Optical rangefinder and optical rangefinding method

An optical rangefinder comprising:

• a light emitting element configured to emit pulsed measurement light;
• a plurality of photon-counting light receiving elements configured to detect reflected light from an object for the measurement light;
• an adder circuit configured to add a voltage pulse output from each of the light receiving elements;
• a time-of-flight measurement circuit configured to calculate, as a time of flight, time from an emission time point of the measurement light to a time point when a sum of the adder circuit reaches a predetermined addition threshold;
• a histogram generation circuit configured to generate a histogram showing a frequency distribution of the times of flight, as calculated by the time-of-flight measurement circuit for repeated emission of the measurement light at predetermined periods, over respective regions on a time axis divided into a plurality of regions at predetermined time intervals;
• a representative value calculation circuit configured to calculate a representative value of the times of flight for the object from the times of flight that are distributed over the regions where the frequency reaches a predetermined histogram threshold based on the histogram;
• and
• a distance calculation circuit configured to calculate a distance to the object based on the representative value of the times of flight calculated by the representative value calculation circuit, wherein
• the distance calculation circuit includes an error correction circuit configured to calculate an error correction value based on a frequency-error characteristic and correct the distance or the representative value of the times of flight with the error correction value, the frequency-error characteristic being determined by an error, the error being obtained from the frequency, as obtained by the histogram generation circuit with intensity of the reflected light varied in advance, and difference between an actual distance and the distance or between an actual time of flight and the representative value of the times of flight,
• the error correction circuit is configured to calculate the error correction value based on a duration-error characteristic and correct the distance or the representative value of the times of flight with the error correction value after the frequency of the frequency-error characteristic is saturated, the duration being a period of time in which the sum of the adder circuit is equal to or greater than the addition threshold, with the intensity of the reflected light varied in advance, and the duration-error characteristic being determined by an error obtained from a difference between the actual distance and the distance, or a difference between the actual time of flight and the representative value of the times of flight.