Abstract:
The invention is a precipitation sensor adapted to detect water upon an automotive glass and a method for its use. The precipitation sensor includes an optical emitter and a first mirror surface in optical communication with the optical emitter. The first mirror surface is adapted to reflect and collimate light emission from the optical emitter. The precipitation sensor also includes an optical receiver and a second mirror surface in optical communication with the optical receiver. The second mirror surface is adapted to focus collimated light upon the optical receiver. The precipitation sensor further includes an intermediate reflector in optical communication with the first mirror surface and with the second mirror surface.

Description:
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/219,170, filed Jul. 19, 2000 and entitled OPTICAL PRECIPITATION SENSOR. The subject matter of this application is incorporated herein by this reference. 

   The U.S. Pat. No. 4,798,956 to Hochstein employed two methods toward overcoming the ambient light problem. For the first method, the receiver was placed at the bottom of a black tube to limit the number of directions from which ambient light could successfully reach the receiver. The use of infrared emitters was central to the second method employed. The &#39;956 patent stated that infrared emitters was used to compensate for ambient light. It indicated that commercially available infrared eminers emitted peak energy at 940 nm, in contrast to solar radiant energy peaking at approximately 500 mn. A filter was then placed in the tube between the opening of the tube and the receiver which passed the infrared light but rejected light of wavelengths shorter than infrared, including the peak solar wavelength of 500 nm. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to precipitation sensors associated with monitoring the accumulation of precipitation upon window glass. More particularly, this invention relates to optical precipitation sensors used in automotive applications. Specifically, this invention relates to the optics used in automotive optical precipitation sensors and a method for their use. 
   2. Description of the Prior Art 
   It is desirable to free the driver, operating an automobile, from the distractions of manually performing certain functions associated with the operation of the automobile. Comfort and safety can be both served by automating these functions. Operation of the wipers for the windshield or other window glass of an automobile, is a function that has been automated. 
   Automating the operation of these wipers requires sensing the presence of water, or precipitation, upon the outer surfaces of the window glass. When water is sensed, a signal is generated, electronic circuitry processes the signal, and the wipers are automatically deployed to clear the water from the window glass surface. Several approaches have been taken toward this sensing of water on window glass. These have included sensing a change in conductivity or capacitance, at a sampling point upon the outer surface, when moisture is present. These have included acoustic effects produced by raindrops hitting the surface of the automobile (e.g. rain landing upon the window glass or some other portion of the vehicle). These approaches have also included various optical techniques. 
   Optical sensors operate on the principle that a light beam is diffused or deflected from its normal path by the presence of water on the outer surface of the window. The systems that use optical sensors have the distinct advantage that they are sensing the same or similar phenomenon, which gives rise to the need for wiper operation, that being the disruption of the light transmissibility of the window glass caused by water residing on the outer surface. 
   Generally, a beam of light, in the infrared or near infrared ranges, is emitted into the window glass, from inside of the automobile, and at an angle giving rise to total reflection at the outer surface. A photoelectric device, such as a photodiode or a phototransistor, then receives the reflected light and produces a representative electrical signal. The light received at the photoelectric device has certain characteristics when the outer surface is dry. The characteristics are altered when water is present on the outer surface, at the point where the light beam comes into contact with the outer surface. Since water has a refractive index close to that of glass, its presence causes a substantial portion of the light, which would otherwise be reflected to the receiver, to dissipate. This change in characteristics results in commensurate change in the electrical signal produced by the photoelectric device. The signal is processed by electronic circuitry to control the operation of the wipers. 
   A recent approach disclosed in U.S. Pat. No. 5,661,303 to Teder, for producing an optical precipitation sensor, includes the use of emission lenses to collimate infrared light emitted from multiple Light Emitting Diodes (LED) and to direct the light upon the outer surface of the window glass at angles giving rise to total reflection. Receiption lenses are then used to direct and focus the reflected emitted light upon receivers. 
   Another recent approach is disclosed in Czech Republic Patent numbered CZ 285,291 B6, to Lan et al., uses a rotational parabolic mirror to collimate and direct near infrared light from multiple LED&#39;s upon the outer surface at an angle giving rise to total reflection. The reflected emitted light is then directed and focused upon a receiver by another rotational parabolic mirror. 
   An issue that arises in connection with the use of optical sensors, for precipitation detection, is desensitization of the photoelectric device of the receiver, by ambient light. Bright ambient light, such as sunlight, impinging upon the photoelectric device of the receiver, causes the device to become relatively insensitive to the emitted light transmitted to the receiver. If enough ambient light is impinging upon the receiver, the signal produced by the receiver may not be adequately different, in response to the presence of water on the outer surface, to be useable by the electronics to reliably control the wipers. 
   The approach using lenses, of the &#39;303 patent, apparently includes opaque members proximate and lateral to the optical axes of the reception lenses to block a portion of the ambient light reaching the receivers. The &#39;291 patent does not discuss nor depict any means for blocking ambient light from reaching the receiver. 
   The U.S. Pat. No. 4,798,956 to Hochstein employed two methods toward overcoming the ambient light problem. For the first method, the receiver was placed at the bottom of a black tube to limit the number of directions from which ambient light could successfully reach the receiver. The use of infrared emitters was central to the second method employed. The &#39;956 patent stated that infrared was used to compensate for ambient light. It indicated that commercially available infrared emitters emitted peak energy at 940 nm, in contrast to solar radiant energy peaking at approximately 500 nm. A filter was then placed in the tube between the opening of the tube and the receiver which passed the infrared light but rejected light of wavelengths shorter than infrared, including the peak solar wavelength of 500 nm. 
   Apparently, none of the approaches disclosed adequately protect the receiver from ambient light to ensure proper sensing of water on an outer surface of a window glass, in all light conditions expected to be encountered by a precipitation sensor. 
   Additionally, the advent of solar or thermal glass, for automotive applications, creates new challenges for the optical precipitation sensor designer. Solar glass includes additives to filter infrared and near infrared light from passing through the glass. Such glass protects the interior of the automobile from heating and other deleterious effects of this wavelength of light. However, it also substantially inhibits the infrared light of the emitter from reaching the receiver. It has been found that at least some infrared optical precipitation sensors are unusable in conjunction with such glass. The problem of ambient light rejection, evident in prior art designs, is exacerbated when the use of infrared emitters is no longer a viable option. 
   Accordingly, there remains the need for an optical precipitation sensor exhibiting improved ambient light rejection particularly when used in conjunction with solar or thermal glass. 
   SUMMARY OF THE INVENTION 
   The present invention has as an object the provision of an optical precipitation sensor with improved ambient light rejection. 
   The present invention has the further object of allowing improved operation of an optical precipitation sensor in the least favorable light conditions expected to be encountered by an automotive precipitation sensor. 
   The present invention has the further object of allowing the effective use of an optical precipitation sensor in conjunction with solar or thermal automotive glass. 
   To achieve the foregoing and other objects in accordance with the purposes of the present invention, as embodied and broadly described herein, an optical precipitation sensor and method is disclosed herein. The invention is a precipitation sensor adapted to detect water upon an automotive glass and a method for its use. The precipitation sensor includes an optical emitter and a first mirror surface in optical communication with the optical emitter. The first mirror surface is adapted to reflect and collimate light emission from the optical emitter. The precipitation sensor also includes an optical receiver and a second mirror surface in optical communication with the optical receiver. The second mirror surface is adapted to focus collimated light upon the optical receiver. The precipitation sensor further includes an intermediate reflector in optical communication with the first mirror surface and with the second mirror surface. 

   
     BRIEF DESCRIPTION OF THE INVENTION 
     The accompanying drawings, which are incorporated in and form part of the specification in which like numerals designate like parts, illustrate preferred embodiments of the present invention and together with the description, serve to explain the principles of the invention. In the drawings: 
       FIG. 1  is a fragmentary perspective depicting an optical precipitation sensor mounted upon a windshield of an automobile; 
       FIG. 2  is a transverse section of the optical precipitation sensor and windshield, taken along line  2 — 2  of  FIG. 1 ; 
       FIG. 3  is a perspective of the glass molding. 
       FIG. 4  is a perspective of the glass molding. 
       FIG. 5  is a fragmentary section showing the field regulator in greater detail. 
       FIG. 6  is a graph showing the effect of the field regulator. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , optical precipitation sensor  10  of the instant invention is shown in relation to automobile  24 , including an opening defined by, hood  12 , side posts  14 , roof  16 , within which is located windshield  18 . Windshield wipers  20  are shown in their rest position with the arcs of their sweep of operation shown by arcs  22 . Optical precipitation sensor  10  is depicted in a preferred location within the reach of wipers  20  in operation. While mounting of optical precipitation sensor  10  is depicted upon windshield  18 , mounting upon any window glass where sensing of precipitation is desired is contemplated, including rear or side windows, sunroofs, or headlamps. 
   Referring to  FIGS. 2 ,  3 , and  4  optical precipitation sensor  10  includes housing  28 , which contains circuit board  30  and glass molding  38 . Circuit board  30  serves as the mounting substrate for all of the electronic circuitry including electronic components  32 , emitters  34  and receiver  36 . These electronic components  32  process the signals related to emitters  34  and receiver  36  and provide an electrical interface to automobile  24  in a conventional manner known to those of ordinary skill in the art and will not be described herein. 
   In this preferred embodiment, molding glass  38  is a single piece of glass and includes all optics of optical precipitation sensor  10 , other than emitters  34  and receiver  36 , and includes emitter optical notches  40 , receiver optical notch  42 , intermediate reflector  44 , first mirror surfaces  52 , and second mirror surface  54 . Locator posts  66  also form part of glass molding  38 , seen in  FIG. 3 , and mate with holes (not depicted) on circuit board  30  to ensure consistent alignment of emitters  34  with emitter optical notches  40  and of receiver  36  with receiver optical notch  42 . 
   As will be discussed below, the configuration of the instant invention, using second mirror surface  54  to shield receiver  36 , very substantially reduces access of ambient light to receiver  36 . However, molding glass  38  preferably includes coloring agents to filter out ambient light  64  at wave lengths other than emitted by emitter  34 , which further excludes ambient light  64  from accessing receiver  36 . The glass composition used in application to clear and tinted windshields  18  is more preferably formulated to transmit the same wavelength of light as is emitted by emitters  34 . Such filtering properties of the glass are achieved by adding the following colorants into the glass:
         CoO (in the range from 0.01 wt. % to 1.0 wt. %)   CeO2 (in the range from 0.0 wt. % to 6.0 wt. %)   TiO2 (in the range from 0.0 wt. % to 1.0 wt. %)   NiO (in the range from 0.0 wt. % to 0.6 wt. %)       

   The CoO is the main functional component of the glass and the three other components improve the filtering function by suppressing the transmission in the visible blue range. The value 0.0 wt. % is used to express that the last three components can be omitted when the transmission in the blue part of the visible spectra can be accepted. The most preferable composition can be found in table 1. This composition results in molding glass  38  being dark blue. 
   
     
       
             
             
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Oxide 
               SiO2 
               CaO 
               K2O 
               Na2O 
               B2O3 
               Al2O3 
               Fe2O3 
               CoO 
               CeO2 
               TiO2 
             
             
                 
             
           
           
             
               Wt. % 
               61.42 
               1.6 
               13.89 
               8.19 
               1.33 
               0.97 
               0.01 
               0.37 
               4.26 
               8.00 
             
             
                 
             
           
        
       
     
   
   It is also contemplated that each of said components, emitter optical notches  40 , receiver optical notch  42 , intermediate reflector  44 , first mirror surfaces  52 , second mirror surface  54 , and locator posts  66 , could be constructed of multiple elements fastened together mechanically or by adhesion. Housing  28  snap fits over circuit board  30  and molding glass  38  to secure the assembly and to maintain the mating relationship of locator posts  66  with the holes on circuit board  30 . Optical precipitation sensor  10  is affixed to windshield  18  at mounting face  68  of molding glass  38  via transparent plastic adhesive tape  56 . Mounting face  68  has a slightly convex shape to largely conform to the curvature of windshield  18 . In this preferred embodiment it is assumed that windshield  18  has a deflection with a radius of approximately 3280 mm and a thickness of 4.7±0.2 mm. 
   Emitters  34  of this preferred embodiment are GaAs LED&#39;s manufactured by OSRAM and designated “SFM 420 TOPLED”. It has the relative spectral emission described in table 2. Its radiation characteristics are that of a cosine emitter and has an active chip area: A=L×W=0.3 mm×0.3 mm=0.09 mm  2 . LED&#39;s of comparable characteristics can also be used. 
   
     
       
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Wavelength 
                 
                 
                 
                 
                 
                 
                 
                 
             
             
               (nm) 
               900 
               920 
               940 
               950 
               960 
               980 
               1000 
               1020 
             
             
                 
             
           
           
             
               I 
               0.04 
               0.18 
               0.87 
               1.0 
               0.90 
               0.55 
               0.20 
               0.06 
             
             
                 
             
           
        
       
     
   
   Emitter optical notches  40  are spherical depressions into molding glass  38  and located over emitters  34  such that emitted light  58  will primarily approach normal to the surface of emitted optical notches  40  for substantially all directions emitted light  58  departs from emitters  34 . In this manner and under ideal conditions, emitted light  58  is not refracted upon passing through the boundary of emitter optical notches  40  and proceeds on a straight path to first mirror surface  52 . 
   First mirror surfaces  52  are parabolic surfaces upon molding glass  38  each with a focal point of 4.7 mm, an axis “a” of 60°, and metalized with a metallic film of aluminum. It is contemplated that other metals can be substituted for aluminum such as gold. Further, the coating does not need to be applied by metalization techniques or even be metal. It is contemplated that reflective plastic or other coatings, which are opaque can be used. The portion of the metallic film closest to mounting base  68  is the leading edge. As can be seen in  FIG. 4 , this preferred embodiment employees three emitter optical notches  40  and three first mirror surfaces  52  over three emitters  34 . This is done to increase the amount of emitted light  58  that can reach receiver  36 . This provides the benefit of improving the signal to noise ratio of emitted light  58  to any stray light that might reach receiver  36  in spite of the shielding techniques that form part of the instant invention. Further, the number of emitters  34 , and associated optical notches  40  and first mirror surfaces  52  can be selected to produce field intensities that optimizes operation of receiver  36 , which is dependent upon system geometry, photoelectric device properties, and the sensor production tolerances. The configuration of first mirror surface  52  results in emitted light  58  being reflected and collimated. 
   Emitted light  58  proceeds on to first reflective region  46  of intermediate reflector  44 . First reflective region  46  deviates from a straight line drawn between emitter optical notch  40  and receiver optical notch  42  by angle “c ”. Angle “c ” is set at 7.50°Intermediate reflector  44  can be metalized or not, depending on application. Not metalizing intermediate reflector  44  provides the benefit of additional ambient light  64  rejection by allowing ambient light  64  that approaches intermediate reflector  44  at less than total reflection angles pass through intermediate reflector  44 . First reflective region  46  and second reflective region  48  each have mean reflective points defined as the average distance of the reflective area of each from mounting face  68 . 
   This embodiment includes field regulators  50 , which take the form of cones protruding from the surface of first reflective region  46  with an apex angle of 90°. Field regulators  50  have the effect of normalizing or otherwise controlling the intensity of emitted light  58  across the width of emitted light  58 . As illustrated in  FIG. 5 , a substantial portion of emitted light  58  that falls upon a field regulator  50  is not reflected leaving only a small portion, suppressed light  59 , to continue on its working optical path toward receiver  36 , with the remainder of emitted light  58  passing through field regulator  50 . Field regulators  50  are placed at the points where it is desired to limit the intensity of emitted light  58 . 
     FIG. 6  is a plot of the field density of emitted light  58  in relation to location of emitters  34  and without the presence of water droplet  60 . The left plot demonstrates the field density when no field regulators  50  are used. The right plot demonstrates the effects of field regulators  50  placed at locations on first reflective region  46  corresponding to the greatest field densities demonstrated in the left plot. As can be seen, the effect of field regulators  50  is to normalize the field densities across emitters  34 . This technique provides the opportunity to normalize the effects of the presence of water droplet  60  upon windshield outer surface  26 , within the later bounds of where emitted light  58  meets windshield outer surface  26 , or the sensed area. Thus, if water droplet  60  lands at various locations upon windshield outer surface  26  and within the sensed area, the level of change of intensity of emitted light  58  caused by the variations of location is normalized. This allows more consistent variation of emitted light  58  intensity regardless of water drop location within the sensed area. 
   This preferred embodiment depicted incorporates field regulators  50  upon first reflective region  46 . However, it is expected that comparable results can be obtained through the placement of field regulators  50  upon second reflective region  48 , or upon a combination of first reflective region  46  and second reflective region  48 . Further, it has been determined that for certain applications, satisfactory performance can be achieved with an optical precipitation sensor  10  of the instant invention without the use of field regulators  50 . 
   After reflecting from first reflective region  46 , emitted light  58  proceeds through transparent plastic tape  56  and into windshield  18 . Transparent plastic adhesive tape  56  is chosen to have a refractive index very close to that of the glass of windshield  18  to avoid losses caused by reflective and refractive effects. Further, for this embodiment, transparent plastic adhesive tape  56  has a thickness of 1.5±0.2 mm. Emitted light  58  proceeds to the boundary of air and windshield outer surface  26  and at angle that gives rise to total reflection. 
   The formula for the calculation of the total reflection is: 
                        
 
where
         α=angle of the light beam going from glass to air   β=angle of the beam after crossing the boundary between glass and air   n 1 =refractive index of the glass (n=1.515)   n 2 =refractive index of air (n=1)   1=glass   2=air
 
The total reflection condition is achieved when the angle βis 90°.
 
         sin   ⁢           ⁢   α     =       sin   ⁢           ⁢   β   *       n   2       n   1         =         sin   ⁡     (     90   ⁢   °     )       *     1   1.515       =   0.66           
   α=41.30°
 
Accordingly, the approach angle “α” must be 41.30° or more from the normal of windshield outer surface  26 . An angle “α” was selected to be 45°.
       

   If windshield outer surface  26  is dry, then emitted light  58  reflects completely according to the principle of total reflection described above. Emitted light  58  then passes through transparent plastic adhesive tape  56  to second reflective region  48  of intermediate reflector  44  and then reflects to second mirror surface  54 . Second mirror surface  54  is a parabolic surface upon molding glass  38  with a focal point of 6 mm, an axis “b ” of 45°, and metalized with aluminum. Second mirror surface  54  focuses emitted light  58  through receiver optical notch  42  and on to receiver  36 . Receiver optical notch  42  is a spherical depression into molding glass  38  and located over receiver  36  such that emitted light  58  will primarily approach normal to the surface of receiver optical notch  42  for substantially all directions emitted light  58  passes from second mirror surface  54  to receiver  36 . In this manner and under ideal conditions, emitted light  58  is not refracted upon passing through the boundary of receiver optical notch  42  and proceeds on a straight path to receiver  36 . 
   Receiver  36  of this preferred embodiment is a Silicon NPN Phototransistor manufactured by VISHAY TELEFUNKEN and designated “TEMT4700”. It has the relative spectral emission described in table 3. Its relative directional sensitivity follows a cosine characteristic and has an active chip area A=L×W=0.74 mm×0.74 mm=0.55 mm 2. . Phototransistors of comparable characteristics can also be used. 
                                                   TABLE 3               Wavelength                                       (nm)   900   920   940   950   960   980   1000   1020                   I   0.94   0.87   0.77   0.71   0.68   0.54   0.43   0.34                    
Relative functional spectral window of a the diode/transistor pair comprising emitter  34  and receiver  36  is described in table 4.
 
   
     
       
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 4 
             
             
                 
             
             
               Wavelength 
                 
                 
                 
                 
                 
                 
                 
                 
             
             
               (nm) 
               900 
               920 
               940 
               950 
               960 
               980 
               1000 
               1020 
             
             
                 
             
           
           
             
               I 
               0.054 
               0.221 
               0.944 
               1.0 
               0.862 
               0.418 
               0.121 
               0.028 
             
             
                 
             
           
        
       
     
   
   Referring to  FIGS. 3 and 4 , it can be seen that only one second mirror surface  54 , receiver optical notch  42 , and receiver  36  are used in this preferred embodiment. A plurality of these can be employed to increase the sensed area upon windshield outer surface  26 . It is believed that any benefit to be derived is outweighed by the additional size and complexity added to optical precipitation sensor  10 . 
   The process described above, where no water droplet  60  is present, creates a predictable field intensity upon receiver  36  and resulting signal from receiver  36 , to the limits of the stability of the electronic devices, including emitters  34  and receiver  36 . When water droplet  60  is present, as depicted in  FIG. 2 , the close relationship of the refractive index of glass and water, optically softens the boundary at windshield outer surface  26  and disturbs the total reflection condition. This, in-turn, causes a substantial portion of emitted light  58  to pass through the boundary as dissipated light  62 . This alters the field density at receiver  36  and thus the signal produced by receiver  36  in a manner processable by the electronic components  32  to produce a signal to operate wipers  20 . 
   As has been referenced above, an issue that arises in connection with the use of optical sensors, for precipitation detection, is desensitization of receiver  36 , by ambient light  64 . Bright ambient light  64 , such as sunlight impinging upon receiver  36 , causes the photoelectric device to become relatively insensitive to emitted light  58 . If enough ambient light impinges upon receiver  36 , the signal produced by receiver  36  is not adequately different in response to the presence of water droplet  60  to be useable by electronic components  32  to reliably control wipers  20 . 
   As has been described, this preferred embodiment uses a combination of choice of wavelength for emitted light  58  and filtering within glass molding  36  to reject a portion of ambient light  64 . However, this alone is inadequate to insure proper operation of optical precipitation sensor  10 . More protection from ambient light  64  is needed. The combination of the opaque nature of second mirror surface  54  caused by the aluminum metalization and its location facilitated by the presence of intermediate reflector  44  effectively rejects a substantial portion of ambient light  64  and thus shields receiver  36 . 
   As can be seen in  FIG. 2 , the aluminum metalization can be continued to a leading edge at a point where emitted light  58  re-enters molding glass  38  after reflecting off of windshield outer surface  26 . Intermediate reflector  44  allows such placement. This results in second mirror surface  54  being intermediate to most sources of ambient light  64  except those sources which produce paths, through the sensed area, that are parallel to emitted light  58  within windshield  18 . Further, that ambient light  64  with approach angles greater to windshield  18  than that which produce the above mentioned parallel paths do not have direct paths, via the combination of intermediate reflector  44  and second mirror surface  54 , to receiver  36 . 
   This optical geometry is so successful at rejecting ambient light  64  that it has provided the opportunity to use optical precipitation sensor  10  in applications involving so-called solar or thermal automotive glass. Such glass contains additives that absorb light in the infrared or near infrared range of wavelengths. When optical precipitation sensor  10 , of the previously described embodiment (or any optical precipitation sensor that uses emitters that emit light in the infrared or near infrared range), is applied to windshield  18  made of such glass, this absorption reduces the field density reaching receiver  36  to an unusable level. 
   This leads to a preferred embodiment where glass molding  38  has no colorants, to filter light, added thereto. Further, the LED of emitter  34  is selected that emits light at wavelengths in the white light range that is not significantly absorbed by solar or thermal glass. In other prior art designs this would not be possible because the receiver would be overly exposed to ambient light. 
   Emitter  34  of this preferred embodiment is an InGaAlP LED manufactured by OSRAM and designated “LA E 6 75 Power TOPLED”. It has the relative spectral emission described in table 5. Other LED&#39;s that have comparable characteristics may also be used. 
   
     
       
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 5 
             
             
                 
             
             
               Wavelength 
                 
                 
                 
                 
                 
                 
                 
                 
             
             
               (nm) 
               590 
               600 
               610 
               620 
               630 
               640 
               650 
               660 
             
             
                 
             
           
           
             
               I 
               0.04 
               0.11 
               0.33 
               1.0 
               0.42 
               0.06 
               0.01 
               0.00 
             
             
                 
             
           
        
       
     
   
   Receiver  36  of this preferred embodiment is also the Silicon NPN Phototransistor manufactured by VISHAY TELEFUNKEN and designated “TEMT4700”, of the previous embodiment. Table 6 describes the relative spectral emissions pertinent to the LED used for emitter  34 , of this embodiment. 
   
     
       
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 6 
             
             
                 
             
             
               Wavelength 
                 
                 
                 
                 
                 
                 
                 
                 
             
             
               (nm) 
               600 
               620 
               640 
               660 
               680 
               700 
               720 
               740 
             
             
                 
             
           
           
             
               I 
               0.43 
               0.47 
               0.56 
               0.60 
               0.62 
               0.65 
               0.69 
               0.78 
             
             
                 
             
           
        
       
     
   
   In all other respects, this embodiment tracks the embodiment previously discussed in detail. 
   The foregoing description and illustrative embodiments of the present invention have been shown on the drawings and described in detail in varying modifications and alternative embodiments. It should be understood, however, that the foregoing description of the invention is exemplary only, and that the scope of the invention is to be limited only to the claims as interpreted in view of the prior art. Moreover, the invention illustratively disclosed herein suitably may be practiced in the absence of any element which is not specifically disclosed herein.