Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-06-28
2003-06-03
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S208100, C250S573000, C340S602000
Reexamination Certificate
active
06573490
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject invention relates to a rain sensor for detecting moisture on the windshield of a vehicle for turning on the windshield wipers, moisture taking the form of mist, water drops, as a film, frost, ice, snow, fog, etc.
2. Description of the Prior Art
The co-pending application Ser. No. 09/835,755 filed Apr. 16, 2001 Attorney Docket No.: 65165.007 entitled DIFFERENTIAL IMAGING RAIN SENSOR discloses a method for achieving greatly improved moisture detection performance through the application of synchronous, differential detection principles. In essence, acquisition and storage of a baseline signal value allows that signal level to be subtracted from a subsequent measurement that includes this background value plus a level indicative of the measured variable. In accordance therewith, that invention provides a method for sensing moisture on the exterior surface of a sheet of glass including the step of capturing an ambient image of light rays passing through the glass from the exterior to the interior surfaces thereof under ambient light conditions. The method is characterized by illuminating the glass with light rays, capturing an illuminated image of the glass and objects on the glass containing the ambient image plus reflected light from the illuminating of the glass, and subtracting the ambient image from the illuminated image to produce a moisture signal.
Ambient light and spurious (optical) signals contribute the main interfering signals in optical rain sensing. Characterizing and storing these signals permits their subsequent effective subtraction.
As disclosed in the co-pending application, a pulsed infrared (IR) light source is used to alternately illuminate image frames for a differential imaging rain sensor. The I.R. illuminant is used to obtain a signal proportional to the density of water drops that collect on the outer surface of a windshield. The use of synchronous detection permits a high degree of selectivity to be achieved since the radiation reflected from the water droplets is unique and can be discriminated over the background light level. Extraneous interfering signals may be subtracted differentially to obtain a net droplet reflection signal and thus a measure of water droplet density. Implicit in this method however, is the requirement that the background-subtracted signal remain invariant during the acquisition of the two images. Naturally, if the reference signal (frame) changes in the interval between successive image capture the basic operating premise is no longer valid. A stable reference requirement implies rapid image acquisition, storage and processing.
Alternately illuminating the area of interest on the windshield surface at high rates also requires that a high intensity pulsed radiant IR source be capable of high frequency modulation. Only solid state (light emitting diode-LED) and pulsed discharge sources are suitable for this purpose, as low cost incandescent lamps cannot be pulsed rapidly enough to be useful.
Differential, alternate image operation of an imaging rain sensor does significantly improve raindrop selectivity at moderate sensitivity. However the technique can be subject to poor false trigger immunity in certain conditions. The net imaging rate for a differential system is at least half that of a conventional (single ended) approach, and from experimental evidence, more like three times slower than the gross image capture rate. The reason for this disparity is that at least two image frames are necessary to yield one net differential frame; and in actual practice a third, wasted interleaving frame is often necessary as well.
Fast framing, typically at sixty net frames per second, is needed to prevent undesired activation (mis-triggering) of the wiping system by spurious, i.e., non-rain, signals. These non-rain artifacts are commonly image components that are not correlated in both differential image frames. Such signals are typically due to moving light sources or pulsed lights that are imaged onto the focal plane array. Given the need for rapid image frame acquisition and processing to prevent false triggering by extraneous lights, the use of differential image processing does in fact reduce rain sensor selectivity under certain conditions. In other words, trying to maintain a high degree of water droplet sensitivity will cause the system to mis-trigger or phantom wipe because it cannot acquire and process images quickly enough to reject spurious artifacts.
In spite of the application of other signal processing enhancements such as narrow band optical filtering and low f
umber optics, the selectivity of prior-art imaging rain-sensors needs to be improved for many operating conditions. Certainly the advent of faster cameras and more rapid image processing will improve this situation in the future, but there is a compelling reason to use the current generation of very low cost integrated imagers, even with their lower framing rates.
SUMMARY OF THE INVENTION AND ADVANTAGES
In accordance with the subject invention, moisture on the exterior surface of a sheet of glass is sensed by illuminating the glass and objects on the glass with light rays at a first wavelength, capturing an illuminated image of the glass containing the first wavelength, simultaneously capturing an ambient image of light rays at a second wavelength passing through the glass from the exterior to the interior surfaces thereof, and comparing the images to produce a moisture signal.
This is accomplished with an assembly comprising an illuminator for illuminating the glass and objects on the glass with light rays at a first wavelength, a first image sensor element having a peak response at the first wavelength for capturing an illuminated image of the glass containing the first wavelength, a second image sensor element having a peak response at a second wavelength for capturing an ambient image of light rays at the second wavelength passing through the glass from the exterior to the interior surfaces thereof simultaneously with the capture by the first image sensor element, and a processor for comparing the simultaneously captured images to produce a moisture signal.
The present invention therefore provides an improvement wherein the imager is capable of capturing high rate, single frames that are then processed to yield a differential signal used to activate a wiper system.
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Allen Stephone B.
Glass Christopher W.
Howard & Howard
Valeo Electrical Systems, Inc.
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