Optical detection of water droplets using light refraction with a mask to prevent detection of unrefracted light

Abstract

An optical detection system for detecting rain or other water droplets on the outer surface of a window and fog on the inner surface with a single photo-detector. The invention measures the accumulation of water droplets on the window by light refraction of a first light beam with droplets to redirect a first light beam to the photo-detector. A masking device prevents light from reaching the photo-detector directly without refraction. As a result the rain measurement output signal of the photo-detector increases with an increasing accumulation of water droplets on the window. The fog accumulation is measured by a second light beam reflected off the inner surface of the window to the photo-detector so that the output signal of the photo-detector decreases with increasing amounts of fog since fog scatters the light to reduce the amount of light reflected to the photo-detector. A third light source is focused directly on the photo-detector to bias it into an operating point of high sensitivity to infrared light and is connected in a negative feedback circuit from the output of the photo-detector amplifier. The photo-detector output is connected to a narrowband amplifier that is tuned to the frequency of an oscillator which pulses the first and second light sources at different times. As a result of this negative feedback, changes in the output signal due to external factors are cancelled so they do not produce errors.

I claim: 
     
       1. An optical droplet detector apparatus for determining the degree of vision impairment through a window due to an accumulation of water or other precipitation on the window comprising: a first light source for illuminating water droplets on said window with a first light beam transmitted through said window to the inside of said window;   a photo-detector located inside said window; and   a masking device positioned in front of said photo-detector to effectively block the direct transmission of light rays from said first light source to said photo-detector, but allowing said photo-detector to receive light rays from said first source which have been refracted by droplets on said window for the purpose of determining the amount of water on said window so that the output signal of said photo-detector increases with an increase in water droplets on said window.   
     
     
       2. Apparatus in accordance with claim 1 wherein the first light beam detects rain droplets on the outside surface of the window, the masking device is positioned between the first light source and the photo-detector and which also includes a second light source positioned inside said window to produce a second light beam for detecting fog on the inside surface of the window which reflects off the inside surface to said photo-detector for the purpose of determining the degree of fog accumulation on said inside surface, said first and second light sources emitting infrared light. 
     
     
       3. Apparatus in accordance with claim 1 which also includes a third light source positioned inside said window with its beam directly aimed at said photo-detector for the purpose of optically driving said photo-detector to its bias operating point and for providing an optical feedback signal to said photo-detector. 
     
     
       4. Apparatus in accordance with claim 3, further comprising a drive circuit for providing pulsed drive current to said first light source for water accumulation measurements and another drive circuit connected to said third light source for gain stabilization, and an output circuit for amplifying the pulsating output current produced by the photo-detector. 
     
     
       5. Apparatus in accordance with claim 4, wherein said output circuit includes a narrowband amplifier and tank circuit closely tuned to the frequency of the pulsed drive current for the purpose of rejecting other, potentially interfering currents produced by said photo-detector. 
     
     
       6. Apparatus in accordance with claim 4, wherein a feedback circuit is connected from said output circuit to the drive circuit for the third light source to provide an out-of-phase, negative feedback signal to modulate said third light source to provide an optical operational amplifier circuit for the purpose of overall gain stabilization. 
     
     
       7. Apparatus in accordance with claim 1, further comprising detector circuit means connected to the output of the photo-detector for peak detection, averaging and temporary storage of the photo-detector output signals to produce output signal data pertaining to visibility impairment of said window, reducing the effects of noise and unwanted signals in the data, and temporarily storing the data for subsequent analog-to-digital conversion or threshold comparison. 
     
     
       8. Apparatus in accordance with claim 2, further comprising an analog-to-digital converter for converting the analog output signal of the photo-detector pertaining to visibility impairment to a pair of digital output signal related to measurements for rain and fog, respectively, and computer interface circuits to interface said digital output signal to a microprocessor for the purpose of making threshold and state-transition decisions to control devices for restoring visibility or for closing windows. 
     
     
       9. Apparatus in accordance with claim 8, wherein a digital offset measurement is made of the output signals at a zero-drive-signal condition obtained when neither said first light source nor said second light source are driven for the purpose of subtracting said offset value from the output signals corresponding to each subsequent rain and fog measurement to eliminate any residual zero-offset errors from said rain and fog measurements. 
     
     
       10. A water droplet detector for determining the degree of vision impairment through a windshield of a vehicle due to an accumulation of water on said windshield comprising: a first sensor incorporating a first light source and a photo-detector for the purpose of determining the degree of water accumulation on the outside surface of said windshield;   a second sensor incorporating a second light source and said photo-detector for the purpose of determining the degree of fog accumulation on the inside surface of said windshield;   a third light source for the purpose of optically driving said photo-detector to its bias operating point;   a drive circuit for providing pulsed drive current to said first light source and to said second light source;   an output circuit connected to the photo-detector including a tuned amplifier for providing narrowband amplification closely tuned to the frequency of said pulsating drive currents for the purposes of amplifying the pulsating output current produced by said photo-detector and rejecting other, potentially interfering currents produced by said photo-detector; and   a feedback circuit connected from said output circuit to said third light source for providing attenuation and additional phase shift to the output signal of said output circuit to provide a negative feedback signal to modulate said third light source for the purpose of gain stabilization of said output circuit.   
     
     
       11. Apparatus in accordance with claim 10, further comprising a detector circuit means connected to the output circuit for peak detection, averaging and temporary storage of the output signals for producing output signal data pertaining to visibility impairment of said windshield, for reducing the effects of noise and unwanted signals in the data, and for temporarily storing the data for subsequent analog-to-digital conversion or threshold comparison. 
     
     
       12. Apparatus in accordance with claim 10, further comprising an analog-to-digital converter for converting the analog output signals of the output circuit pertaining to visibility impairment to a pair of digital output signals relating to measurements for rain and fog, respectively, computer interface circuits to interface said digital output signals to a microprocessor for the purpose of masking threshold and state-transition decisions to control devices for restoring visibility or for closing windows, and offset means for measuring digital offset of the output signals at a zero-drive-signal condition obtained when neither said first light source nor said second light source are driven, for the purpose of subtracting said offset value from the output signals corresponding to each subsequent rain and fog measurement to eliminate any residual zero-offset errors from said rain and fog measurements. 
     
     
       13. Apparatus in accordance with claim 10 in which the light sources and their drive circuits together with the photo-detector and its output circuit and feedback circuit are connected as an optical operational amplifier for gain stability. 
     
     
       14. A method of optical detection of water or other precipitation on a window to determine the degree of vision impairment through the window due to the accumulation of water on said window, comprising the steps of: transmitting a first light beam from a first light source through said window to the inside of said window;   detecting the first light beam with a photo-detector inside said window;   blocking the direct transmission of light rays from said first light source to said photo-detector by a light masking device; and   refracting said first light beam with water droplets or other precipitation on said window to cause a portion of said first light beam to be redirected from said water droplets to said photo-detector so that the detector output signal of said photo-detector increases with an increase in water droplets on said window.   
     
     
       15. A method in accordance with claim 14 in which the first light beam is transmitted from a first light source outside the window and is refracted by rain droplets on the outside surface of the window, and which also includes the step of: transmitting a second light beam from a second light source inside the window so that said second light beam is reflected off the inside surface of the window to the photo-detector for determining the amount of fog accumulation on said inside surface by decreases in the detector output signal with increases in fog on the inner surface of said window due to diffusion of the light beam by the fog.   
     
     
       16. A method in accordance with claim 14 which also includes the step of: transmitting a third light beam from a third light source directly to the photo-detector to optically drive the photo-detector to its bias operating point.   
     
     
       17. A method in accordance with claim 15 which also includes the step of: applying pulsed drive current selectively to the first light source and to the second light source to cause them to emit light which is pulsed at a predetermined frequency to cause the photo-detector to produce a pulsed output signal.   
     
     
       18. A method in accordance with claim 17 which also includes the step of: transmitting the pulsed output signal of the photo-detector through an output circuit including a narrowband amplifier and tank circuit tuned to the frequency of the pulsed drive current.   
     
     
       19. A method in accordance with claim 18 which also includes the step of: transmitting a negative feedback signal from the photo-detector output circuit to the drive circuit of the third light source.   
     
     
       20. A method in accordance with claim 18 which also includes the steps of: peak detection of the analog photo-detector output signal at the output of the photo-detector output circuit;   converting the analog photo-detector output signal to a digital output signal; and   processing said digital output signal with a computer to produce digital data corresponding to measurements of the amount of water on the window. .Iadd.   
     
     
       21.  An optical droplet detector apparatus for determining the degree of vision impairment through a window due to an accumulation of water or other precipitation on the window comprising: a first light source for illuminating water droplets on said window with a first light beam to refract a portion of said first light beam by said droplets;   a photo-detector located inside said window; and   a masking device positioned in front of said photo-detector to effectively block the direct transmission of light rays from said first light source to said photo-detector, but allowing said photo-detector to receive light rays from said first source which have been refracted by droplets on said window for the purpose of determining the amount of water on said window so that the output signal of said photo-detector increases with an increase in water droplets on said window. .Iaddend..Iadd.   
     
     
       22.  Apparatus in accordance with claim 21 wherein the first light beam detects rain droplets on the outside surface of the window, the masking device is positioned between the first light source and the photo-detector and which also includes a second light source positioned inside said window to produce a second light beam for detecting fog on the inside surface of the window which reflects off the inside surface of the window to said photo-detector for the purpose of determining the degree of fog accumulation on said inside surface, said first and second light sources emitting infrared light. .Iaddend..Iadd. 
     
     
       23.  Apparatus in accordance with claim 21 which also includes a third light source positioned inside said window with its beam directly transmitted to said photo-detector for the purpose of optically driving said photo-detector to its bias operating point and for providing an optical feedback signal to said photo-detector; a drive circuit for providing pulsed drive current to said first light source for water accumulation measurements and another drive circuit connected to said third light source for gain stabilization, and an output circuit for amplifying the pulsating output current produced by the photo-detector;   said output circuit includes a narrowband amplifier for the purpose of rejecting other, potentially interfering currents produced by said photo-detector. .Iaddend..Iadd.24. Apparatus in accordance with claim 23 wherein a feedback circuit is connected from said output circuit to the drive circuit for the third light source to provide a negative feedback signal to modulate said third light source to provide an optically coupled operational amplifier circuit for the purpose of overall gain   
     
     
        stabilization. .Iaddend..Iadd.25.  A water droplet detector for determining the degree of vision impairment through a windshield of a vehicle due to an accumulation of water on said windshield comprising: a first sensor incorporating a first light source and a photo-detector for the purpose of determining the degree of water accumulation on the outside surface of said windshield;   a reference light source for the purpose of optically driving said photo-detector to its bias operating point;   a drive circuit for providing pulsed drive current to said first light source;   an output circuit connected to the photo-detector including an amplifier for the purpose of amplifying the pulsating output current produced by said photo-detector and rejecting other, potentially interfering currents produced by said photo-detector; and   a feedback circuit connected from the output of said output circuit to said reference light source to provide a negative feedback signal to modulate said reference light source which optically couples said feedback signal to said photo-detector for the purpose of gain stabilization of said output circuit. .Iaddend..Iadd.26. Apparatus in accordance with claim 25, further comprising an analog-to-digital converter for converting the analog output signals of the output circuit pertaining to visibility impairment to digital output signals relating to measurements, computer interface circuits to interface said digital output signals to a microprocessor for the purpose of making threshold and state-transition decisions to control devices for restoring visibility or for closing windows, and offset means for measuring the digital offset of the output signals at a zero-drive-signal condition for the purpose of subtracting said offset value from the output signal corresponding to each subsequent measurement to eliminate any residual zero-offset errors from said measurements. .Iaddend..Iadd.27. Apparatus in accordance with claim 25 in which the light sources and their drive circuits together with the photo-detector and its output circuit and feedback circuit are connected as an optically coupled operational amplifier for gain stability. .Iaddend..Iadd.28. A method of optical detection of water or other precipitation on a window to determine the degree of vision impairment through the window due to the accumulation of water on said window, comprising the steps of:   transmitting a first light beam from a first light source to illuminate water droplets on said window;   detecting the first light beam with a photo-detector inside said window;   blocking the direct transmission of light rays from said first light source to said photo-detector by a light masking device; and   refracting said first light beam with water droplets or other precipitation on said window to cause a portion of said first light beam to be redirected from said water droplets to said photo-detector so that the detector output signal of said photo-detector increases with an increase   
     
     
        in water droplets on said window. .Iaddend..Iadd.29.  A method in accordance with claim 28 in which the first light beam is transmitted from a first light source and is refracted by rain droplets on the outside surface of the window, and which also includes the step of: transmitting a second light beam from a second light source inside the window so that said second light beam is reflected off the inside surface of the window to the photo-detector for determining the amount of fog accumulation on said inside surface by decreases in the detector output signal with increases in fog on the inner surface of said window due to   
     
     
        diffusion of the light beam by the fog. .Iaddend..Iadd.30.  A method in accordance with claim 29 which also includes the steps of: applying pulsed drive current selectively to the first light source and to the second light source to cause them to emit light which is pulsed at a predetermined frequency to cause the photo-detector to produce a pulsed output signal; and   transmitting the pulsed output signal of the photo-detector through an output circuit including a narrowband amplifier. .Iaddend..Iadd.31. An optical droplet detector apparatus for detecting the presence of water droplets on the windshield of a motor vehicle, comprising:   a first light source for illuminating water droplets on the outer surface of the windshield with a first light beam to refract a portion of said first light beam by said droplets;   a photo-detector located inside the vehicle; and   a masking device positioned to prevent the direct transmission of light from said first light source to said photo-detector to cause the photo-detector to receive light from said first light beam when said first light beam is refracted by water droplets on said windshield so that the output signal of said photo-detector increases in amplitude with an increase in the amount of water droplets on said windshield.   
     
     
        .Iaddend..Iadd.32.  Apparatus in accordance with claim 31 which also includes a reference light source for producing a reference light beam which is directly transmitted to the photo-detector to bias its operating point. .Iaddend..Iadd.33. Apparatus in accordance with claim 32 in which the photo-detector is connected to the input of a detector circuit which has a negative feedback circuit connected from the output of said detector circuit to said reference light source to provide an optical feedback signal to said photo-detector. .Iaddend..Iadd.34. Apparatus in accordance with claim 33 in which the first light source and the reference light source are connected to drive circuits for producing pulsed drive current which pulses both light sources on and off. .Iaddend..Iadd.35. Apparatus in accordance with claim 31 which also includes a second light source positioned inside the vehicle for producing a second light beam which is transmitted to the inner surface of the windshield where it is redirected to the photo-detector by fog on said inner surface in order to detect said fog. .Iaddend.