Data processing: measuring – calibrating – or testing – Measurement system – Dimensional determination
Reexamination Certificate
2000-01-25
2001-04-03
Shah, Kamini (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system
Dimensional determination
C356S005010, C342S095000
Reexamination Certificate
active
06212480
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to the field of precision ranging instruments. More particularly, the present invention relates to an apparatus and method for utilizing a light based speed and/or range finder apparatus to determine absolute reflectivity of reflective objects such as highway directional and informational signs.
2. Description of Related Art
Retroreflective sheeting is commonly used to provide reflective informational signs along highways, on roadways and on safety barriers along roadways. For example, stop signs, caution signs and highway directional signs often have such sheeting materials or coatings to enhance the visibility of the signs and especially the lettering to motorists traveling at night.
The only illumination of these signs is provided by the oncoming vehicle headlights. The retroreflective sheeting suffers from the disadvantage that its reflectivity deteriorates with age and exposure to the environmental effects of sun, wind, and precipitation. These signs must therefore be periodically inspected, refurbished and/or replaced. Anticipated state and/or federal government regulations for reflectivity of road signs may soon require that periodic measurements of sign reflectance or more frequent sign replacements be undertaken to meet reflectivity standards.
It is currently extremely difficult to accurately measure reflectivity of such signs along roadways in the field. Measurements of a sign's reflectivity require very precise instrument locations close to the sign, precise lighting conditions, and often require either sign removal or closure of the highway portion immediately in front of the sign while the required lighting and sensing instruments are set up and measurements taken. Such road closures or driving restrictions placed on motorists are inconvenient and generate a safety concern not only for the instrument operators taking measurements, but also to motorists in the vicinity. Currently, a sign must be relocated to a test facility where a light source and suitable filters may be accurately positioned in order to perform the measurements. Alternatively, the test setup may be implemented at the sign location, but such field measurements are prone to errors which may be unacceptable to meet new standards which may be forthcoming. Consequently, there is a need for a convenient and safe method for remotely ascertaining absolute reflectivity of a sign surface.
Another problem is that the reflectivity of many signs deteriorates nonuniformly. Portions of the sign which receive more direct sunlight or are colored with more light sensitive paint materials deteriorates faster than other portions. Consequently, accurate determinations of sign reflectivity are difficult to make and reflectivity determinations of important portions of the sign, such as the letters and numbers, as opposed to background portions, may be highly inaccurate. Therefore there is a need for a method of accurate reflectivity determinations for selected portions of signs rather than the entire sign.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method for measuring accurately the absolute reflectivity of a selected surface such as a sign surface that has retroreflective material thereon.
It is another object of the present invention to provide a hand held measuring apparatus for determining the absolute reflectivity of a retroreflective surface.
It is another object of the present invention to provide an apparatus and method for determining absolute reflectivity or a retroreflective surface of a roadway sign in the field with a hand held instrument.
It is a still further object of the present invention to provide an apparatus and method for remotely determining absolute reflectivity of a retroreflective surface.
The modified range finder apparatus used in the present invention preferably is a modified conventional hand held range finder such as an Impulse 100 manufactured by Laser Technology, Inc. This range finder utilizes a laser diode to emit pulses of infrared light toward a target. The range finder also may utilize a less expensive LED in place of the laser diode. Accordingly, throughout this specification it is to be understood that use of the term “laser” is used for convenience in explanation only. The LED may also be utilized with suitable excitation circuitry.
The Impulse type of speed and/or range finder includes a received (RX) pulse pulse width determining circuit to develop and apply, via the instrument's microprocessor, a correction factor to distance measurements. This correction is conventionally necessary since the distance measurement circuitry operates from the leading edges of the transmit and receive pulses to determine the time of flight of the pulse to and from the target. Since the strength of the received pulses may vary in amplitude, and hence pulse width varies, depending upon target distance, reflectivity, atmospheric conditions, etc., correction is necessary to achieve truly accurate distance measurements. This correction is accomplished by incorporating a database in ROM in the apparatus containing empirically determined correction factors for given pulse widths of the return signals.
The apparatus in accordance with the present invention may be a single purpose instrument for measuring reflectance or may be a ranging and/or speed measuring instrument which includes a separate mode of operation which utilizes, in addition to the distance correction factor data base and pulse width correction described above, another data base to measure reflectance of a reflective surface and indicate the absolute coefficient of retroreflection for an unknown reflective surface being measured by comparison of the measurement to a reading with the same instrument of a known reflectance standard. This reflectance mode of operation is made possible by implementation of a procedure to determine a compilation of power attenuation factors (Ka), utilizing the same fixed distance calibration setup as was utilized in the distance correction factor calibration mentioned above, which are stored in the apparatus and utilized to compute absolute values of the coefficient of retroreflection, RA, of a target surface. One further lookup table is preferably provided in the apparatus in accordance with the present invention for use in the reflectance mode of operation. This is a lookup table of reference standard reflectances for various colors and reflective surfaces, which have currently been standardized into classes.
The modified range finder apparatus in accordance with the present invention preferably includes EEPROMs which contain the distance correction and reflectance databases as well as the lookup table of standard material reflectances as well as a storage register for storing the reading of a known standard retroreflective surface of the type to be evaluated in the field.
The modified range finder further includes software in ROM to analyze the return signal strengths and compute the absolute retroreflectance coefficient for the sign or sign surface being measured. The resulting retroreflectance coefficient is indicated directly on a display and may optionally be ported through a UART interface connection for downloading to a personal computer or other processor device for further data manipulation and/or storage.
Basically the method of measuring an absolute reflectivity value of a retroreflective surface utilizing a light emitting range finder (e.g. laser or LED) in accordance with the present invention comprises, in the reflectance mode of operation, either recalling a previously stored reference reflectance factor from the instrument database or, if increased accuracy is desired, entering into the instrument database the known reflectance for the surface type to be measured in the field and taking a reflectance reading on a known standard retroreflective surface of the type to be measured in the field in order to determine an accurate reference reflec
Burton Carol W.
Hogan & Hartson LLP
Kubida William J.
Laser Technology, Inc.
Shah Kamini
LandOfFree
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