Abstract:
The present invention relates to a rain sensor that adaptively functions in a variety of different modes when deployed on vehicle windows of different thicknesses and compositions. The arrangement of multiple lens segments and reflecting surfaces in a nonsequential configuration allows utilization of a greater proportion of light rays from the at least two emitters. Further, a greater portion of the light rays emitted by the at least two emitters is captured by the lens segments and reflectors arranged about at least two detectors, and directed to the detectors. Connection to analytical circuitry then allows interpretation of electrical signals, which in turn control, for example, window wiper systems.

Description:
RELATED APPLICATION 
       [0001]    This application is claiming the benefit, under 35 U.S.C. 119(e), of the provisional application filed Nov. 16, 2006 under 35 U.S.C. 111(b), which was granted Ser. No. 60/859,555. This provisional application is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present invention relates generally to an optical rain sensor for detecting water on an automotive window, and more particularly, to a rain sensor that is capable of operating efficiently when mounted on glass of varying thickness and composition. 
         [0003]    In recent years, it has been increasingly common for motor vehicles to incorporate optical rain sensing wiper control systems that adjust the speed of the wipers in response to the accumulation of water on the outside surface of, for example, the windshield. The rain sensors of such systems typically employ beams of light directed through the windshield at an angle of 45 degrees. The presence of rain or snow on the outside surface of the windshield disrupts the beams, and the optical rain sensor uses that effect to determine an appropriate speed for the vehicle wipers. A practical implementation of such a system was taught by Teder in U.S. Pat. No. 5,059,877, and the teachings thereof are incorporated herein by reference. 
         [0004]    An important factor in the success of a commercial rain sensor is the optical configuration of the sensor. Specifically, the sensor should efficiently couple rays into a vehicle window, and should yield a large sensed area. The sensor should require few opto-electronic devices in order to implement the required sensed area, and to keep the size small and the cost low. Additionally, it is desirable that the rain sensor be compatible with the windows of passenger cars, as well as the windows of larger trucks, recreational vehicles, and other specialty vehicles. Such windows are constructed with a variety of thicknesses and constructions, resulting in different infrared transmittances of the subject structures. 
         [0005]    The rain sensor taught in U.S. Pat. No. 5,898,183 to Teder shows that a rain sensor may be made in a very compact and inexpensive form, and yet still operate in a highly efficient manner. The rain sensor of the &#39;183 patent features two emitters and two detectors, each mounted on a planar circuit board, and couples high obliquity rays into the windshield. A consequence of the approach of the &#39;183 patent is that the design must be nominally optimized for each different glass thickness on which the sensor is mounted. By using a sufficiently large aperture, good performance of the rain sensor of the &#39;183 patent can be achieved for the range of thicknesses typically used in passenger cars, i.e., between 4 to 6 mm thick. But, if the same configuration is to be used for windshields deployed on, for example, recreational vehicles, which are typically 8 mm thick, the optical design, must be made proportionately larger. This makes for a physically larger rain sensor. A larger rain sensor is more costly, because of the need for more materials, and is less aesthetically pleasing. 
         [0006]    A rain sensor for thick windshields may be made more compact if reflecting surfaces are used to fold the beams toward the inside surface of the rain sensor. Such an approach is taught by Stanton in U.S. Pat. No. 5,414,257. This approach is particularly advantageous for thick windshields, where the thickness of the windshield would dictate that the optical devices would be too far apart if implemented with the refractive approach of the &#39;183 patent. However, the approach is less suitable when deployed on a thin windshield, as an optimal design places the emitter and detector so close together as to risk them interfering with one another. The rain sensor of the &#39;257 patent is thus suitable for only a modest range of windshield thicknesses. A common aspect of the aforementioned rain sensors is that they operate in a single mode of operation. That is, the rays from the emitter strike an optical element, are coupled into the glass, deflect off the outside surface, and so on. Optical systems wherein all rays of interest progress from each surface to the next are known in the field of optical engineering as “sequential”, or “deterministic.” The other major class of optical systems is known as “nonsequential”. In nonsequential systems, a ray emanating from a surface may subsequently strike any of several surfaces, depending on the position and direction of the ray. All of the aforementioned rain sensors utilize sequential optical systems. The rays follow this same deterministic sequence, or mode of operation, regardless of whether the sensor is deployed on thin glass or thick. By the nature of this approach, the rain sensor must be optimized for a particular thickness of windshield. If the sensor is placed on a windshield that is much thicker or much thinner than the design optimum, the rays from the emitter do not strike the optical structure that is supposed to guide the rays to the detector. The result is that the rain sensor either functions poorly or ceases to work at all. It would be better if a sensor could operate in a different fashion for different material thicknesses. 
         [0007]    U.S. Pat. No. 6,232,603 describes a device for detecting the presence of moisture on an outside surface of a windshield, which device includes an emitter for transmitting energy, a sensor for receiving energy, an energy absorbing member and a controller for monitoring energy. 
         [0008]    U.S. Pat. No. 6,311,005 describes a sensor device for determining the degree of wetting and/or soiling of a pane in a motor vehicle. The sensor device is said to detect moisture on the outer side of the pane via an optical beam which is arranged in the area of the pane. The sensor device includes a reflector positioned in the pane, the reflector intended to direct the beam through the pane under conditions of total reflection or reflection at the outer side of the pane and at the reflector. The pane additionally includes a light filter which absorbs a selected wavelength of sunlight. Attenuation of the light is said to be reduced via an optically more absorbent layer of the light filter. 
         [0009]    U.S. Pat. No. 7,230,260 describes a raindrop sensor provided on a first surface of a transparent body for sensing water attached to a second surface of the transparent body, the raindrop sensor including a light emitting element, a light guide body, a light receiving element and an abnormality determining device. The functions of these various components is also described. 
         [0010]    A further issue with prior art rain sensors is that they are subject to misalignment. It was shown in the &#39;183 patent that a rain sensor may be made with segmented lenses. Such lenses, known also as Fresnel lenses, offer compact size. Like conventional lenses, they share a common focal point and focal power across the surface of, for example, a glass pane. Thus, if the optical device is misaligned, or the mounting of the rain sensor induces too much deviation to the optical path, it may be possible for a significant portion of emitter rays to miss the detector lens. The effect is controllable in the sensor of the &#39;183 patent, but there remains room for improvement. 
         [0011]    Conventional surface mount emitters of infrared radiation radiate into a hemisphere, more strongly on-axis, and decreasing off-axis. The strength of the radiation is, generally, decreased by as much as 50% at 60 degrees off-axis, and decreases even more rapidly when greater than 60 degrees off-axis. Thus, surface mount emitters, are typically said to have a 60 degree half-angle, or 120 degree cone angle. This angle, times the surface area of the emitter, may be a thought of as the “extent” of the emitter. Similarly, the sensitivity of a surface mount detector of infrared radiation, for example, a photodiode drops rapidly beyond 60 degrees off-axis, and are usually also specified as having a 120 degree acceptance cone. 
         [0012]    These emitter and detector extents are each in three-dimensions. So, in examining the figures of the present application, one must realize that most of the rays emitted from the emitters are not in the plane of the page on which the figures is printed. The emitters additionally radiate into and out of the plane of each page, and in most known rain sensors this radiation is not utilized. Even if an emitter captures light rays over a cone angle of 40 degrees, taken in three dimensions, this in total utilizes only 12% of the available angular extent of the emitter. Similarly, a cone angle of 40 degrees for the detector side, utilizes but 12% of the available acceptance angle of the detector. The mechanical and optical constraints of rain sensor design make it very difficult to utilize a high extent. Even the best of known rain sensors would approach using only 25% of the available extent on either the emitter of detector side optics. More typical extent utilization in rain sensors is well under 10% 
         [0013]    The rain sensor disclosed in the &#39;183 patent seeks to use as many of the light rays that emanate from the emitter as possible. Only those rays that are ultimately coupled into the detector are of value in sensing rain. Similarly, it is desirable to use all of the available angles to direct rays into the detector, it is desirable to utilize the entire available sensed surface area of the detector. The product of detector area and angle is known in the field of optical engineering as the as the “extent” of the detector, and good use of the available extent allows the least expensive detector for the purpose. While the sensor of the &#39;183 patent represented an advance over the art that came before, it will be appreciated that there is room for improvement in the utilization of emitter rays and target extent. 
         [0014]    A sensor utilizing multiple passes, or deflections, through the windshield, may readily be constructed for windshields with high infrared transmittance. Most modern passenger car windshields, however, strongly absorb infrared light, rendering the multi-pass approach inoperative. It would be better if a rain sensor could work as a single-pass sensor when deployed on infrared absorbing glass, and still function as a multi-pass sensor when deployed on clear glass which allows transmittance of fairly high percentages of infrared radiation. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention is a rain sensor which is compatible with vehicle windows having a wide range of thicknesses and compositions. In the subject rain sensor, an optical coupler, into which optical elements are molded, is optically and mechanically coupled to a vehicle window using a suitable coupling method. A housing is detachably mounted to the coupler, and holds a circuit board generally parallel to the window. Electronic components are mounted on the circuit board, along with at least two emitters and two receivers or detectors. Each emitter is disposed to direct rays through the coupler and vehicle window and onto each of the at least two detectors, thus resulting in a minimum of four similar optical paths. 
         [0016]    Optical elements including lens surfaces and reflecting surfaces guide the infrared light from each emitter, through the vehicle window, and into each detector. All of these optical elements are preferably molded into a single optical coupler. The optical elements for the detectors are substantially identical to the corresponding elements for the emitters. Each of the at least four optical paths created by the arrangement of emitters and detectors are substantially identical and symmetric about the center of the rain sensor. 
         [0017]    To implement each optical path from emitter to detector, a group of optical elements is molded into the coupler above the level of the emitter relative to the circuit board. This group includes a set of lens segments or lenslets that generally gather and collimate a fan of rays from the emitter, and direct them at an angle of about 45 degrees into the window. Additionally, a “folder” structure gathers some rays that would otherwise travel toward the outside of the rain sensor. The folder structure reflects these light rays back toward the inside of the rain sensor, and thus they are available to travel through the window at an angle of about 45 degrees. A symmetric group of lens segments similar to those arranged about the emitter exists in proximity to the detector. In a fashion analogous to the emitter collimator, the group of emitter lens segments focuses rays traveling through the window toward the detector at about 45 degrees. A second folder structure gathers some light rays that would otherwise miss the detector to the outside, and reflects them back toward its corresponding detector. 
         [0018]    The present invention will operate in one of three possible modes, depending upon the characteristics of the window upon which it is deployed, particularly the thickness of the window and the level of infrared radiation transmittance of the window. The three modes of operation may generally be referred to as the “collimator to focuser” mode, the “folder to focuser/collimator to folder mode” and the “folder to folder” mode. 
         [0019]    When the present invention is deployed on typical automotive windshields, which are 4.0-6.5 mm in thickness and in the range of 17% to 75% transmissive to infrared radiation, the rain sensor operates predominantly in “collimator to focuser” mode. When on thicker windshields suitable for recreational vehicles, which are typically between 6.5 and 9.5 mm thick and 20% to 75% IR transmissive, the present rain sensor invention operates predominantly in “folder to focuser/collimator to folder” mode. When deployed on exceptionally thick windshields of 9.5-12 mm, the present invention operates predominantly in “folder to folder” mode. When deployed on exceptionally thin and transmissive windshields of 4-4.5 mm and &gt;70% IR transmittance, the sensor will further function in an additional mode that may be considered “folder to folder multi-bounce mode.” 
         [0020]    By changing modes of operation with deployment on differing windows, the present invention allows a single rain sensor to offer exceptional performance in a wide variety of applications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims taken in conjunction with the accompanying drawings in which: 
           [0022]      FIG. 1  is a transverse section view of the rain sensor showing the optical path deployed on a windshield having a thickness such as are typically found on passenger cars. 
           [0023]      FIG. 2  is a transverse section view of the optical components of the rain sensor when deployed on a thin windshield such as may be found on sports cars. 
           [0024]      FIG. 3  is a transverse section view of the optical components of the rain sensor when deployed on a thick windshield such as may be found on recreational vehicles. 
           [0025]      FIG. 4  is a transverse section view of the optical components of the rain sensor when deployed on a very thick windshield such as may be found on tractors. 
           [0026]      FIG. 5  is a top view presentation showing the relationship of the two emitter and two detectors and connecting optical paths. 
           [0027]      FIG. 6  is a perspective view of the underside of the coupler showing all of the optical surfaces. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The rain sensor of the present invention allows a single rain sensor to efficiently detect the amount of moisture on vehicle windows of widely varying thickness, ranging from unusually thin windows, especially windshields, found on, for example, sports cars to exceptionally thick windows found on, for example, farm tractors. Achieving such efficiency, while keeping the physical size of the rain sensor small, and the cost of manufacturing same low, is, generally accomplished by maximizing the “extent” of the detector(s) of light rays which can then utilize a greater fraction of the light rays generated by the emitter(s). Deflection and direction of light rays by an assemblage of lenses adjacent the emitter(s) and detector(s) is also part of the present invention. 
         [0029]    The amount of infrared (IR) energy transmittance through a window also affects the efficiency of operation of rain sensors, as earlier discussed herein. In order to account for not only variations in IR energy (or radiation) transmittance, as well as variations in glass thickness, requires that a rain sensor be capable of operating in more than one mode, for reasons which will be explained in greater detail hereinafter. The structure of the present rain sensor further allows for greater tolerance in the manufacturing process, as well as expanding the number of acceptable methods of mounting the rain sensor to the vehicle window. 
         [0030]    The rain sensor  10  of the present application is shown, generally, in a transverse, cross sectional view in  FIG. 1 . The rain sensor  10  is mounted upon a vehicle window  12  of thickness T. Window  12  is of conventional laminated construction, comprising two layers of glass with an intervening interlayer of an adhesive plastic material, e.g., polyvinyl butyral. The laminated structure has an outside surface  13  and an inside surface  14 . The presence or absence of the plastic interlayer in the laminated structure has no bearing on the invention and is thus omitted for clarity. Rain sensor  10  is preferably mounted to the window  12  using a suitable optically transparent adhesive  15 , for example, adhesive tape, glue or other bonding agents may be used. The invention includes a coupler piece  16 , into which optical elements, including refracting and reflecting surfaces, are molded. Coupler  16  is typically of an injection-moldable material, such as polycarbonate or acrylic plastic. It is contemplated that coupler  16  could be constructed using multiple-shot molding techniques to incorporate ambient light blocking, but this is not necessary to implement the invention and is not shown here. In  FIG. 1 , the optical elements molded into coupler  16  are shown generally at  18 . A housing  20  is detachably affixed to the coupler. Within the housing a planar printed circuit board  22  is mounted generally parallel to windshield  12 . 
         [0031]    Mounted upon an upper surface  26  of printed circuit board  22  are conventionally affixed electronic components  24 . Such components  24  perform the signal processing and control functions required of the rain sensor. Also mounted on upper surface  26  of the planar circuit board is a representative infrared emitter  30 . Each such emitter  30  produces rays over a 180 degree hemisphere, in a nominally Lambertian pattern. That is, the intensity is strongest normal to the circuit board  22 , and declines with the cosine of the angle to the normal. The emitter  30  may also be implemented as a chip bonded directly to the circuit board  22 . Rays from emitter  30  are shown, generally, at  32 . Also on upper surface  26  of circuit board  22  and spaced apart from emitter  30 , is a representative infrared detector  40 . Each such detector receives rays over a 180 degree hemisphere. A photosensitive die  42  within detector  40  acts to form the active area of the detector  40 . The detector  40  is preferably, a photodiode, but a phototransistor, photovoltaic cell, or cadmium sulfide photo-resistor may alternatively be used. A portion of housing  20  forms crosstalk-prevention walls  28  that prevent emitter rays  32  from directly reaching detector  40  without first passing through coupler  16 . A representative rain drop  34  is also shown on the outside surface of the laminated window  12 . 
         [0032]    Coupler  16  largely consists of a planar substrate, having an inside surface  46 . An “on-axis” collimator lens  50  is molded onto the coupler substrate at an angle of nominally 45 degrees to the angle normal to emitter  30 . On-axis collimator lens  50  has a convex curvature relative to emitter  30 . Other optical elements to be described are similarly also molded into coupler  16 . On-axis collimator  50  has the effect of substantially collimating rays  32  from emitter  30 . The surface of on-axis collimator lens  50  is nominally spherical. The spherical shape extends into and out of the plane of  FIG. 1 . As will be examined later, emitter rays  32  are not, however, fully collimated, and thus some light rays are slightly divergent. Put differently, emitter  30  is located slightly to the inside of a focal point of on-axis collimator lens  50 . 
         [0033]    Also disposed on coupler inside surface  46  is an “above-emitter” collimator lens  52 . Above-emitter collimator lens  52  has the effect of substantially collimating rays  32  that are closer to an angle normal to the printed circuit board  22 , but slightly less than normal. The above-emitter collimator lens also directs the rays reflected from it to be roughly 45 degrees with respect to normal to printed circuit board  22 . The surface of above-emitter collimator lens  52  is nominally toroidal, so that the lens may have a sharper radius of curvature into and out of  FIG. 1 , and a more gradual radius in the plane of  FIG. 1 . The toroidal aspect of the lens has the effect of reducing aberration of the light rays, compared with a purely spherical shape. Thus, this lens, as well as others described below, extend into and out of the plane of  FIG. 1 . 
         [0034]    On-axis collimator lens  50  and above-emitter lens  52  together comprise a collimator region  54  that, generally, contains a fan of rays  32  emanating from emitter  30 , and on-axis collimator lens  50  and above-emitter collimator lens  52  capture and direct rays  32  to an angle of approximately 45 degrees as they enter window  12 . Collimator region  54  is shown implemented as two lens sections, but may alternately be implemented in more sections, including segments into and out of the plane of  FIG. 1 . Preferably, each of the collimator lens sections is kept slightly divergent. That is, it may be seen that rays  92  splay slightly apart, rather than remaining perfectly parallel, as they would were above-emitter collimator lens  52  a perfect collimator. Such divergence is achieved by placing emitter  30  slightly inside the nominal focal point of above-emitter lens  52 . It is contemplated that the entire collimator region  54  could be molded as a single surface, combining on-axis collimator lens  50  and above-emitter lens  52  into a single, larger and deeper surface. This would not be as desirable as the lens segments ( 50 ,  52 ) as shown, because it would make for a more difficult-to-mold part. Also molded onto coupler  16  is an inside-reflector structure or “folder” structure shown generally at  60 . This structure has the general effect of partially collimating emitter rays  32  moving toward the outside of the rain sensor as represented by reflector surface  64  and deflecting them at a roughly 45 degree angle toward the inside of the rain sensor represented by surface  46 . Such a structure may also be considered a “folder”, in that it redirects light rays toward the inside of the rain sensor capturing such rays that would otherwise not be utilized and additionally allowing a more compact optical assembly. Reflecting surfaces are often introduced into, for example, binoculars, to render the optical systems more compact. Such surfaces are typically referred to as “folders” in the field of optical engineering. In particular, emitter folder structure  60  includes an emitter-facing lens surface  62 . This convex lens surface  62  partially collimates emitter rays  32 , as just described. Also molded into folder structure  60  is a reflector surface  64 , generally normal to the coupler substrate. Reflector surface  64  is kept as close to perfectly normal as good injection molding practices permit, with a draft angle of nominally 1 degree. As shown in  FIG. 1 , rays  32  from the emitter  30  perform a total internal reflection (TIR) off reflector surface  64 . The reflecting surface  64  and emitter facing lens  62  are joined by material as required to make folder  60  a contiguous volume. 
         [0035]    It will be appreciated that the optical configuration shown in  FIG. 1  makes use of nearly the full fan of rays  32  emanating from emitter  30 . High obliquity rays directed toward the inside of the sensor, for example toward inside surface  46 , are coupled into the windshield with only modest deflection. In addition to those rays, however, the rays moving toward the outside of the rain sensor, reflector surface  64  are also coupled into the vehicle window after being re-directed by folder structure  60 . The aforementioned optical features, on-axis lens  50 , over-emitter lens  52 , and folder  60 , form a group of emitter optical elements  66 . Coupler  16  snaps into housing  20  with rib features  68 . Alternatively, clip or screw features may be used to affix the coupler. The top surface of coupler  16  forms an optically transparent bond with adhesive  15 , and rays traveling through this bond are essentially undeflected. The adhesive  15  forms a similar optically transparent bond with windshield  12 . 
         [0036]    Also shown in  FIG. 1 , a group of receiver optical elements  70  is molded into coupler  16  in proximity to detector  40  the optical elements arranged generally symmetrically. An on-axis receiver lens  72  guides light rays moving out of window  12  toward the detector  40  at an angle of approximately 45 degrees. On-axis receiver lens  72  is preferably of identical shape to on-axis collimator lens  50 . Thus, the on-axis receiver lens  72  acts as a receiver and focuser lens, gathering a “column” of rays into a ray “pencil” that focuses to a finite-sized spot upon detector die  42 . The detector die is large enough to permit some degree of tolerance for misalignment of the beams. Directly analogous to the emitter side on-axis lens  50 , on-axis receiver lens  72  accepts a fan of rays that is slightly convergent. That is, the detector die  42  is located slightly inside a focal point of an-axis receiver lens  72 . This has the effect of accepting a wider fan of rays than it would if the detector die  42  were located precisely at the focal point. Thus deployed, it can be seen that optical elements  70  for the detector  40  are identical to the corresponding elements for the emitter optical elements  66 . 
         [0037]    Also included in the group of detector optical structures  70  is an above-detector receiver lens  74 . Above-detector lens  74  gathers the column of rays traveling at 45 degrees but generally above and slightly inside detector  40 , focusing them to a spot on detector die  42 . Further included in detector optical structures  70  is a receiver folder structure  76 , deployed to focus rays that are traveling at about 45 degrees within windshield  12 , and near the outside of the rain sensor, e.g., surface  46 , and reflect them back to the detector die surface  42 . Receiver folder structure  76  comprises a convex detector-facing lens  78 , a receiver side inside-reflecting surface  80 , and material to make the structure a contiguous volume. All of the receiver side group  70  optical elements surfaces are preferably of a shape identical to the emitter-side  66  counterparts. So designed, the coupler may be deployed such that the emitter and detector groups are interchangable. In a fashion analogous to the emitter optical structure  66 , a full fan of rays is directed into the detector group of optical elements  70 . Thus, nearly the full extent of the detector target region is utilized: the entire surface area of the detector is illuminated by rain sensing rays, and the rays come from a full splay of angles Also analogous to the emitter side, each set of receiver elements  70  accepts a slightly convergent fan of rays, rather than being perfectly focused. The collection of rays that ultimately strike the detector die  42  forms a receiver ray fan  82 . 
         [0038]    Still with reference to  FIG. 1 , rays  32  emanating from emitter  30  may take several paths. “On-axis” rays  90  may be considered those that travel from emitter  30  at nominally close to 45 degrees toward the inside of the rain sensor, and may be, at least, partially collimated by on-axis collimator lens  50 . That is, they are on the 45 degree axis, but not the axis normal to the board and the emitter. On axis rays  90  are weaker than those straight out of emitter  30 , and thus normal to board  22 . This is because of the intensity pattern of the emitter, which shines brightest straight out. On axis rays  90  are already traveling at 45 degrees with respect to board  22 , and thus require little deflection from on-axis collimator  50 . So, on-axis rays  90  encounter little loss due to surface reflection. The resulting rays  90  are thus still quite strong after coupling into the windshield. Slightly-inside rays  92  are close to the normal of emitter  30 , and are thus stronger out of the emitter than are on-axis rays  90 . However, above-emitter collimator lens  52  significantly deflects slightly inside rays  92 , and the rays thus encounter significant loss due to surface reflection. The effects roughly balance and the slightly-inside rays  92  are intense enough to usefully sense rain. Reflected useful rays  94  initially travel toward the outside of the device, but are deflected back toward the inside of the rain sensor by the emitter reflecting surface  64 , ultimately to be directed through window  12  onto detector die  42 . Other rays such as  96  emanate from the emitter  40 , but miss any of the optical elements of the emitter lens group  66 . Such rays do no harm, but are not useful in sensing rain. Other rays, omitted from the figure for clarity, are gathered by the emitter focusing structures, and may undergo one or more reflections, but are not ultimately directed to the detector die  42 . Each of the varying paths from emitter  30  to detector  40  may be thought of as “modes”. That is, the rain sensor employs several quite different paths for the light rays, and the order of rays passing through the surfaces is not predetermined. Such a system is known in optics as “nonsequential”, and the analysis of such systems is more complex than that of a deterministic system. This ability to function in different modes will be examined further later in this document. 
         [0039]    Window  12  of  FIG. 1  represents a vehicle window 6 mm thick which is, generally, the maximum thickness for windows in typical passenger cars, including laminated structures, such as windshields. T=6 mm may be considered an optimum design thickness for those lenses deployed along the optical path without deflection, e.g., on-axis collimator lens  50  and above-emitter collimator lens  52 . That is, coupler  16  is designed such that the following lenses function optimally with, for example, a relatively thick passenger car windshield: on-axis collimator  50 , above-emitter collimator  52 , on-axis receiver lens  72 , and above-detector receiver lens  74 . It will be appreciated that with the lens arrangements as presented, light rays following the path of no single optical axis necessarily travels from emitter to detector without some amount of deflection along its path. 
         [0040]    The optical components of the present invention, deployed on vehicle window  12  of 4 mm thickness T is shown in  FIG. 2 . Components such as the housing and circuitry are omitted for clarity. A 4 mm thick window such as that of  FIG. 2 , which is quite thin for, e.g., a windshield, might be used on a sports car, mounted at a very shallow angle for minimum wind resistance and for weight savings. Just as in deployment of the present rain sensor shown in  FIG. 1 , the coupler  16  deflects on-axis rays  90  and slightly inside rays  92 . It can be seen in  FIG. 2  that some of the highest obliquity rays are coupled from emitter to detector. Additionally, it may be seen from  FIG. 2  that a two-pass, or multi-deflection ray  100  undergoes one or more deflections off a center region  102  of the coupler  16  before being directed to the detector  40 . If windshield  12  is relatively transmissive to IR energy, then two-pass ray  100  will be of sufficient amplitude to usefully sense rain. Thus configured, the present invention allows for multi-deflection operation when the glass thickness and IR energy transmittance characteristics are suitable. 
         [0041]    The present invention is shown in  FIG. 3 , on vehicle window  12  of nominally T=8 mm. Such a window thickness is typical of a windshield for a recreational vehicle, for example, large, single-piece windshields. In this application a ray  104  travels at an angle slightly off of an angle normal to the emitter  30  to slightly past detector  40 , if not redirected. This ray  104  thus traverses both above-emitter collimator lens  52  and above-detector receiver lens  74  which does redirect it into detector  40 . Additionally, a ray  106  passes through folder  60  on the emitter  30  side, through windshield  12 , and is focused onto the detector  40  by on-axis receiver lens  72 . A similar ray  108  traverses through on-axis collimator lens  50 , windshield  12 , and receiver folder  76  where it is redirected to detector  40 . It may be seen that under such conditions the folder structure ( 60 ,  72 ) becomes a more important means of coupling emitter rays to the detector  40 . Further, the groups of rays surrounding ray  104 , ray  106 , and  108  form relatively widely spaced disparate target regions. This increases the sensed area of the windshield  12 . The present invention is particularly compact in operation with such a thick windshield, yet offers a very highly sensed area. 
         [0042]    The present invention is further presented in  FIG. 4  on a very thick T=10 mm vehicle window. Windows, for example windshields, may be found in some specialty vehicles that are not used on highways, such as tractors or other farm or constructions equipment. The emitter reflector to on-axis receiver ray  106  exists in a similar fashion to that shown in  FIG. 3 . Additionally, an emitter ray  110  is redirected by folder  60  to above-detector receiver lens  74 . Similarly, ray  112  travels through above-emitter lens  52 , through window  12  and is redirected by the detector folder  76  to detector  40 . Further, ray  114  travels from emitter side folder  60  to detector side folder  76  where it is redirected to detector  40 . The emitter folder to detector folder ray  114  exists on windshields as thick as 12 mm, but such deployments are not illustrated here because the operation of the device is similar to that shown in  FIG. 4 . 
         [0043]    The explanation of the optical path herein has thus far been concerned with the optical path from a single emitter to a single detector. In a preferred embodiment of the invention, there are deployed two such emitters and two detectors, forming four such optical paths. The arrangement is shown in a top view in  FIG. 5 . That is,  FIG. 5  shows coupler  16  looking through the windshield, to best illustrate the optical paths of the invention. In this arrangement, emitter  30  shines rays of infrared light onto detector  40 , by way of a first optical path  121  through coupler  16 . Coupler  16  forms, from the plan view, a square. The optical elements of the path are omitted for clarity. Each of  FIGS. 1 through 4  may be taken to be a transverse section along first optical path  121 . Additionally, emitter  30  shines rays along a second optical path  122 , through coupler  16  to a second detector  40 B. The elements of the coupler are shown in a perspective view in  FIG. 6 . Returning again to  FIG. 5 , a second emitter  30 B is disposed in the corner of coupler  16  opposite emitter  30 . Second emitter  30 B directs light to detector  40  along a third optical path  123 , and onto detector  40 . Additionally, second emitter  30 B directs light along a third optical path  124  to second detector  40 B. Thus configured, each emitter ( 30 ,  30 B) directs light onto two detectors ( 40 ,  40 B), making an exceptionally efficient use of optical devices. Thus deployed each of the four optical paths is substantially identical and symmetric about the center of the rain sensor. 
         [0044]      FIG. 6  is a perspective view of the underside of the coupler. This figure shows the arrangement of optical elements  18  on the inside surface of coupler  16 . A first group of emitter side optical elements  66  is disposed in one corner of the coupler. A second group of emitter side optical elements  66 B is disposed in an opposite corner of coupler  16 . Similarly, first and second groups ( 70 ,  70 B) of receiver side optical elements are deployed in the two opposite corners as shown. The location of emitter  30  is shown at “X”, just above emitter group  66 . Locations of other emitter  30 B and the detectors ( 40 ,  40 B) are similarly located with respect to their corresponding groups of optical elements. It is clear from the figures that regions above the optical elements are densely occupied. Virtually all of the available space surrounding the optoelectronic devices is utilized by structures that gather rays and direct them in a fashion useful for sensing rain. The present rain sensor invention is thus very efficient, making use of a high percentage of available rays. 
         [0045]    In operation, the control circuitry (formed with electronic components  24 ) as shown in  FIG. 1 , pulses infrared emitters  30  and  30 B. This sends rays  32  generally through window  12  to detectors  40  and  40 B. The details of the optical coupling will be explained below. These rays give rise to signals in the detector ( 40 ,  40 B), which is processed by the circuitry  24 . The presence of rain drop  34 , or other water droplets, mist, or melted snow on outside surface  13  of window  12  alters the strength of the received beams. In a similar fashion to other optical rain sensors, the circuitry  24 , preferably also including a microprocessor with suitable software, interprets the change in signal strength as a rain event. Algorithms within the microprocessor determine an appropriate response from the wipers, and a suitable interface and wiper system implements the commands. 
         [0046]    In the present invention, the optical paths between emitters ( 30 ,  30 B) and detectors ( 40 ,  40 B) vary significantly as the rain sensor is deployed on different window thicknesses. Thus, in a departure from the prior art, operation varies with deployment. The most common deployment, on nominally thick windshield glass, is shown at  FIG. 1 . In operation, the majority of useful rays are transmitted from emitter to detector in a largely on-axis mode, with certain axis shifts. In detail, rays  92  emanate from the emitter  30 , generally traveling toward above-detector collimator lens  52 . Rays  92  are deflected by above-emitter collimator lens  52  and largely collimated, and travel through the windshield at about 45 degrees. It can be seen by the slight splay of rays  92  after above-emitter collimator lens  52  that the above-emitter collimator lens  52  is slightly divergent. This slight divergence allows rays  92  to strike a larger area of the receiver side optics group  70  than they would if they were perfectly collimated. Rays  92  are then largely gathered by on-axis receiver lens  72 , and focused to a spot on detector die  42 . At the window thickness portrayed in the figure, a few of rays  92  also strike above-detector receiver lens  74 . Rays  90  may be thought of as functioning in “collimator to focuser mode.” That is, they travel generally from collimator region  54  to focuser regions ( 72 ,  74 ) In a converse fashion, rays  90  from the emitter to emitter on-axis lens  50  are largely received by above-detector receiver lens  74 . Rays  90  are also slightly divergent, thus striking a significant area of receiver optics group  70 . As can be seen from the figure, emitter rays  92  from the on-axis fan inside are directed to the on-axis receiver fan  82  outside. As may be seen from  FIG. 1 , however, the folder structures ( 60 ,  76 ) also contribute to the operation of the rain sensor in this configuration. Ray  94  from emitter  30  is partially collimated by the emitter-facing lens surface  62 , and totally-internally-reflects (TIR&#39;s) off emitter-side reflector surface  64 . Rays  94  travel through the window at about 45 degrees, and some are ultimately gathered by detector-side on-axis lens  72 . These rays are at high obliquity and thus less strong, but nonetheless strike detector  40  and are of some value in sensing rain. Such rays may be thought of as operating in “folder to focuser/collimator to folder” mode. Similarly, and in the same mode some of the rays  90  from the emitter side on-axis lens are captured by receiver folder  76 , and usefully directed toward detector die  42 . It will be appreciated that the presence of the reflecting structures therefor makes use of a wider fan of rays  32  from emitter  30 . Further, the present invention makes greater use of the available extent or acceptance angle of detector  40 , gathering a wide detector ray fan  82 . In such fashion, the overall efficiency of the rain sensor is increased. Additionally, the folding structures ( 60 ,  76 ) allow for additional useful target regions on the outside surface of the glass. 
         [0047]    The operation of the system on a thin (4 mm) window  12  is shown in  FIG. 2 . The rays ( 90 ,  92 ) through emitter-side collimator region  54  function, for the most part, similarly to that of the more common  6 mm glass deployment of  FIG. 1 . Because of the thinner glass, on-axis rays  90  do not strike the detector folder  76 . Similarly, there is no path from emitter folder  60  to detector on-axis collimator  72 . However, the folders ( 60 ,  76 ) aid the operation of the invention in cases where the window is relatively IR energy transmissive, and can thus permit multi-pass or multi-deflection operation. For transmissive windshields, e.g., &gt;65% transmissive to IR, a two-pass ray  100  emanates from emitter  30 , is partially collimated (i.e., rays are slightly divergent) by emitter-facing lens  62 , reflected by reflecting surface  64 , and directed into windshield  12  at about a 45 degree angle. Ray  100  then strikes outside surface  13  of windshield  12 , and is reflected back into windshield  12 . It then strikes a center region  102  of coupler  16 , where it reflects back up into the windshield. After a second pass through windshield  12 , two-pass ray  100  is gathered by above-detector receiver lens  74  and focused onto detector die  42 . Ray  100  thus forms widely separate target regions that are responsive to the effects of rain. A symmetrical and analogous ray  100 D passes through detector-side folder  76  as shown. Thus, the present invention is capable of functioning as a multi-deflection rain sensor, with the associated increase in sensed area, where the characteristics of the glass are suitable. 
         [0048]    The present rain sensor invention is shown again on thicker,  8 mm (Recreational Vehicle) glass in  FIG. 3 . It will be appreciated that the mode of operation of the rain sensor differs significantly from the mode of operation when the rain sensor is deployed on 5 to 6 mm passenger car glass. In deployment of the rain sensor on thicker glasses the more significant portion of the useful emitter rays pass through the folders ( 60 ,  76 ). That is, the “folder to focuser/collimator to folder” mode” dominates the operation of the device. Specifically, rays  106  through emitter folder  60  are transmitted to on-axis receiver lens  72 . In a similar and symmetric fashion, rays are transmitted through on-axis collimator lens  50  to detector folder  76 . Each of these groups of rays ( 106 ,  108 ) are strong and at relatively low obliquity, and thus are important to the ability of the sensor to detect rain. Still referring to  FIG. 3 , a ray  104  exists to travel between above-emitter lens  52  and above-detector lens  74 , a mode which is absent in the, for example, 4-6 mm glass. Rays  104 , however, still contribute to the operation of the rain sensor. Thicker windshields are typically of less IR energy absorptive materials, permitting satisfactory transmittance of the light rays. Additionally, it can be seen from  FIG. 3  that the rays form a relatively large sensed area on the outside surface, owing to the differing paths of the rays. Thus, the present rain sensor invention can take advantage of the thicker glass, providing a larger sensed area. 
         [0049]    Turning to  FIG. 4 , deployment of the present invention on very-thick (10 mm) glass, it may be seen that the “collimator to focuser mode” dominates operation of the invention in deployment of the sensor on, for example, 4-6 mm glass are absent. In a fashion similar to deployment on 8 mm glass, a ray  106  is transmitted from emitter  30  to detector  40  by way of emitter folder  60 , and on-axis receiver lens  72 , in “folder to focuser/collimator to folder mode.” Additionally, a ray  110  travels a similar path, but by way of above-detector receiver lens  74 . This is possible because of operation of emitter folder  60  i.e., not complete collimation, but slight divergence of some defected rays. On the very-thick glass (10 mm) deployment, a ray  114  also travels from emitter folder  60  to detector folder  76 . This mode of operation may be thought of as “folder to folder”. These multiple rays result in widely spaced targets, allowing for greater probability of a sensed area being hit by rain drops. The present invention permits operation on very thick glass without the need to adopt a larger, more complex on-axis system, that is, the present invention is exceptionally compact, despite the ability to work with thick windshields.  FIG. 4  also shows the virtue of divergent beams at each optical element; rays through the emitter folder  60  splay far enough apart to travel through both of the on-axis and folder structures of the detector side. 
         [0050]    The present invention has achieved good performance and compact size by operating in different modes for different thicknesses of glass. For the thinnest windshields, the most important are high obliquity rays, generally directed at 45 degrees towards the inside of the rain sensor. For windshields that exhibit high light transmittance, rays may bounce off the inside surface of the coupler and pass through the windshield twice, further enhancing the sensed surface area. For exceptionally thick windshields, the reflected rays are utilized. These rays are also at fairly high obliquity, but in this case directed at close to 45 degrees to the outside of the rain sensor. At various thicknesses in between, multiple modes of operation are utilized. 
         [0051]    As shown in  FIGS. 1 through 6 , emitter side collimator lens region  54  is preferably made up of two lens surfaces, or lenslets. This arrangement differs from a single Fresnel lens with two segments, in that each of the lens regions is independently optimized. Additionally, each of the sections is allowed to be slightly divergent relative to one another. Were the regions mathematically coupled in a traditional Fresnel design approach, the extent of divergence would be uniform across the entire aperture area. As shown in the present invention, however, each divergent region directs a splay of rays into the windshield. This allows rays from above-emitter collimator lens  52  and the on-axis collimator lens  50  rays to travel toward each other, a situation precluded in normal Fresnel design. This is best illustrated in  FIG. 1  (rays  90  and  92 ). The multi-lens design may be thought of as a “bug-eye” lens approach, where each of the regions does not share a common focal point. The system shown is implemented in just two sections for the predominantly on-axis region. Two lenses were found to be sufficient, but the invention may be implemented with a larger number of lens sections. A beneficial effect of these divergent sections is greater tolerance to misalignment. 
         [0052]    It may be appreciated that the optical surfaces of the coupler lenses must all be optimized simultaneously, that is, the above-device and on-axis lenses must function properly for the nominal case mode (6 mm glass), properly directing rays at approximately 45 degrees. These same lenses, however, must also exhibit sufficient efficiency when coupling rays from or to the reflecting structures. Thus, it is necessary in the design of such a system to optimize both modes of operation simultaneously. Recent advances in computational power and optical design software, such as that offered by Zemax, Incorporated, of Tuscon, Ariz., have made such simultaneous optimization possible. In a practical implementation of the invention, a dual-processor computer was set about the task of optimizing all surfaces simultaneously, tracing many trillions of rays over a period of days. The resulting system is thus able to function well in the multiple modes, with differing windshield thickness.  FIGS. 1 through 4  presented herein are derived from computer-generated ray tracings, and represent the actual performance of a practical rendition of the invention. 
         [0053]    The foregoing discussion discloses and describes exemplary embodiments of the present invention. One skilled in the art will readily recognize from this discussion and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.