Patent Description:
Traditional methods for determining the retroreflectivity of different objects at a distance, for example, road signs have included handheld devices that are placed up against the sign and measure a small spot, typically <NUM>-inch diameter on the objects/signs. In these conventional approaches, the reflection plane mirrors reflect incident light back toward the source only if the incident light beam is perpendicular to the mirror surface. This type of device requires the operator to walk up to the object/sign to measure it. Many signs are mounted higher than can easily be reached and therefore the instrument is attached to a long pole or a lift system is used to lift the operator to the sign level. More recently, a system integrated into vehicles utilizes fixed light sources and a camera mounted on the vehicle. The system takes a series of photos of the sign and determines when it is at the correct distance from the sign for the observation angle to be correct.

American Society for Testing and Materials (ASTM)standards E1709 and E2540 specify that measurements shall be performed with a device that has an observation angle of <NUM>° or <NUM>°, respectively. Moreover, sign sheeting material specifications, outlined in ASTM D4956, require minimum performance values of the sign sheeting materials at specific observation angles. However, this type of system requires a dedicated vehicle, driving around at night, and is cost prohibitive for most users.

A retroreflectometer with moving mirror assemblies is disclosed in <CIT>.

The present disclosure is directed to a retroreflectometer for non-contact measurements of optical characteristics of retroreflective materials from a range of distances. According to the invention, and as claimed in claim <NUM>, a retroreflectometer is provided for non-contact measurements of optical characteristics of a surface of a device under test (DUT). The retroreflectometer includes a light source for emitting a light beam; a first moving mirror assembly for scanning the light beam; a collimating lens for collimating the scanning light on an illumination spot on the surface of the DUT; an imaging lens for receiving a reflected scanning light comprised of the collimated scanning light reflected from the surface of the DUT; a second moving mirror assembly for controlling a predetermined observation angle, wherein the first moving mirror assembly and the second moving mirror assembly moved in synchronization to maintain concentricity of the illumination spot on the surface of the DUT; a light collector for collecting the reflected light from the second moving mirror assembly; a processor including a memory for determining the optical characteristics of the surface of the DUT responsive to the collected reflected light.

In some embodiment, the retroreflectometer may also include one or more of a display for displaying information about the optical characteristics of the surface of the DUT; a camera for capturing an image of the illumination spot on the surface of the DUT and in conjunction with a display for visual alignment of the illumination spot on the surface of the DUT; a measurement circuit for converting the collected reflected light into an electrical signal; a light trap for trapping ambient lights; and/or a laser range finder for determining a distance and orientation of the retroreflectometer to the DUT.

A more complete appreciation of the disclosed invention, and many of the attendant features and aspects thereof, will become more readily apparent as the disclosed invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate like components.

Embodiments of the present invention are directed to a portable retroreflectometer for non-contact measurements of optical characteristics of retroreflective materials from a range of distances. An example of the optical material is the material typically used to make highway and street signs and safety markers that are elevated and not easily reached by operators. Unlike conventional reflection plane mirrors, which reflect incident light back toward the source only if the incident light beam is perpendicular to the mirror surface, the retroreflective materials reflect a non-perpendicular incident beam back toward the source. Elevated highway signs and markers illuminated by vehicle headlights can thus be seen and understood by a driver whose eyes are positioned above the illuminating headlights of the vehicle. The retroreflectometer enables verification of the optical characteristics of the signs and markers, and the level of degradation of retroreflectivity of a worn, weathered, dirty, or otherwise partially obscured sign or marker.

According to the present disclosure, a non-contact system and method for measuring the nighttime photometric and colorimetric retroreflectivity of retroreflective sheeting used commonly on road signs and traffic delineators from many different distances. Measurements can be performed during the day or night, but the results always provide the nighttime performance characteristics of the sign.

According to the invention the retroreflectometer makes non-contact measurements of the characteristics of light reflected from a retroreflective surface of an object, for example, a road sign, marker, or similar surfaces, from a distance, to verify that incident light, for example, from vehicle headlights, and the desired color are visible to a driver positioned above the headlights. The retroreflectometer determines the orientation of the retroreflective object (DUT) in space by triangulating the distance to three points on the object. It then sets the angle between the light source and detectors based on the measured distance from the DUT, measures the intensity and characteristics of the reflected light, provides display and storage of the resulting data and information for the retroreflective performance of the DUT.

In some embodiments, the angle between the light source and detectors, defined as the observation angle by the ASTM, is set by utilizing a digital distance finder and automatically adjusting the physical angle between the optical axis of the Illuminating System in <FIG> and collection system <FIG> and a convergence between them. In some embodiments, the orientation of the retroreflective object in space is determined by utilizing three digital distance finders and the well-known triangulation method.

<FIG> is an exemplary block diagram of an illuminating optical system for a retroreflectometer, according to the present invention. As shown, the system comprises of a light source <NUM>, for example, a broadband pulsed light source with an optical pathway along which an illumination light beam (<NUM> and <NUM>) from the broadband pulsed light source travels and ends at the retroreflective surface of a device under test (DUT) <NUM> to be measured.

In some embodiments, the light source is a high-power broadband white light source, which is pulsed, for example, at a frequency between <NUM> and <NUM>, with a low duty cycle of <NUM>-<NUM>%. The light beam from the light source exits from a field aperture <NUM> and emitted to a moving (e.g., rotating) mirror <NUM> actuated by a synchronized motor, such a step motor <NUM> causing the light beam reflected from the moving mirror <NUM> to change its direction move (scanned). The moving light is then collimated using a collimating lens <NUM>, such as multi-element objective lens with, for example, a <NUM> focal length.

The collimated moving light <NUM> is then scanned on an illumination spot <NUM> on the DUT <NUM>. In some embodiments, the location of the illumination spot <NUM> on the DUT <NUM> is selected by pointing the retroreflectometer via a viewfinder at the DUT. The illumination spot <NUM> changes its location during scanning. In some embodiments, the light source <NUM> is mounted to a linear stage/platform <NUM> to adjust the distance between it and the collimating lens <NUM> to set the size of the spot <NUM> properly based on the measured distance. The observation angle is defined to be the angle between the light source and the collection optics (<NUM> in <FIG>), which mimics the angle between the vehicle headlights and the driver's eyes. The physical angle <NUM> between the optical axis of the light collector <NUM> and illuminator <NUM> and their convergence point <NUM> at the DUT is adjusted by a synchronized motion of the linear stage <NUM> and goniometer <NUM>. The angle <NUM> is set by the Goniometer <NUM> and the convergence point is controlled by the linear stage <NUM>. (see, for example, <FIG>), utilizing the well-known law of tangents.

To deal with the challenge of keeping the instrument steady while taking a measurement, in some embodiments, the illumination beam is swept by the moving (motorized) mirror <NUM> in small angular increments in the horizontal plane and multiple measurements (e.g., between <NUM> and <NUM>) are taken in a rapid succession burst mode, for example, a step angle of between <NUM> and <NUM> degrees and a full angular spread of +/- <NUM> to <NUM> degrees. Also, to help keep the instrument steady, some embodiments place the instrument onto a tri-pod or a mono-pod.

<FIG> is an exemplary block diagram of a receiving (collection) system for a retroreflectometer, according to some embodiments of the present disclosure. As depicted, a second optical pathway along which a retroreflected beam <NUM> travels back from the measurement spot <NUM> on the retroreflective surface of the DUT <NUM> to an analysis aperture <NUM> in front of a detector-amplifier <NUM>, for example, a photometrically corrected detector/amplifier or a color measurement detector/amplifier. The retroreflected beam <NUM> first goes through a lens aperture/iris <NUM> and then reflected by a moving (motorized) mirror/scanner <NUM> that controls the position of the measurement spot <NUM> on the DUT <NUM>. To maintain the concentricity of the illumination spot <NUM> and measurement spot <NUM> on the DUT <NUM> (see, e.g., <FIG>), the motorized mirrors <NUM> and <NUM> are moved in synchronization (controlled by a processor <NUM>). The convergence of the spots <NUM> and <NUM> is maintained by adjusting the angle of the steering mirror <NUM> and the position of the goniometer <NUM> on the linear stage <NUM> as the distance to the DUT <NUM> changes.

In some embodiments, an imaging lens <NUM>, for example, a zoom lens system includes a variable focal length between <NUM> and <NUM>. This achieves a constant magnification for any distance to the DUT between <NUM> and <NUM> meters. A camera <NUM> provides a live image of the DUT, and in conjunction with a display <NUM>, aids in selecting a scan / measurement location on the DUT. For example, the retroreflectometer may be pointed at the DUT and the display is the viewfinder allowing the alignment.

The retroreflected beam from the motorized scanner and beamsplitter <NUM> enters an analysis aperture <NUM> and enters a detector-amplifier <NUM> that is coupled in conjunction with the pulsed illumination source (e.g., <NUM> in <FIG>). The retroreflected light reaching the detector-amplifier <NUM> produces a photocurrent proportional to the intensity of the received light. A high-gain current-to-voltage amplifier is used to provide voltage signal to an analog-to-digital converter (ADC) (shown as parts of the detector-amplifier <NUM>) that converts a voltage to a digital number. Although the detector and ADC are shown in the same block <NUM> for simplicity, one skilled in the art would recognize that these are separate electrical devices and may be implemented separately.

The detector is positioned on a linear stage <NUM> which in some embodiments assists in controlling the measurement spot size on the DUT <NUM>. The scanner with a beam splitter (shown as part of <NUM>) allows for scanning of the image plane by the analysis aperture <NUM>, for example, at a <NUM>-<NUM> rate. The measurement circuit <NUM> converts the raw optical signal into an electrical signal. To eliminate the effects of ambient light, the measurement circuit is AC-coupled to the pulsed light source. In some embodiments, a processor <NUM> is electrically coupled to the ADC (part of <NUM>) with an accompanying memory to store operating logic (firmware and/or software) to determine the photometric intensity of a predetermined pattern of the retroreflected beam incident to the detector which defines the retroreflected light which propagates from the retroreflective surface, using know methods. For example, output from the detector-amplifier <NUM> is delivered to the processor <NUM> with associated controls, memory, power supply, and analog-to-digital converter. The processor <NUM> is coupled to user-interface and visual-display components <NUM>. The processor <NUM> is also coupled to a global position sensor <NUM>. One or more sensors (e.g., <NUM> in <FIG>) enable recordation of the location of the sign or marker being measured. The processor <NUM> provides the information for the retroreflective performance of the DUT, for example, on the display <NUM>.

<FIG> is an exemplary block diagram of a retroreflectometer, according to some embodiments of the present disclosure. As shown the retroreflectometer includes a light source (e.g., <NUM> in <FIG>) for emitting a light beam; a first moving mirror assembly (e.g., <NUM> in <FIG>) for scanning the light beam; a collimating lens (e.g., <NUM> in <FIG>) for collimating the scanning light on an illumination spot on the surface of the DUT; an imaging lens (e.g., <NUM> in <FIG>) for receiving a reflected scanning light comprised of the collimated scanning light reflected from the surface of the DUT; a second moving mirror assembly (e.g., <NUM> in <FIG>) for controlling a predetermined observation angle, wherein the first moving mirror assembly and the second moving mirror assembly moved in synchronization to maintain concentricity of the illumination spot on the surface of the DUT; a light collector <NUM> for collecting the reflected light from the second moving mirror assembly; a processor <NUM> including a memory for determining the optical characteristics of the surface of the DUT responsive to the collected reflected light.

In some embodiments, the distance and orientation to a DUT <NUM> is determined, for example, by a laser range finder <NUM>. Next, a moving illuminator <NUM> moves to the proper spacing away from the light collector based on the distance from the DUT to maintain a proper observation angle <NUM> by invoking the law of tangents to calculate the proper observation angle between the illuminator <NUM> and the light collector <NUM> for example, according to ASTM specifications. A light collector with scanner <NUM>, similar to the receiving (collection) system of <FIG> collects the reflected light from the illuminator <NUM> and reflected off the DUT <NUM>. In some embodiments, both the moving illuminator <NUM> and light collector <NUM> scans the DUT surface within the pre-defined angular range <NUM>, for example, utilizing a steering mirror <NUM>, in the horizontal plane using burst mode while data is being collected (<NUM>).

A measurement spot <NUM> on the DUT, for example, a STOP sign <NUM>, is selected from a measurement field <NUM>, by scanning <NUM> the measurement field. For small areas, like the legend on a STOP sign, scanning the measurement field and selecting the peak measurement across all the measurements ensures that the measurement field is directly on the legend and not partially off of it. The light collector with scanner <NUM> collects the reflected light, for example, via a beamsplitter <NUM>, a goniometer <NUM> and a linear stage <NUM> (similar to <NUM> in <FIG>). A processor <NUM> with an accompanying memory determines the photometric intensity of a predetermined pattern of the retroreflected beam incident to the detector which defines the retroreflected light which propagates from the retroreflective surface and provides the information regarding the retroreflective performance of the DUT, as described above with respect to <FIG>.

In some embodiments, the retroreflectometer rejects the light from the illuminator <NUM> after it passes through the beamsplitter <NUM>, for example using a light trap <NUM>, so that the light collector <NUM> will not see the light scattered inside the instrument.

<FIG> depict various views of a retroreflectometer, according to some embodiments of the present disclosure. <FIG> shows the left side view, <FIG> shows the rear view, <FIG> shows the right view, <FIG> shows the top view and <FIG> shows the front view. The front side view shows the optics and the rear side view shows the display and control switches. In some embodiments, there are two handles - one at each side of the portable retroreflectometer for eases of handling and, moving and targeting the portable retroreflectometer.

Mounting Feature <NUM> and <NUM> facilitate mounting the retroreflectometer on a vehicle/platform. Measurement handles <NUM> & <NUM> help secure the device housing <NUM>. Rear panel control buttons <NUM> are part of the user interface to input commands to the device and display <NUM> displays information, while a thermal printer module <NUM> is used to print reports and other information. A rechargeable battery <NUM> powers the device. Instrument measurement windows <NUM> and <NUM> are used to perform the measurements and laser range finder <NUM> is used to measure the distance to the DUT. Carrying handles <NUM> and <NUM> are used to carry the device. A GPS antenna <NUM> receives (and transmits) GPS signals. A power inlet <NUM> connects the device to a vehicle (or other platforms), and a power switch <NUM> is used to turn the power on/off. A laser range finder <NUM> is used to measure the distance to the DUT.

Claim 1:
A retroreflectometer for non-contact measurements of optical characteristics of a surface of a device under test (DUT) (<NUM>) comprising:
a light source (<NUM>) for emitting a light beam, in a first optical path;
a first moving mirror assembly (<NUM>) for scanning the light beam;
a collimating lens (<NUM>) for collimating the scanning light on an illumination spot (<NUM>; <NUM>) on the surface of the DUT;
an imaging lens (<NUM>) for receiving a reflected scanning light comprised of the collimated scanning light reflected from a measurement spot (<NUM>; <NUM>) on the surface of the DUT, in a second optical path different from the first optical path;
a second moving mirror assembly (<NUM>) for controlling a predetermined observation angle (<NUM>), wherein the first moving mirror assembly (<NUM>) and the second moving mirror assembly (<NUM>) move in synchronization to maintain concentricity of the illumination spot (<NUM>) and the measurement spot (<NUM>) on the surface of the DUT, and wherein the retroreflectometer is arranged so that the predetermined observation angle (<NUM>) is determined by determining the orientation of the DUT in space by triangulating the distance to three points on the surface of the DUT and setting the observation angle (<NUM>) based on the measured distance from the DUT;
a light collector (<NUM>) for collecting the reflected light from the second moving mirror assembly (<NUM>); and
a processor (<NUM>) including a memory for determining the optical characteristics of the surface of the DUT responsive to the collected reflected light, wherein the processor (<NUM>) controls the synchronization of the first moving mirror assembly (<NUM>) and the second moving mirror assembly (<NUM>).