Multipath and noise reduction for time-of-flight cameras
A time-of-flight, ToF, camera for measuring distance information for objects of a scene. The ToF camera comprises a light source for illuminating the objects, an image sensor for acquiring light signals reflected from the objects, a processing unit for processing first portions of the reflected light signals modulated with oscillations of a first frequency, and for processing second portions of the reflected light signals modulated with oscillations of a second frequency. The second frequency is higher than the first frequency and a second distance measurement value range is smaller than a first distance measurement value range. The second frequency is a non-integer multiple of the first frequency. The processing unit is adapted to restrict, on the basis of a spacing, the determining of the second distance measurement value range to the zeroth repetition of a predetermined number of repetitions of the first distance measurement value range.
1. A time-of-flight (“ToF”) camera for measuring distance information for objects of a scene, the ToF camera comprising:
a light source for emitting modulated light signals for illuminating the objects;
an image sensor for acquiring light signals reflected from the objects;
a processing unit for processing first portions of the reflected light signals modulated with oscillations of a first frequency for determining first distance measurement values in a first distance measurement value range that is dependent on the first frequency and for processing second portions of the reflected light signals modulated with oscillations of a second frequency for determining second distance measurement values in a second distance measurement value range that is dependent on the second frequency, wherein the second frequency is higher than the first frequency and the second distance measurement value range is smaller than the first distance measurement value range, wherein the processing unit is adapted, in determining the second distance measurement values, to resolve ambiguities of the second distance measurement value range within the first distance measurement value range on the basis of the first distance measurement values and to generate a distance measurement image on the basis of only the second distance measurement values; and
an output or storage unit for outputting or storing the distance measurement image;
wherein the second frequency is a non-integer multiple of the first frequency, such that the end of a zeroth repetition of the first distance measurement value range lies within a repetition of the second distance measurement value range and a spacing from the end thereof, wherein the processing unit is adapted to restrict, on the basis of the spacing, the determining of the second distance measurement values to the zeroth repetition of a predetermined number of repetitions of the first distance measurement value range.
2. The ToF camera according to claim 1 , wherein, in resolving the ambiguities, a repetition of the second distance measurement value range is selected within the first distance measurement value range value by means of a scaling factor that is dependent on a ratio value.
3. The ToF camera according to claim 2 , wherein the ratio value is determined on the basis of the second distance measurement value range and a difference between the first distance measurement value and an intermediate value of the associated second distance measurement value, wherein for the intermediate value it is not yet determined in which repetition of the second distance measurement value range it lies.
4. The ToF camera according to claim 2 , wherein the restricting the determining of the second distance measurement values to the zeroth repetition of the first distance measurement value range occurs on the basis of the ratio value.
5. The ToF camera according to claim 4 , wherein the restricting the determining of the second distance measurement values to the zeroth repetition of the first distance measurement value range comprises a comparison with a threshold value, wherein the threshold comparison is dependent on the value of the decimal portion of the ratio value.
6. The ToF camera according to claim 5 , wherein the restricting the determining of the second distance measurement values to the zeroth repetition of the first distance measurement value range comprises a shifting of the ratio value by an offset, wherein the threshold comparison is dependent on the value of the decimal portion of the shifted ratio value, wherein the offset corresponds to half of the threshold value.
7. The ToF camera according to claim 1 , wherein the spacing of the ends lies in a range from one half to one tenth, one half to one fifth, or one half to one third of the second distance measurement value range.
8. The ToF camera according to claim 1 , wherein the first frequency lies in a range from 5 MHz to 25 MHz and the second frequency lies in a range from 50 MHz to 150 MHz.
9. The ToF camera according to claim 1 , wherein the oscillations of the first frequency or the oscillations of the second frequency are periodic sequences of square pulses, with a pulse-pause ratio of 1/2.
10. The ToF camera according to claim 9 , wherein the ToF camera is adapted, by integration of the reflected light signals over a plurality of integration phases that are offset in time with respect to one another, to generate a plurality of partial measurement images at different phase positions from one or more first sections of the reflected light signals or from one or more second sections of the reflected light signals, wherein the first distance measurement values and the second distance measurement values are determined from the partial measurement images.
11. The ToF camera according to claim 10 , wherein the integration phases each correspond to the duration of the square pulses or the time offsets each correspond to two thirds of the duration of the square pulses.
12. The ToF camera according to claim 10 , wherein three partial measurement images are generated from the one or more second sections of the reflected light signals and only one partial measurement image is generated from the one or more first sections of the reflected light signals.
13. The ToF camera according to claim 1 , wherein the light source is further adapted to emit one or more second sections, which are to be processed for determining the second distance measurement values, temporally before one or more first sections, which are to be processed for determining the first distance measurement values.
14. The ToF camera according to claim 1 , wherein the light source is further adapted to emit one or more first sections, which are to be processed for determining the first distance measurement values, and one or more second sections, which are to be processed for determining the second distance measurement values, in a temporally interleaved sequence.
15. The ToF camera according to claim 1 , wherein the ToF camera is adapted, in emitting the modulated light signals, to operate the light source during the one or more second portions with a longer exposure time or with a higher current or voltage than during the one or more first sections.
16. The ToF camera according to claim 1 , wherein the processing unit is adapted, in determining the first distance measurement values or the second distance measurement values, to perform a correction with respect to a frequency-dependent offset value.
17. The ToF camera according to claim 1 , wherein the processing unit is further adapted to process third portions of the reflected light signals modulated with oscillations of a third frequency for determining third distance measurements in a third distance measurement value range that is dependent on a third frequency, wherein the third frequency is higher than the first frequency and lower than the second frequency and the third distance measurement value range is smaller than the first distance measurement value range and larger than the second distance measurement value range, wherein the processing unit is adapted, in determining the third distance measurement values, to resolve ambiguities of the third distance measurement value range within the first distance measurement value range on the basis of the first distance measurement values and, in determining the second distance measurement values, to resolve ambiguities of the second distance measurement value range within the third distance measurement value range on the basis of the third distance measurement values.
18. The ToF camera according to claim 1 , wherein the ToF camera further comprises an interface for parameterizing the ToF camera, wherein a number of frequencies used for distance measurement or a ratio of at least two of the first frequency, the second frequency and, if provided, the third frequency can be set by the parameterization.
19. A time-of-flight (“ToF”) method for measuring distance information for objects of a scene, the ToF method comprising:
emitting modulated light signals for illuminating the objects;
acquiring light signals reflected from the objects;
processing first portions of the reflected light signals modulated with oscillations of a first frequency for determining first distance measurement values in a first distance measurement value range that is dependent on the first frequency, and processing second portions of the reflected light signals modulated with oscillations of a second frequency to determine second distance measurement values in a second distance measurement value range that is dependent on the second frequency, wherein the second frequency is higher than the first frequency and the second distance measurement value range is smaller than the first distance measurement value range, wherein the processing, in determining the second distance measurement values, resolves ambiguities of the second distance measurement value range within the first distance measurement value range on the basis of the first distance measurement values and generates a distance measurement image on the basis of only the second distance measurement values; and
outputting or storing the distance measurement image;
wherein the second frequency is a non-integer multiple of the first frequency such that the end of a zeroth repetition of the first distance measurement value range lies within a repetition of the second distance measurement value range and a spacing from the end thereof, wherein the processing restricts, on the basis of the spacing, the second distance measurement values to the zeroth repetition of a predetermined number of repetitions of the first distance measurement value range.