Apparatus and method for determining precision reflectivity of highway signs and other reflective objects utilizing an optical range finder instrument

• US 6,212,480 B1
• Filed: 01/25/2000
• Issued: 04/03/2001
• Est. Priority Date: 07/27/1998
• Status: Expired due to Term

• Current Assignee: Kama-Tech (HK) Limited
• Original Assignee: Laser Technology Incorporated (Laser Technology)
• Inventors: Dunne, Jeremy G.
• Primary Examiner(s): SHAH, KAMINI S
• Application Number:
  US09/491,291
• Time in Patent Office: 434 Days
• Field of Search: 702/159, 342/23, 342/54, 342/94, 342/132, 342/95, 356/5.01, 358/475, 358/445, 359/885
• US Class Current: 702/159
• CPC Class Codes: G01S 17/88 Lidar systems specially adapted for specific applications
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating

Abstract

An apparatus and method for measuring coefficients of retroreflectance of retroreflective surfaces such as road signs involves use of a modified light based range finder. The apparatus includes a power attenuation factor data base which relates pulse width of received pulses to power attenuation of the transmitted pulses. The range finder calculates target range based on time of flight of light pulses. The apparatus automatically calculates the absolute coefficient of retroreflectance for an unknown reflective surface being measured by comparison of the measurement to a reading with the same instrument of a known reflectance standard. The method involves either recalling a stored standard reference reflectance factor or determining a reflectance factor via the range finder for a sample of retroreflective material with a predetermined coefficient of retroreflectance, and then measuring the distance to an unknown target, determining a power attenuation factor from the received pulse width from the unknown target and then calculating the absolute coefficient of retroreflectance based upon these determined values of power attenuation factor, distance and the reference reflectance factor.

Claims

1. A method of measuring an absolute coefficient of retroreflectance for a reflective surface utilizing a signal transmitting and receiving range finder comprising the steps of:

providing a reference reflectance factor in said range finder for an unknown target retroreflective surface;

measuring a power attenuation factor for an unknown target;

calculating a coefficient of retroreflectance of said unknown target from said measured power attenuation factor and said reference reflectance factor.

2. The method according to claim 1 wherein said step of providing comprises:

providing a database of power attenuation factors (Ka) for relative measurements of return energy of received return signal pulses in said range finder;

measuring a first distance (d_ref) to a sample of said standard retroreflective surface by transmitting a signal toward and receiving a return signal with said range finder;

determining a first relative measurement of return energy from said return signal;

determining a first power attenuation factor (Ka_ref) for said first relative measurement of return energy from said power attenuation factor database; and

computing said reference reflectance factor (K_tref) of said sample from said first power attenuation factor, said first measured distance and a predetermined coefficient of retroreflectance (RA_ref) for said sample.

3. The method according to claim 2 wherein said step of computing is performed according to the equation K_tref=RA_ref*Ka_ref/d_ref².

4. The method according to claim 2 wherein said step of measuring includes aiming said range finder and transmitting a second signal at an unknown target and determining a distance (d_unknown) to said umnknown target from a second return signal received in said range finder.

5. The method according to claim 4 wherein said step of measuring further includes determining a second power attenuation factor (Ka_unknown)for said unknown target from said power attenuation factor database.

6. The method according to claim 5 wherein said step of calculating is performed according to an equation RA=K_tref*d_unknown²/Ka_unknown.

7. The method according to claim 1 wherein said step of providing includes recalling a reference reflectance factor (K_tref) for said unknown target from a database in said range finder.

8. The method according to claim 7 wherein said step of measuring comprises the steps of:

measuring a distance (d_unknown) to said unknown target by transmitting a signal toward said unknown target and receiving a return signal from said target;

determining a relative measurement of return energy of said return signal from said unknown target; and

determining a power attenuation factor (Ka_unknown) for said relative measurement of return energy of said return pulse.

9. The method according to claim 8 wherein said step of calculating comprises computing said coefficient of retroreflectance according to the equation RA=K_tref*d_unknown²/Ka_unknown.

10. A method of remotely determining a coefficient of retroreflectance of an unknown retroreflective surface comprising:

providing a signal transmitting and receiving range finder for transmitting a signal toward a target and receiving a return signal from said target, said range finder having a processor for determining distance from said range finder to said target and including a return relative measurement of return energy determining circuit therein and an attenuation factor database containing power attenuation factors for various return relative measurements of return energy;

transmitting a first signal toward a standard target having a known coefficient of retroreflectance;

receiving a first return signal from said standard target;

determining a first relative measurement of return energy for said first return signal;

determining a distance from said range finder to said standard target from a time of flight of said transmitted signal to and from said target;

determining a first power attenuation factor for said return signal from said first relative measurement of return energy and said power attenuation factor database;

calculating a reference reflectance factor for said standard target from said distance to said standard target, said known coefficient of retroreflectance and said first power attenuation factor;

transmitting a second signal toward an unknown target;

receiving a second return signal from said unknown target;

determining a second relative measurement of return energy for said second return signal from said unknown target;

determining a distance from said range finder to said unknown target;

determining a second power attenuation factor for said return signal from said unknown target; and

calculating an absolute coefficient of retroreflectance for said unknown target from said reference reflectance factor, said second power attenuation factor and said distance to said unknown target.

11. A method of remotely determining a coefficient of retroreflectance of an unknown retroreflective surface comprising the steps of:

providing a signal transmitting and receiving range finder for transmitting a signal toward a target and receiving a return signal from the target, said range finder having a processor for determining distance from said range finder to said target and including a return relative measurement of return energy determining circuit therein, an attenuation factor database containing power attenuation factors for various return relative measurements of return energy and a reference reflectance factor database for various target retroreflective surfaces;

recalling from said reference reflectance factor database a reference reflectance factor for an unknown target;

transmitting a signal toward said unknown target;

receiving a return signal from said unknown target,

determining a relative measurement of return energy for said return signal from said unknown target;

determining a distance from said range finder to said unknown target;

determining a power attenuation factor for said unknown target from said relative measurement of return energy and said power attenuation factor database; and

calculating an absolute coefficient of retroreflectance for said unknown target from said reference reflectance factor, said power attenuation factor and said distance to said unknown target.

12. An apparatus for determining an absolute coefficient of retroreflectance of a retroreflective surface comprising:

a signal transmitter operable to transmit a signal toward a target;

a signal receiver operable to receive a reflected signal from said target;

a processor connected to said transmitter and receiver for determining a distance to said target from a time of flight of said transmitted and reflected signals to and from said target;

a relative measurement of return energy determining circuit connected to said receiver and said processor for determining a relative measurement of return energy of said reflected signal; and

a power attenuation factor database connected to said processor storing power attenuation factor data for various reflected relative measurements of return energy.

13. The apparatus according to claim 12 wherein said processor is operable to calculate an absolute coefficient of retroreflectance of a target from a reference reflectance factor for said target, said measured distance to said target and a power attenuation factor for said reflected relative measurement of return energy from said power attenuation factor database.

14. The apparatus according to claim 13 further comprising a database of reference reflectance factors coupled to said processor for various retroreflective surfaces.

15. The apparatus according to claim 14 further comprising a display connected to said processor for displaying said distance and said coefficient of retroreflectance.

16. The apparatus according to claim 15 wherein said transmitter and receiver are in a laser range finder.

17. The apparatus according to claim 16 wherein said range finder includes said database of reference reflectance factors.

18. A signal transmitting and receiving range finder adapted for use in determining coefficients of retroreflectance of retroreflective target surfaces, said range finder comprising:

a signal transmitter for transmitting a signal toward a target;

a signal receiver for receiving a return signal reflected from said target;

a microprocessor operably connected to said transmitter and said receiver programmed to compute coefficients of retroreflectance;

a relative measurement of return energy measuring circuit coupled to said microprocessor for measuring a relative measurement of return energy of a return signal; and

a power attenuation factor database operably coupled to said microprocessor for determining a power attenuation factor (Ka) for a relative measurement of return energy, wherein said microprocessor utilizes measured return signals and power attenuation factors from said database to calculate a coefficient of retroreflectance (RA) for said target.

19. The apparatus according to claim 18 wherein said range finder includes a database of reference reflectance factors (K_tref) for retroreflective surfaces.

20. The apparatus according to claim 18 wherein said microprocessor is programmed to compute a coefficient of retroreflectance (RA) according to an equation RA=K_tref*d_unknown²/Ka_unknown.