Patent Publication Number: US-2018029563-A1

Title: Windshield monitoring system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a windshield monitoring system, particularly, but not exclusively, for a vehicle. Aspects of the invention relate to a windshield monitoring system, a windshield clearing system, a vehicle including the same, a method of monitoring a windshield, and a method of clearing a windshield. 
     BACKGROUND 
     A vehicle such as a car or the like includes a frame supporting several windshields. A windshield is a term of art covering a front windscreen, or a rear windscreen, or a side window, of which there are several. A windshield may comprise a laminate construction or it may be a singular piece of glass. A windshield serves several functions including segregating interior and exterior environments of the vehicle whilst providing visible communication between the two. 
     Under ideal conditions, interior and exterior surfaces of the windshield are characterised by a glass to air interface. In practice, various sources of contamination can build up on either surface affecting the visibility for the vehicle occupants. 
     Windshields are prone to having their surfaces contaminated which can result in a degradation of visibility. Water based contaminants are most frequent and result from differences in humidity and temperature between interior and exterior environments of the vehicle. For instance, on a very cold day, moisture from occupants&#39; respiration and/or wet clothing can condense on an interior surface of a windshield. Such condensation can be problematic from a visibility perspective when condensation covers large areas of a windshield. Another example of problematic windshield contamination is ice on an external surface of the windshield on a particularly cold day. 
     Typically, vehicles are provided with heating provisions which can be operated to clear such contamination. One such heating provision includes air blowers which can be manually oriented by a vehicle occupant towards the interior surface of the windshield. 
     Another heating provision includes a series of electroconductive parallel wires which heat up due to electrical resistance when an electrical current is passed across them, and which are embedded into the laminated glass. The wires heat the glass by an energy transfer from electrical energy to thermal energy. Such a system can be operated manually by an occupant pressing a push button on an instrument panel. Such manually operated heating elements are inherently inefficient as they require an occupant to monitor the windshield themselves, to actively detect the contamination, and to activate the heating elements accordingly. Typically, a driver would not activate the heating elements unless the source of contamination was affecting their visibility. Hence at the time of detection of any contamination, the area of contamination is likely to be large resulting in excessive power being required to clear the contamination. In addition, the user will inevitably always over compensate and leave the heating element on for much longer than is required. Accordingly, energy operating the heating element will be wasted unnecessarily. 
     Various attempts have been made to provide automatic contamination detection systems. 
     With reference to  FIGS. 1A and 1B , one known system includes a windshield  10  having a combined emitter/detector  12  mounted on a reverse (non-reflective) side of a rear-view mirror  14 . The emitter shines infrared light  18  onto an interior surface  16  of the windshield  10 . Since the windshield  10  is oriented at an acute angle of inclination relative to the rear-view mirror, any reflected light  20  from the emitter reflects away from the detector  12 . However, the presence of contamination  22  in the area of the windshield  10  where the light  18  is shone is scattered on reflection. The scattered light  20   a  is likely to impinge on the detector  12 . Any detected light is determined to be attributable to the presence of surface contamination  22 . 
     Such a system is not ideal since the scattered light  20   a  may not impinge on the detector  12  and in addition only contamination in the area of the windshield  10  where light is shone can be detected. Accordingly, such attempts to automatically heat the windshield to clear the contamination are not energy efficient since it is likely that large areas of contamination can develop before being detected. In addition, the entire windshield may be heated if a small amount of condensation develops in the region of the sensor. 
     Another known contamination detection system uses an infrared light detector within the vehicle interior in an attempt to detect any rain water on the exterior surface of the windshield by monitoring for reflection of the infrared light resulting from the light passing through the rain drops. Such systems are not very popular. Primarily, this is due to the fact that it is desirable to include an infrared reflective (IRR) layer as part of the laminated windshield to reflect away heat energy from the vehicle cabin. IRR glass has an internal conductive metallic layer which is reflective to infrared light. Accordingly, the infrared light for the rain sensor cannot pass the wind shield to detect the presence of rain water. In order to make the system usable, a window has to be cut in to the IRR layer by laser cutting. The rain detection system is thus limited to the size of the window cut in to the glass. This is a common trait with rain detection systems, which are generally limited to detecting rain in a small area of the windshield. Cutting the window in to the IRR layer removes any of the benefits associated with using IRR glass, though in practice the affected area is relatively small. In addition, such a laser cutting process is expensive. 
     A further known windshield contaminant detection system uses a dew point detection system for detecting dew formation on an interior surface of a windshield. Such dew point detection systems work by estimating dew formation by calculation based on humidity and temperature. Since these dew point detection systems are based on calculation rather than being detected directly, they are inherently inaccurate. As a result, any windshield clearing mechanisms, such as heating mechanisms, used to clear the windshield based on the detection of dew in this way may be activated in an untimely manner. Untimely activation of the clearing mechanisms wastes energy since the windshield may either be heated when no dew is present or be activated later than required allowing a relatively large area of dew to form, which would be harder to clear. 
     It is an aim of the present invention to further improve on the prior art. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention there is provided a windshield monitoring system comprising a windshield for a vehicle. The system may comprise a plurality of light emitters forming an array. Each emitter may be arranged to direct a beam of light into an edge of the windshield at an angle of incidence arranged to subject the beam of light to total internal reflection at a windshield to air interface to propagate the beam of light across the windshield to an opposing edge thereof. The angle of incidence may be arranged to refract the light beam away from the windshield at a windshield to contaminant interface The system may comprise a light detector arranged to detect that light from the beam of light has been refracted away from the windshield. The emitters may be separated substantially across a length of the windshield. 
     Contamination is defined as any impurities at a windshield surface resulting in an interface which could alter visibility through the windshield. Using total internal reflection and refraction of light beams provides for effective detection of any such contamination. Using an array of light emitters separated substantially along a length of the windshield provides a large area of coverage over the windshield. Accordingly, there is a higher likelihood of detecting the contamination early. 
     The light detector may be arranged to detect the presence of any refracted light directly. An alternative is to monitor the reflected light and determine the presence of refracted light by an intensity drop in the reflected light. Light measured directly is either present or not present. Detecting the presence of reflected light is thus more reliable. 
     The windshield may be a front windscreen, or a rear windscreen, or a side window. Front and rear windscreens are most likely to benefit from such a monitoring system since any contamination on those windscreens would more greatly affect a driver&#39;s vision or distract their attention. Side windows, especially front side windows can also affect a driver&#39;s vision. 
     The contamination may be selected from the list of moisture, ice, and liquid water. 
     The or each light emitter may comprise a Light Emitting Diode (LED). An LED is relatively cheap and simple compared to other light emitters and emit light over a narrow bandwidth which may be advantageous for detection purposes. 
     Each light emitter may comprise a pulsar to pulsate, strobe, or shutter the light beam according to a predefined pattern. Pulsating the light beam would provide for a level of noise filtering since any light detected which isn&#39;t pulsating according to the predefined pattern would not be attributable to the light emitter. 
     Each light beam may be pulsated at a unique predefined pattern or may be phase shifted relative to the other light pulse patterns. Each light beam being unique in these ways would allow the emitter and/or the location within the windscreen of the light beam it emits to be identified. 
     The array may substantially span along a side edge of the windshield. A side edge as opposed to a top or bottom edge would result in fewer light emitters being required to provide equivalent windshield coverage due to the aspect ratio of a windshield. 
     The light source may comprise two arrays of light emitters, the arrays being mutually orthogonal. Two mutually orthogonal arrays provides for a 2 dimensional co-ordination system which is particularly advantageous when using detectors which have difficulty in distinguishing direction of the source of the light. The 2 dimensional array thus allows for improved fidelity of locating the contamination. 
     The light detector may comprise a camera arranged to monitor the windshield. The camera may be configured to be sensitive only to the light wavelength, or band of wavelengths produced by the light emitters. Cameras are reliable and easy to install. In addition, location of the contamination can be detected to a relatively high degree of accuracy by associating the location of refracted light with a particular pixel of the camera. 
     The camera may be arranged to monitor the interior face of the windshield. Any refracted light resulting from interior contamination may thus be detected directly. 
     The light detector may comprise a photodiode. Photodiodes are cheap and reliable and would be particularly advantageous when used with the 2 dimensional array of light emitters. 
     The system may comprise an association module arranged to determine a coordinate position of the contamination by associating the light with one or more emitters from each array. 
     The system may comprise a noise filter to distinguish light emitted from the light emitter from other light. The noise filter thus reduces the risk of false light detections. 
     The noise filter may comprise a lock-in amplifier. A lock-in amplifier is reliable and easy to integrate. 
     The angle of incidence of the light beam may be between about 43° and about 65°. The critical angle of a glass to air interface may be about 43° and the critical angle of a glass to water interface may be about 65°. The difference in critical angle is due to the difference in refractive index between air and water. Therefore, the system would detect most water based contamination over this range of incidence angles. 
     According to a further aspect of the present invention there is provided a windshield clearing system comprising; the aforementioned windshield monitoring system; a windshield clearing element on the windshield; and a control module arranged to control the windshield clearing element to clear the windshield in response to the detection of refracted light. 
     The windshield clearing element may comprise a heating mechanism. The heating mechanism may comprise a plurality of heating elements, wherein the control module may be arranged to heat a heating element at a location corresponding to the detected contamination. 
     Each heating element may comprise a resistance heating element. A resistance heating element converts electrical energy to heat energy and are relatively efficient compared to other heating types. 
     The resistance heating element may comprise a metallic layer. A metallic layer is easy to provide on a windshield for instance by deposition. 
     The system may comprise a termination module to terminate heating of the heating element in response to the windshield monitoring system ceasing to detect refracted light or after a predetermined time period. 
     Terminating heating in response to no refracted light being detected provides for an increase of efficiency in terms of power required to power the system. 
     According to a further aspect of the present invention there is provided a vehicle comprising the aforementioned windshield clearing system. 
     According to a further aspect of the present invention there is provided a method of monitoring a windshield. The method may comprise emitting a plurality of light beams into an edge of the windshield at an incidence angle arranged to subject the light beams to total internal reflection at a windshield to air interface to propagate across the windshield. The angle of incidence may be arranged to refract the light beam away from the windshield at a windshield to contaminant interface. The plurality of light beams may be separated so as to substantially span a length of the windshield. The method may comprise detecting the presence of light refracted away from the windshield. 
     The method may comprise pulsating and/or strobing of the or each light beam according to a pre-defined pattern. 
     The method may comprise pulsating, strobing or shuttering each light beam according to a unique pattern. 
     The method may comprise emitting the plurality of light beams substantially along adjacent mutually orthogonal edges of the windshield. 
     The method may comprise filtering out noise detected by the light detector. 
     The angle of incidence of the light beam may be between about 43° and about 65°. 
     The method may comprise associating the refracted light with one of the light emitters. 
     According to a further aspect of the present invention there is provided a method of clearing a windshield comprising; detecting surface contamination using the aforementioned method of monitoring a windshield; and heating the windshield in response to detecting any contamination. 
     The method may comprise dividing the windshield into a plurality of zones and heating the zone corresponding to the location of detected contamination. 
     The method may comprise terminating the heating of the area in response to ceasing to detect refracted light or after a predetermined time period. 
     According to an aspect of the invention there is provided a windshield monitoring system. The system may comprise a laminated windshield having an infrared reflective (IRR) layer. The system may comprise an infrared light source arranged to direct a beam of light into the windshield at an incidence angle arranged to subject the beam of light to total internal reflection at a windshield to air interface to propagate the beam of light across the windshield towards an opposing edge thereof. The angle of incidence may be arranged to refract light away from the windshield at a windshield to contaminant interface, wherein the infrared light is arranged to reflect off the infrared reflective layer. The system may comprise an infrared light detector arranged to detect that light has been refracted away from the windshield. 
     The windshield monitoring system according to embodiments may allow for contamination to be detected before it obscures a relatively large area of the windshield which saves energy in removing the contamination by heating the glass. It is counterintuitive to use infrared light for detecting contamination of a windshield surface since the windshield has the infrared reflective layer not allowing the infrared light to pass. However, in this case, the infrared reflective layer is used to benefit the propagation of the infrared light beam across the windshield. 
     The light detector may be arranged to monitor intensity of the internally reflected light to detect refracted light by a drop in light intensity. 
     The light source may be arranged to direct the beam of light on an interior side of the infrared reflective layer, and/or the light source may be arranged to direct the beam of light on an exterior side of the infrared reflective layer. 
     A beam of light directed on the interior side of the infrared reflective layer means the contamination on the interior surface of the wind shield can be detected. In a similar way, a light beam directed on the exterior side of the infrared reflective layer will detect surface contamination on the exterior side of the wind shield. 
     The light detector may comprise a photodiode. 
     The light source may comprise a plurality of light emitters forming an array substantially spanning along an edge of the windshield and each emitter may be arranged to direct a beam of infrared light across the windshield towards an opposing edge thereof. 
     The array of light emitters substantially spanning along an edge of the windshield provides for a wide area of coverage for the detection system. 
     The windshield monitoring system may comprise a locating module arranged to locate the contamination based on which light detector has detected the contamination. 
     The locating module provides for more accurate contamination detection which in turn allows for a more energy efficient system since the area affected by the contamination can be targeted for clearing before the contamination spreads. 
     The light source may comprise two mutually orthogonal arrays of emitters wherein each array may substantially span along adjacent edges of the windshield and may be arranged to direct light to a respective opposing edge thereof. 
     The two mutually orthogonal arrays provides for two dimensional positioning of any detected contamination such that the contamination can be assigned a coordinate on the windshield. 
     The light detector may comprise a plurality of light detectors each being associated with a different light emitter. 
     The system may comprise a noise filter to distinguish light emitted from the light source from other detected light. 
     Reducing the detection of other detected light reduces the risk of erroneous readings when detecting the presence of refracted light since light can be detected from sources other than the emitters of the windshield monitoring system. 
     The noise filter may comprise a lock-in amplifier. 
     A lock-in amplifier is a reliable component which lends itself particularly well to this application. 
     The system may comprise a pulsar arranged to pulsate the light according to a predefined pattern. 
     Pulsating the light makes it easier to detect the source of the detected light since light coming from other sources would not be pulsated in the same way. 
     Each light emitter may be pulsed, strobed, or shuttered according to a unique pattern or may be phase shifted relative to the other light emitters. 
     A unique pattern per light emitter or phase shifted light relative to the other emitters is easier to identify the source of the light which can further aid in locating the contamination on the wind shield. 
     The windshield may be a front windscreen, a rear windscreen, or a side window. 
     The front and rear windscreens would benefit particularly well from this windshield monitoring system since it is these windshields which have the biggest impact on driver visibility. Side windows, particularly front side windows also have an effect on driver visibility. 
     The contamination may be selected from the list of moisture, ice, liquid water. 
     These contaminants are particularly suited to this application since they are most easily treated with other ancillary systems of the vehicle such as heaters and/or air blowers. Treatment is more difficult for other contaminants, for example, dirt. 
     The angle of incidence of the light beam may be between about 43° and about 65°. 
     The critical angle for a glass to air interface is about 43° and the critical angle for a glass to water interface is about 65° due to the difference in refractive index of air and water. Accordingly, an angle of incidence of the light in this range would be reliable enough to refract away light only at a glass to water interface but not at a glass to air interface. 
     According to a further aspect of the present invention there is provided a windshield clearing system comprising the aforementioned windshield monitoring system; a windshield clearing element; and a control module arranged to control the windshield clearing element to clear the contamination in response to detecting refracted light. 
     The windshield clearing element may comprise a heating mechanism. The windshield clearing element may comprise a windshield wiper system. 
     The heating mechanism may be divided into a plurality of heating elements and the control module may be arranged to control the heating elements to heat the windshield at a location corresponding to the detected contamination. 
     Dividing the heating element into a plurality of heating elements, or zones, and only heating the heating element corresponding to the location of detected contamination reduces the energy consumption by the vehicle since no energy is wasted clearing uncontaminated areas. 
     The heating element may comprise a resistance heating element. 
     The resistance heating element may comprise a metallic layer. 
     A metallic layer is easy to deposit on to a windshield at relatively low cost. 
     The windshield clearing system may comprise a blower control unit for electrical communication with an air blower arranged to selectively direct air towards the windshield. 
     Alternatively, the windshield clearing system may comprise a wiper system. 
     The windshield wiper system may be arranged to cycle selectively over an area of the windshield where exterior contamination has been detected. 
     The windshield clearing system may comprise a termination module to stop operation of the clearing system in response to contamination no longer being detected. Terminating clearing leads to further energy efficiencies since attempting to clear an already clear windshield is just wasted energy. This is particularly important for electric vehicles and hybrid electric vehicles. 
     Alternatively or in addition, the termination module may be arranged to stop operation of the clearing system after a predetermined time period. This is important for false triggers resulting from, for instance, windshield cracks since the clearing system would not be able to ‘clear’ the crack and so allowing the system to continue attempting to clear the crack would waste energy. 
     The control module may be arranged to emit a warning or indication in response to stopping operation of the clearing system after more than a predefined number of times. The control module may be arranged to emit a warning or indication after a predefined number of times of stopping operation of the clearing system, in the case of stopping after a predetermined time period. The indication can be used off-line, for instance during manufacture, or on the vehicle as a dashboard indication or a signal available during a maintenance inspection, to indicate that the windshield requires inspection since such reoccurrences may be due to the presence of windshield cracks, which cracks are not clearable by the clearing system. 
     According to a further aspect of the present invention there is provided a vehicle comprising the aforementioned windshield clearing system. 
     According to a further aspect of the present invention there is provided a method of detecting surface contamination on a windshield having an infrared reflective (IRR) layer. The method may comprise emitting an infrared light beam into an edge of the windshield at an angle of incidence arranged to subject the light beam to total internal reflection at a windshield to air interface to propagate the light beam towards an opposing edge of the windshield. The angle of incidence may be arranged to refract the beam of light away from the windshield at a windshield to contaminant interface. The method may comprise reflecting the light off the IRR layer. The method may comprise detecting surface contamination by monitoring light from the beam of light being refracted away from the windshield. 
     The method may comprise monitoring light reflected internally within the windshield. The method may comprise determining the presence of surface contamination by a drop in light intensity due to light being refracted away from the windshield. 
     The method may comprise filtering out noise from other detected light not associated with the emitted light. 
     The method may comprise pulsating the light beam according to a predefined pattern. 
     The method may comprise emitting a plurality of light beams substantially spanning along an edge of the windshield. The method may comprise detecting any refracted light from each of the light beams. The method may comprise associating the location of the refracted light with one of the light beams to determine the location of the contamination. 
     The method may comprise;
         emitting the plurality of light beams as two arrays, each array substantially spanning along adjacent edges of the windshield. These adjacent edges may be arranged orthogonally to one another.       

     The method may comprise emitting an infrared light beam across an interior side of the IRR layer of the windshield. The method may comprise emitting an infrared light beam across an exterior side of the IRR layer of the windshield. 
     The angle of incidence of the light beam may be between about 43° and about 65°. 
     According to a further aspect of the present invention there is provided a method of clearing a windshield having an infrared reflective (IRR) layer comprising;
         detecting the presence of surface contamination using the aforementioned method; and   using a windshield clearing system, clearing the windshield in response to detecting refracted light. Clearing of the windshield may be achieved by heating the windshield in response to detecting refracted light.       

     Heating the windshield may include dividing the windshield into a plurality of zones and heating a zone corresponding to an area of the windshield where contamination has been detected. 
     According to a further aspect of the present invention there is provided a windshield monitoring system, a windshield clearing system, a vehicle, a method of detecting surface contamination on a windshield, and a method of clearing a windshield, substantially as described above. 
     Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1A  shows a windshield monitoring system according to the prior art; 
         FIG. 1B  shows a similar view to  FIG. 1A ; 
         FIG. 2  shows a windshield monitoring system according to a first embodiment of the present invention; 
         FIG. 3  shows a block diagram of the windshield monitoring system of  FIG. 2 ; 
         FIG. 4  shows a schematic of a windshield clearing system including the windshield monitoring system of  FIG. 2 ; 
         FIG. 5  shows a section view of the windshield monitoring system of  FIG. 2  in operation; 
         FIG. 6  shows a similar view to  FIG. 2  of a windshield monitoring system according to a second embodiment of the present invention; 
         FIG. 7  shows a similar view to  FIG. 3  of a second embodiment of the present invention; 
         FIG. 8  shows part of a windshield monitoring system according to a third embodiment of the present invention; 
         FIG. 9  shows a block diagram of the windshield monitoring system of  FIG. 8 ; 
         FIG. 10  shows a windshield clearing system according to another aspect of the third embodiment of the present invention; 
         FIG. 11  shows a section view of the windshield monitoring system of  FIG. 8 , in operation not detecting contamination; 
         FIG. 12  shows a similar view to  FIG. 11  of the windshield monitoring system detecting contamination; and 
         FIG. 13  shows a windshield clearing system of a windshield monitoring system according to a fourth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 2 , a vehicle  100  includes a vehicle frame  102  supporting at least one windshield  104  dividing the vehicle interior and the vehicle exterior. The term windshield covers a front windscreen, a rear windscreen, and a side window. Each windshield  104  either comprises a plurality of sheets of laminated glass or a singular sheet of glass. For instance, a laminated glass windshield  104 , as shown in  FIG. 5 , is constructed using an inner sheet of glass  105 , an outer sheet of glass  107 , and an intermediate layer  109  of Polyvinyl Butyral (PVB) sandwiched between the two. The intermediate layer  109  is made from PVB to prevent shards of glass flying off in a crash and because PVB has the same refractive index as the inner  105  and outer  107  sheets of glass. 
     The vehicle  100  also includes a windshield monitoring system.  FIG. 2  shows part of the windshield monitoring system with  FIG. 3  showing a more detailed version in the form of a block diagram. 
     With continued reference to  FIG. 2 , the windshield monitoring system includes an array of light emitters  108 . The light emitters are provided in the form of Light Emitting Diodes (LEDs). The emitters  108  are separated from one another to span over a length, such that the light they emit spans the width of the  104 . The emitters  108  are separated substantially equidistantly from one another. The emitters  108  are arranged to emit a narrow beam of light  106 , specifically narrow beams of monochromatic light. Monochromatic light is used in contrast to white light since white light would disperse within the windshield. The array of emitters  108  is provided along one side of the windshield. In particular, the emitters  108  are encapsulated along the side of the windshield  104  within a refractive index matched medium. As will be described in more detail below, the emitters are arranged to direct the beams of light  106  into the windshield at an angle of inclination of between about 43° and about 65°. 
     The windshield monitoring system also includes a detector. Specifically, the detector is a camera  110 . The camera  110  is a digital camera able to distinguish a position of a detected image by pixels. The camera  110  may be located in the vehicle interior. The camera  110  may be sensitive to all wavelengths of visible light, or may alternatively be sensitive only to a specific wavelength or waveband corresponding to the beams of light produced by the light emitters  108 . When monitoring the front windscreen, the camera  110  is integrated into a ceiling panel of the vehicle  100  in between a rear view mirror (not shown) and the windshield  104 . Alternatively, the camera may be mounted at the top of the dashboard. The camera is oriented so as to monitor an interior surface  112  of the front windscreen. As will be described in more detail below, the camera  110  can thus detect any refracted light  114  as a result of surface contamination  116 . Due to the camera  110  dividing a recorded image up by pixels, the location of the refracted light  114 , and thus the location of contamination  116 , can be determined directly by mapping the pixel(s) detecting the reflected light onto a map of the windshield  104 . 
     With reference to  FIG. 3 , the example shown of the windshield monitoring system also includes a noise filter  119 , which noise filter  119  includes a pulsar  120  and a lock-in amplifier  122 . The function of each of these components will be described in more detail below. However in summary, the pulsar  120  includes a motor  124  and a disc  126 . The disc is rotatable about an axis (not shown). The motor  124  is coupled to the disc  126  so as to rotate the disc  126  in operation. The disc  126  includes successive transparent  128  and opaque  130  regions. Light from a light beam  106  is allowed to pass though the transparent regions  128 . However, light beams  106  are blocked by the opaque regions  130 . The axis of rotation of the disc  126  is misaligned with the beam of light  106  such that the beam of light  106  impinges on whichever of the transparent  128  or opaque  130  regions it is aligned with. 
     As an alternative to pulsating the light beam using the disc  126 , the light beam can be pulsated by strobing or shuttering by flickering the light beam on and off using control logic stored on a computer on board the vehicle. 
     The lock-in amplifier  122  communicates with the motor  124  so as to obtain the rotational speed of the disc  126 . The lock-in amplifier  122  also has a data store having information regarding the geometric profiles of the transparent and opaque regions  128 ,  130 . In this way, the lock-in amplifier can associate light detected as a result of the beam of light  106  from other light  132 , for instance natural light or light from oncoming vehicle headlights. 
     With reference to  FIG. 4 , the vehicle  100  also includes a windshield clearing system  140 . The windshield clearing system  140  includes a control module  142  and a heating mechanism  144 . 
     The control module  142  is provided as electronic data on a non-volatile memory component of an on-board vehicle computer. The computer also includes a processor for executing the electronic data. Operation of the control module  142  is described in more detail below. However, the control module  142  receives inputs from the windshield monitoring system. Specifically, the control module  142  receives an indication as to the presence and location of any detected refracted light  114 . The control module  142  then commands a heating zone  144  at a location of the detected refracted light to heat up to clear the responsible contamination. 
     The control module  142  includes a termination module  145  as part of the electronic data stored on the memory component of the on-board computer. The termination module  145  is arranged to send a termination command to the heating zone  144  to terminate heating in response to the camera  110  ( FIG. 2 ) ceasing to detect refracted light  114 . 
     The heating mechanism  144  includes a plurality of active heating elements  146  and passive heating elements  148 . The active heating elements  146  include resistance heating elements arranged to increase in temperature when an electrical current is passed across them. The resistance heating elements may be provided as vapour deposited metallic layers. The metallic layers can be fabricated from any metallic element or alloy suitable for conducting heat in by electrical resistance. One such metal allow is an alloy including silver. However, other electrically conductive materials may be used. For instance, an organic material, such as graphene, may be used. The passive elements  148  are boundaries of the active heating elements  146  and are provided as areas of uncoated glass to provide insulated areas intermediate the active heating elements  146 . These uncoated areas are relatively narrow so as to prevent a pattern of unclear windshield. It can be visualised that the windshield is thus divided into a plurality of zones by virtue of the active and passive heating elements  146 ,  148 . 
     With reference to  FIG. 5 , in operation, light from an emitter  108  is shone as a beam of light  106  into an edge of the windshield  104 . The light beam  106  is directed into the windshield  104  at an angle of incidence θ i  relative to a normal line of incidence N of the windshield  104 . The angle of incidence θ i  is between about 43° and about 65°. It will be appreciated that the windshield in  FIG. 5  is of a laminated glass construction though the same angles of incidence would be applicable to a windshield made from a singular sheet of glass. 
     The lower limit of about 43° is chosen since this is the critical angle θ c  of a glass to air interface. The critical angle θ c  is the angle at which light reflects internally with no light escaping from the glass to the air. This phenomenon is known as total internal reflection. Any angle of incidence greater than the critical angle θ c  also subjects the light beam to total internal reflection. The angle of incidence, θ i , being greater than the critical angle of 43° is sufficient to subject the beam of light  106  to total internal reflection at a glass to air interface to propagate the beam of light  106  across the windshield  104  to an opposing edge thereof. In particular, the light beam  106  is reflected off both interior and exterior surfaces to propagate the light beam  106  across the windshield  104 . 
     The upper limit of about 65° is the critical angle, θ c , for a glass to water interface. In the same way, a beam of light  106  impinging on a glass to water interface at an angle of incidence, θ i , greater than 65° would reflect internally whereas an angle of incidence lower than 65° would refract light away from the windshield  104  as refracted light  114 . An initial angle of incidence θ i  is selected to be lower than the critical angle θ c  of glass to water in order to intentionally refract light  114  away from the windshield  104  when in contact with any water based contamination  116 . In this way, the angle of incidence θ i  is arranged to refract the light beam  106  away from the windshield  104  at a glass to water/contaminant interface. 
     Water based contamination benefits most from this clearing system since water is more easily cleared up by heating than other types of contamination such as dirt. Water based contamination  116  includes water vapour, liquid water, and ice. 
     Any refracted light  114  is detected by the camera  110 . Any light detected by the camera  110  which has not been pulsated by the pulsar  120  ( FIG. 3 ) will be filtered out by the noise filter  119 . 
     Since the camera  110  can distinguish the location of refracted light  114  on the windshield  104 , the control module ( FIG. 4 ) configures the corresponding active heating element  146  covering the area of contamination  116  to heat up. Heating the area of contamination  116  will evaporate the water responsible for the contamination  116 . 
     The termination module  145  sends a termination command to stop passing electrical current through the active heating element  146  when the camera  110  ceases to detect refracted light  114 . In addition, the termination module  145  sends a termination command to stop passing electrical current through the active heating element  146  after a predetermined time period. This is particularly important in instances where false positives occur. For instance, a false positive may be triggered for windshield defects such as chips or cracks which will never be cleared by any of the vehicle&#39;s on board ancillary systems. Providing the termination command after the predetermined time period thus prevents wasted energy for non-treatable windshield contamination scenarios. 
     The advantages of the previous embodiment are numerous. One particular advantage is that by providing detection over a wide area of the windshield  104 , less energy is ultimately consumed since any contamination  116  is detected early. This is particularly important for electric or hybrid electric vehicles where power consumption is critical to the range of the vehicle. In addition, dividing the windshield into zones of separate heating elements provides further efficiencies since uncontaminated areas are not directly heated. 
     With reference to  FIGS. 6 and 7 , a second embodiment of the windshield monitoring system will now be described.  FIG. 6  shows part of the windshield monitoring system with  FIG. 7  showing a more detailed version in the form of a block diagram. It is possible to transfer those features described specifically with reference to the first embodiment to the second embodiment. Those features of the second embodiment which are common to the first embodiment are labelled  100  greater. 
     The windshield monitoring system includes two arrays of emitters  208 . The arrays are mutually orthogonal and are provided along adjacent edges of the windshield  204 . Specifically, the arrays are positioned along a bottom edge and a right side edge of the windshield  204 . The emitters  208  are again separated so as to span along each edge of the windshield to provide coverage over the majority of the windshield  204 . The emitters  208  are oriented to emit beams of light  206  into the windshield  204  at an angle of incidence of between about 43° and about 65°. These angles are selected for the same reasons as described above. The emitters  208  are also oriented so that the light beams  206  are substantially parallel. 
     The light detector in this embodiment is a photodiode  210  as opposed to a camera. The photodiode  210  can detect the presence of light, and depending on the type of photodiode used, may tuned to be sensitive only to a specific wavelength or band of wavelengths of light as may be produced by the light emitters, but cannot distinguish the location of light, like the camera  110 . In order to determine the location of contamination  216 , the windshield detection system associates the detected light with one light emitter  208  from each array. 
     With reference to  FIG. 7 , the windshield monitoring system again includes a pulsar  220  and a lock-in amplifier  222  which work in the same way as the previous embodiment. However, each light beam  206  is pulsated at a unique predefined pattern and/or frequency of pulsation. The windshield monitoring system also includes an association module  270 . The association module  270  is provided as electronic data on the memory component of the on-board computer, like the control module. 
     The association module  270  includes a data store for storing data relating to the pulse patterns for each emitter  208 . In response to the photodiode  210  detecting a light pulse, the association module  270  compares the pulsed refracted light beam with those stored for each emitter  208 . By matching the light pulse to the responsible emitter  208  in each array, the association module locates the position of the contamination  216  with a 2 dimensional coordinate. For instance, the association module may determine that contamination exists in the centre of the windshield  204  if a central emitter  208  from each array is identified as being responsible for the refracted light  214  ( FIG. 6 ). This coordinate is then fed into the control module ( FIG. 4 ) to control a heating zone corresponding to an area at which contamination  216  has been detected. 
     As an alternative to pulsating the light beam  206 , the light beams  206  could be phase shifted relative to the other beams of light  206 . 
     A third embodiment of the invention will now be described. With reference to  FIG. 8 , a vehicle  300  includes a frame  302  supporting a plurality of windshields  304  (one shown). The term windshield  304  includes a front windscreen, or a rear windscreen, or a side window, of which there are several. Each windshield  304  either comprises a plurality of sheets of laminated glass or a singular sheet of glass. 
     With brief reference to  FIG. 11 , a laminated windshield  304  includes a plurality of sheets. Said sheets include outer and inner sheets of glass  306 ,  307 , an infrared reflective (IRR) film  308 , and an intermediate layer  309 . The intermediate layer  309  is made from Polyvinyl Butyral (PVB). PVB is selected because it has the same refractive index as the inner  307  and outer  306  sheets of glass. The windshield  304  separates an exterior environment  310  from an interior environment  312 . Accordingly, the windshield  304  has an exterior surface  316  and an interior surface  314 . 
     With reference to  FIGS. 8 and 9 , the vehicle  300  also includes a windshield monitoring system, part of which is shown in  FIG. 8  with a more detailed version in the form of a block diagram being provided in  FIG. 9 . 
     With specific reference to  FIG. 8 , the windshield monitoring system includes a light source in the form of a plurality of light emitters  320 . The light emitters are Light Emitting Diodes (LEDs). The light emitters are configured to emit beams  322  of infrared light. The beams  322  are narrow beams of light. The emitters  320  are provided as two arrays of emitters. The arrays of emitters are arranged along adjacent, mutually orthogonal edges of the windshield  304 . In particular, one array is provided along a right hand side edge of the windshield  304 . The other array is provided along a lower horizontal edge of the windshield  304 . The two arrays of light emitters  320  are thus mutually orthogonal. Each emitter  320  is arranged to direct a beam of infrared light  322  in to the windshield  304  at an angle of incidence, θ i , relative to a normal line of incidence N between about 42° and about 65°. The reasons for this are provided in more detail below. The light emitters  320  span along each respective edge of the windshield  304 . Beams of light  322  from the same array are substantially parallel to one another. In this way, almost complete coverage of the windshield can be provided by the two mutually orthogonal arrays of light emitters  320 . Each emitter  320  is encapsulated in a refractive index matched medium along the edge of the windshield. 
     The windshield monitoring system also includes a plurality of light detectors  324 . Each light detector is a photodiode. The light detectors  324  are also provided as two mutually orthogonal arrays. The two arrays of light detectors  324  are provided along a left side edge of the windshield  304  and a top edge of the windshield  304  respectively. Each light detector  324  is associated with a unique light emitter  320  such that there are equal numbers of light detectors and emitters. The light detectors  324  have the same separation distance as the light emitters  320 . As will be described in more detail below, the light detectors  324  are arranged to detect the intensity of light reflected internally within the windshield  304  from the corresponding emitter  320  at the opposing edge of the windshield  304 . 
     As will be described in more detail below, any contamination  326  on the windshield may result in a light beam  322  being refracted away from the windshield  304 . Refracted light  328  causes a drop in intensity of the internally reflected light  322  detected by the light detector  324 . 
     With specific reference to  FIG. 9 , the windshield monitoring system includes a noise filter  329  to distinguish light emitted from the light source from other detected light. The noise filter comprises a pulsar  330  and a lock-in amplifier  332 . 
     The pulsar  330  includes a motor  334  and a rotatable disc  336 . The motor  332  is arranged to rotate the disc about a rotation axis (not shown) at a variable speed measure in revolutions per minute (RPM). The disc includes successive transparent  338  and opaque  340  regions. A light beam  322  emitted from the light emitter  320  is directed in to the disc  336 . The light beam  322  impinges on the successive transparent  338  and opaque  340  regions since the axis of rotation of the disc  336  is misaligned with the light beam  322 . The light beam  322  is allowed to pass through the transparent regions  338  but is blocked by the opaque regions  340 . By opaque, it is meant that any material can be used which blocks out infrared light. The transparent  338  and opaque  340  regions are not consistent in area so as to vary the duration of light passing the disc  336 . In this way, the pulsar  330  is arranged to pulsate the light according to a predefined pattern corresponding the sizes of the transparent  338  and opaque  340  regions and the speed at which the motor  334  drives the disc  336 . 
     The disc  336  and/or the speed of the motor  334  are varied across the array of light emitters  320  such that each light beam  322  is pulsated according to a unique pattern. In this way, light can be identified as originating from a specific light emitter  320 . Another way in which this can be achieved is to phase shift light from each of the light emitters  320  relative to the other light emitters  320 . 
     The lock-in amplifier  332  is connected to the motor  334  to monitor the RPM thereof. The lock-in amplifier  332  also includes a data store for storing the geometric profiles of the rotatable discs  336 . The lock in amplifier can compare the received light with that calculated as being an expected pattern of light such that the light beam  322  originating from the light emitter  320  can be distinguished from other light  342  originating from a variety of other sources. 
     As an alternative, the pulsar may use strobing effects or shuttering to pulsate the light beam rather than using the disc  336  as described above. 
     The windshield monitoring system also includes a locating module  344  which is linked to the light detectors  324 . Any light detected by a specific light detector  324  can be used to locate the position of the contamination as a two dimensional coordinate by identifying which detector  324  in each array has detected the refracted light. 
     With reference to  FIG. 10  the vehicle  300  also includes a windshield clearing system  350 . The windshield clearing system  350  includes the windshield monitoring system, a control module  352 , and a clearing element, in this embodiment in the form of a heating mechanism  354 . 
     The control module  352  is provided as electronic data on a non-volatile memory component of an on board computer system. The computer system also includes a processor to execute the electronic data to operate the control module  352 . The locating module  344  is also provided in the same way, namely, electronic data stored on the non-volatile memory component of the on board computer. The control module  352  is arranged to apply a voltage across the heating mechanism  354  in response to a demand to heat the heating element to clear the contamination. 
     The control module  352  includes a termination module  353  as part of the electronic data stored on the memory component of the on-board computer. The termination module  353  is arranged to send a termination command to the heating mechanism  354  to terminate heating in response to the detectors  324  ( FIG. 9 ) ceasing to detect refracted light  328 . 
     The heating mechanism  354  is divided in to a plurality of zones, or heating elements. The zones are divided in to active heating elements  356  and passive heating elements  358 . Active heating elements  356  are defined as those that can be heated and are provided with electrically conductive layers. These passive zones  358  are very small in dimension so as to prevent a pattern of unclear glass during operation. These passive zones  358  are considered as boundaries to the active zones  356 . The control module  352  can apply a voltage across the metallic layer of any of the active heating elements  356 . The corresponding electrical current heats a zone of the windshield corresponding to the location of the detected contamination. 
     The heating mechanism  354  comprises a resistant heating element which is a metallic layer. The metallic layer is vapour deposited on to the windshield  304 . The metallic layer is silver alloy. However, other electrically conductive materials would suffice, for instance some organic materials like graphene, or even the IRR layer itself. 
     With reference to  FIG. 11 , in operation, the light emitter  320  emits a beam of infrared light  322  in to an edge of the windshield  304  such that the angle of incidence of the light on the glass to air interface is between about 43° and about 65°. These angles are measured relative to a normal angle of incidence N relative to the interior and exterior surfaces  314  and  316  of the windshield  304 . 
     The critical angle, θ c , for a glass to air interface is about 43°. The critical angle, θ c , for a glass to water interface is about 65°. The difference in critical angle is due to the difference in refractive index between air and water. Any light below an angle of incidence, θ i , lower than a critical angle, θ c , would refract away from the glass. Any angle of incidence, θ i , above the critical angle, θ c , would reflect internally within the glass. This phenomenon is total internal reflection. 
     This windshield clearing system is most suitable for use in clearing water based contamination such as moisture (in the form of fogging or misting of the windshield), ice, and liquid water since water evaporates relatively easily through heat. Accordingly, an angle of incidence, θ i , of a beam of light  322  between about 43° and about 65° would reflect internally at a glass to air interface and refract away from the glass at a glass to contaminant interface. 
     The light detector  324  constantly monitors the intensity of light reflected internally across the windshield  304 . Since the light beam  322  is infrared light, the light naturally reflects off the IRR layer  308  inside the laminated windshield  304 . In this way, a light beam  322  can be directed on an interior side of the IRR layer  308 . Additionally, or alternatively, the light beam  322  can be directed on an exterior side of the IRR layer  308 . This can be achieved by directing adjacent emitters  320  to direct their beam of light  322  on opposite sides of the reflective IRR layer  308 . Alternatively, a pivoting mechanism can be used to pivot an emitter  320  such that one pulse of light  322  can be directed on the interior side of the IRR layer  308  and a subsequent pulse of light  322  from the same emitter can be directed to an exterior side of the IRR layer  308 . 
     With reference to  FIG. 12 , any contamination  326  changes the critical angle, θ c , at the interface as described above. When the light beam  322  impinges on a glass to contaminant interface, light is refracted away from the windshield  304  as refracted light  328 . The refracted light  328  results in a reduction in intensity of the internally reflected light beam  322 . The detector  324  detects this reduction in intensity of light. A drop in intensity of light monitored by the light detector  324  is attributable to the presence of contamination  326  on the windshield  304 . 
     The noise filter  329  filters out any light detected by the detector  324  attributable to other sources than the light source  320 . The locating module  344  provides a two dimensional coordinate of the contamination  326  based on which of the detectors  324  have detected the contamination. This coordinate will distinguish between the interior and the exterior of the windshield  304  using the same approach since each emitter  320  can be configured to direct light either down and interior or exterior side of the IRR only. Using this coordinate, the control module  352  passes a voltage to one of the active heating zones  356  corresponding to the location where contamination has been detected. 
     When the detectors  324  experience an intensity of the received light returning to expected levels, the control module  352  determines that contamination  326  has cleared. The termination module  353  reduces the voltage across the heating element  354  to zero Volts. The temperature of the heating element  354  reduces accordingly. 
     There are various energy efficiencies in using this system since heating the windshield  304  has been made automatic in response to detecting any contamination  326  rather than letting the contamination  326  propagate to a point which might affect the driver&#39;s visibility. In addition, heating only an area of the windshield  304  corresponding to the location of contamination  326  is more energy efficient compared to a case where the entire windshield  304  is heated even when contamination  326  is localised. 
     Various other alternative embodiments are also possible within the scope of the appended claims. A fourth embodiment of the invention will now be described. Those features in common with the previous third embodiment are labelled with like reference numerals. 
     In this alternative embodiment, the windshield monitoring system is the same as the previously described third embodiment and so is not described in any further detail here. 
     With reference to  FIG. 13 , the windshield clearing system  450  of the alternative fourth embodiment includes the control module  352  having a termination module  353  installed therewith. The windshield clearing system also includes a windshield wiper system  454 . The windshield wiper system  454  includes a motor  456  powering a wiper blade  458 . 
     There are two wiper arms  458  shown in  FIG. 13 , however this may be reduced depending on the windshield incorporating the windshield clearing system  450 . The wiper arms  458  are of conventional design incorporating being coupled to the motor  456  in a known manner. The wiper arms  458  each support a wiper blade. The wiper blade is made from rubber and is arranged to contact the exterior surface of the windshield  304 . 
     There are two motors  456  shown in  FIG. 13 , each for powering a wiper arm  458 . However, other configurations may be employed such as a single motor  456  driving a common linkage attached to both wiper arms  458 . The motor  456  is arranged to rotate the wipers arm  458  back and forth over the exterior surface of the windshield  304  in a rotary fashion. The motor  456  can be configured to operate at several speeds. 
     In one mode of operation, when the windshield monitoring system ( FIG. 12 ) detects exterior surface contamination on the windshield  304 , the control module  352  ( FIG. 13 ) configures the motors  456  to rotate the wiper arms  458  over the windshield. The wiper blades clear the windshield  304  by wiping away water. Once the windshield monitoring system ( FIG. 12 ) ceases to detect the presence of water on the windshield  304 , the termination module  353  ( FIG. 13 ) commands the motors to terminate stop, after a cycle has been completed. 
     In another mode of operation, when the locating module  344  ( FIG. 9 ) has detected localised contamination, the controller  352  ( FIG. 13 ) will configure the motors  456  to move the wiper arms  458  to the area where contamination has been detected. Next, the motor  456  will move the wiper arm  458  back and forth over the contamination. Again, wiping will terminate once the windshield monitoring system ceases to detect contamination. 
     This alternative windshield clearing system has been described with reference to water based contamination. However, other contaminants such as dirt can be cleared in this way too by wiping away the dirt using the wiper blades. The wiper cycle can be a wash/wipe cycle if the contamination remains after an acceptable period of time. That said, some contaminants such as windshield cracks may cause false triggers. In this case, the termination module  353  over-rides the control module  352  after a pre-determined time period to save energy. If a predefined number of false triggers occur then the control module  352  is arranged to emit an indication to have the windshield  304  checked. The indication can be used on the vehicle  300  or off-line during manufacture or maintenance inspection. Doing so will allow the windshield to be checked since such reoccurrences may be due to cracks in the windshield  304 . If the windshield  304  is found to cracked, then the windshield  304  can be replaced. 
     It will be appreciated that in addition, or as an alternative to clearing the interior of the windshield  304  using electrically conductive heating elements, the windshield clearing system may be arranged to generate a control signal for communication to a vehicle based heating ventilation and air conditioning (HVAC) unit comprising an air blower. The HVAC unit is arranged to condition the air in the vehicle cabin. When contamination of the interior surface of the windshield is detected, the windshield clearing system generates a signal indicative of the location and severity of detected water based contamination on the interior surface of the windshield. In response to this signal, a fan or blower located in the vehicle HVAC unit may be activated, directing moving air across the interior surface of the windshield to accelerate clearing. Depending on detected ambient conditions, such as cabin air temperature and humidity and outside air temperature and humidity, the HVAC unit blower may be used in addition to the heating elements located on the windshield  304 . In this way, the termination module  353 , as described above, may also have the authority to terminate an action of the HVAC unit that had been previously requested in response to a detected water based contamination on the windshield.