lens  250 . 
     Referring now to FIG. 8, an illustrative optical system incorporating the lens of FIG. 7 is shown. Baffle  300  is placed between scene  24  and lenses  106  and  108 . In a preferred embodiment, baffle  300  is part of housing  100 . Baffle  300  is angled at an angle θ of approximately 45° with vehicle horizontal. Baffle  300  defines opening  302  opening towards scene  24  in front of the vehicle. Opening  302  may be trapezoidal such that the projection of aperture  302  onto a vertical surface would form a rectangle on the vertical surface similar to aperture  102 . Aperture  302  is as small as possible without restricting light projected by lens  106  to any point in upper subwindow  94  or by lens  108  to any point in lower subwindow  96 . 
     Lens  250  is mounted in one side of opening  302 . The width of lens  250  is approximately the same as the diameter of lens  106  or  108 . Lens  250  is oriented such that ray  251  comes from approximately above the vehicle and ray  260  comes from approximately in front of the vehicle. Lens  250  is positioned so that a blurred, inverted image of lens  250  is projected by red lens  106  onto one edge of image sensor  34  between line  304  and line  306  to form red sky image  312 . Lens  250  is also positioned so that a blurred, inverted image of lens  250  is projected by red complement lens  108  onto one edge of image sensor  34  between line  308  and line  310  to form red complement sky image  314 . Due to parallax error, line  306  is above the lower edge of upper subwindow  94  and line  308  is below lower subwindow  96 . The active length of lens  250  is made short enough to permit the entire active length to be projected on the regions between lines  304  and  306  and between lines  308  and  310 . 
     Red sky image  312  and red complement sky image  314  are scanned into processing and control system  36 . Since only a coarse image is required for headlamp on/off control, it is not a great detriment that red sky image  312  and red complement sky image  314  are not in focus. In one embodiment, a threshold is compared to the light levels detected by image sensor  34 . If the light levels are above the threshold, headlamp  22  is turned off. If the light levels are below the threshold, headlamp  22  is turned on. 
     The pixel locations for red sky image  312  and red complement sky image  314  are correlated so that readings can be compared for each 10° elevational increment. A higher ratio of red complement indicates that blue sky is being viewed. In one embodiment, a lower threshold point may be used to turn headlamp  22  on or off for a blue sky than for a cloudy sky. 
     In another embodiment, the threshold is hysteretic. In still another, a time delay after the last on/off transition is used. These two embodiments may prevent headlamp  22  from frequent on/off transitions around the switch point. 
     While the best modes for carrying out the invention have been described in detail, other possibilities exist within the spirit and scope of the present invention. Those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.