Apparatus and method for measuring retroreflectivity of a surface

1. An apparatus for measuring retroreflectivity of a surface, the apparatus comprising:

a light source for illuminating an area of said surface at an incident angle relative to the surface;

a detector for detecting light reflected back from an observation area of the surface at an observation angle relative to the surface;

a signal processing unit adapted to receive a detector signal from the detector and to determine, from at least the received detector signal, a measure of the retroreflectivity of at least a part of the surface;

at least one device for measuring a distance between the apparatus and the illuminated area, the device for measuring a distance between the apparatus and the illuminated area comprising the light source, the detector and the signal processing unit;

wherein the signal processing unit is further adapted to adjust the measure of the retroreflectivity responsive to the measured distance;

characterised in that the detector comprises a position-sensitive detector for detecting light as a function of a position within a light receiving area of the detector, wherein the signal processing unit is adapted to determine the distance from a position within the light receiving area of the detected light reflected back from the surface.

2. The apparatus according to claim 1 , wherein the detector comprises a 2-dimensional detector adapted to receive an image of the illuminated area of the surface; and wherein the signal processing unit is adapted to determine the distance from a position of the image on the 2-dimensional detector.

3. The apparatus according to claim 1 , wherein the signal processing unit is adapted to determine from a captured image of the illumination area the distance between the apparatus and the illuminated area by me of a triangulation operation.

4. The apparatus according to claim 1 , further comprising an optical system for directing a predetermined portion of a light beam emitted by the light source to the detector.

5. The apparatus according to claim 1 , wherein the signal processing unit is further adapted to perform a compensation of the detected reflected light for ambient light based on a detection of light reflected from the observation area.

6. The apparatus according to claim 1 , wherein the observation angle is smaller than the incident angle.

7. The apparatus according to claim 3 , wherein the signal processing unit is adapted to determine said distance from a position of a retroreflected stripe of light on the detector, using trigonometric relations.

8. The apparatus according to claim 4 , wherein the optical system is configured to direct a predetermined portion of the light beam emitted by the light source to a first position on the light receiving area, and wherein the optical system is further configured to direct at least a portion of the light reflected back from the surface to a second position, different from the first position, on the light receiving area.

9. The apparatus according to claim 5 , wherein the signal processing unit is adapted to perform the compensation of the detected reflected light for ambient light based on a detection of light reflected from a portion of the observation area different from the illuminated area.

10. The apparatus according to claim 5 , wherein the light source is configured to illuminate the illuminated area by flashes of light leaving the illuminated area unilluminated during an interval between consecutive flashes; wherein the detector is further adapted to detect light reflected back from the observation area during said interval between consecutive flashes; and wherein the signal processing unit is adapted to perform the compensation of the detected reflected light for ambient light based on a detection of light reflected from the illuminated area during said interval between consecutive flashes.

11. The apparatus according to claim 6 , wherein the apparatus measures retroreflective property of road markings.

12. The apparatus according to claim 11 , wherein the retroreflective property is measured from a moving vehicle.

13. A method for measuring retroreflectivity of a road surface while moving across the road surface, the method comprising:

providing an apparatus for measuring the retroreflectivity of the road surface while moving across the road surface, the apparatus comprising a light source for illuminating an area of said road surface at an incident angle relative to the road surface and a detector for detecting light reflected back from an observation area of the road surface at an observation angle relative to the surface;

receiving a detector signal indicative of an intensity of light reflected back from the illuminated area of the road surface, and detected by the detector at the observation angle relative to the surface responsive to the illumination of the illuminated area at an incident angle relative to the road surface;

determining, from at least the received detector signal, a measure of the retroreflectivity of the road surface; and

receiving from at least one measurement device at least one measurement indicative of a distance between the apparatus and the illuminated area;

wherein determining the measure of the retroreflectivity comprises adjusting the measure of the retroreflectivity responsive to the measured distance relative to a nominal distance.

14. The method according to claim 13 , wherein the at least one measurement device is different from said apparatus for measuring the retroreflectivity.

15. The method according to claim 14 further comprising determining an inclination of a vehicle relative to the road surface and computing the distance between the apparatus and the illuminated area from at least the determined inclination.

16. The method according to claim 14 , wherein receiving from at least one measurement device comprises receiving a measurement of the distance between the apparatus and the illuminated area from a range finder configured to directly measure said distance; wherein the range finder is separate from the apparatus.

17. An apparatus for measuring retroreflectivity of a road surface, the apparatus comprising:

a light source for illuminating an area of said road surface at an incident angle relative to the surface;

a detector for detecting light reflected back from an observation area of the road surface at an observation angle relative to the road surface;

a signal processing unit adapted to receive a detector signal from the detector and to determine, from at least the received detector signal, a measure of the retroreflectivity of at least a part of the road surface; and

at least one device for measuring a distance between the apparatus and the illuminated area,

wherein the signal processing unit is further adapted to adjust the measure of the retroreflectivity responsive to the measured distance;

wherein the observation angle is smaller than the incident angle.