Time of flight camera
A continuous wave time of flight (CW-TOF) camera that modulates sensitivity of its photosensor during an exposure period at a frequency of modulation of structured light that the camera transmits to illuminate a scene that it images but phase shifted relative to phase of the transmitted light by a sampling phase offset modified by a sequence of N perturbation phase shifts λ n =2π(n−1)/N, (1≤n≤N), and modifies the structured light in synchrony with the sequence of perturbation phase shifts to reduce error due to multipath interference in determining distances to features in the scene.
1. A continuous wave time of flight (CW-TOF) camera operable to determine distances to features in a scene, the CW-TOF camera comprising:
a light source operable to transmit light to illuminate the scene;
a photosensor having pixels configured to register amounts of light reflected by features in the scene from the transmitted light; and
a controller configured to:
control the light source to transmit structured light modulated at a modulation frequency to illuminate the scene with a structured illumination pattern, wherein the structured illumination pattern comprises three illumination zones, wherein each of the three illumination zones comprises different intensity or wavelength of transmitted light;
turn on the photosensor for an exposure period to register light reflected by the features from the transmitted structured light;
modulate sensitivity of the photosensor during the exposure period at the frequency of modulation of the transmitted light but phase shifted relative to phase of the transmitted light by a sampling phase offset modified by a sequence of perturbation phase shifts λ n ;
control the light source to modify the modulated structured light with a sequence of optical pattern modifiers in synchrony with the sequence of perturbation phase shifts; and
determine distances to the features based on the light registered by the pixels.
2. The CW-TOF camera according to claim 1 wherein the structured illumination pattern generated by the structured light comprises a plurality of illumination zones comprising at least two different types of illumination zones and wherein during the exposure period features in the scene located in a same illumination zone and features in the scene located in different illumination zones of a same type are illuminated in accordance with a same sequence of optical pattern modifiers and features in the scene located in different types of illumination zones are illuminated in accordance with different sequences of optical pattern modifiers.
3. The CW-TOF camera according to claim 2 wherein the sequences of optical pattern modifiers for the different types of illumination zones are chosen so that multipath (MP) light that a given feature in the scene receives from other features in the scene is substantially the same for each perturbation phase shift in the sequence of perturbations that modify structured light that the feature receives from the light source.
4. The CW-TOF camera according to claim 3 wherein a pitch between different types of illumination zones is determined so that MP light that a given feature in the scene receives from other features in the scene is substantially the same for each perturbation phase shift in the sequence of perturbations that modify structured light that the feature receives from the light source.
5. The CW-TOF camera according to claim 4 wherein the different types of illumination zones are discrete illumination zones that are contiguous along boundaries.
6. The CW-TOF camera according to claim 5 wherein the optical pattern modulators are multiplicative constants.
7. The CW-TOF camera according to claim 6 wherein the constants are binary constants.
8. The CW-TOF camera according to claim 7 wherein the sequences of optical pattern modifiers associated with different illumination zones are cyclic permutations of each other.
9. The CW-TOF camera according to claim 8 wherein a number of perturbation phase shifts is equal to two and the sequence of perturbation phase shifts is (0°, 180°).
10. The CW-TOF camera according to claim 9 wherein the illumination zones comprise two different types of illumination zones, Z1 and Z2, for which the synchronized sequences of perturbation phase shifts and optical pattern modifiers during an exposure period are respectively [Z1: (0°, 180°); (1, 0)] and [Z2: (0°, 180°); (0, 1)], where in a set of brackets the first entry followed by a colon identifies the illumination zone, and the entries following the colon give the sequence of perturbation phase shifts and synchronized sequence of optical pattern modifiers for the identified zone respectively.
11. The CW-TOF camera according to claim 8 wherein a number of perturbation phase is equal to three and the sequence of perturbation phase shifts is (0°, 120°, 240°).
12. The CW-TOF camera according to claim 11 wherein the illumination zones comprise three different types of illumination zones, Z1, Z2, and Z3, for which the synchronized sequences of perturbation phase shifts and optical pattern modifiers during an exposure period are respectively [Z1: (0°, 120°, 240°); (1, 0, 0)], [Z2: (0°, 120°, 240°); (0, 1, 0)], and [Z3: (0°, 120°, 240°); (0, 0, 1)].
13. The CW-TOF camera according to claim 8 wherein a number of perturbation phase shifts during an exposure period is equal to four and the sequence of perturbation phase shifts is (0°, 90°, 180°, 270°).
14. The CW-TOF camera according to claim 13 wherein the illumination zones comprise four different types of illumination zones, Z1, Z2, Z3 and Z4 for which the synchronized sequences of perturbation phase shifts and optical pattern modifiers during an exposure period are respectively [Z1: (0°, 90°, 180°, 270°) (1, 0, 0, 1)], [Z2: (0°, 90°, 180°, 270°); (1, 1, 0, 0)], [Z3: (0°, 90°, 180°, 270°); (0, 1, 1, 0)] and [Z4: (0°, 90°, 180°, 270°); (0, 0, 1, 1)].
15. The CW-TOF camera according to claim 6 wherein the illumination zones comprise two different types of illumination zones, Z1 and Z2.
16. The CW-TOF camera according to claim 15 wherein a number of perturbation phase shifts during an exposure period is equal to two and the sequence of perturbation phase shifts is (0°, 180°).
17. The CW-TOF camera according to claim 16 wherein the synchronized sequences of perturbation phase shifts and optical pattern modifiers for the illumination zones during an exposure period are respectively [Z1: (0°, 180°); (1,0.5−E)] and [Z2: (0°, 180°); (0, 0.5+E)], where E<0.5.
18. The CW-TOF camera according to claim 16 wherein the synchronized sequences of perturbation phase shifts and optical pattern modifiers for the illumination zones during an exposure period are respectively
[Z1: (0°,180°); (1+k cos(0°+ξ), (1+k cos(180°+ξ)], and
[Z2: (0°,180°); (1+k cos(180°+ξ), (1+k cos(0°+ξ)].
19. The CW-TOF camera according to claim 15 wherein the number N of perturbation phase shifts is greater than two and the sequences of perturbation phase shifts and optical pattern modifiers for the illumination zones during an exposure period are respectively,
[Z1: (λ 1 , λ 2 , . . . λ N ); (1+cos λ 1 , 1+cos λ 2 , . . . , 1+cos λ N )] and
[Z2: (λ 1 , λ 2 , . . . λ N ); (1−cos λ 1 , 1−cos λ 2 , . . . , 1−cos λ N )].
20. The CW-TOF camera according to claim 1 wherein the different types of illumination zones are continuous illumination zones for which illumination of features in the scene are continuous functions of locations of the zones.
21. The CW-TOF camera according to claim 20 wherein the number N of perturbation phase shifts is greater than two and the illumination zones are defined by a phase ξ L that is a continuous function of location in the scene and a sequence of perturbation phase shifts and optical pattern modifiers during an exposure period for an illumination zone having phase ξ L , is one of the following:
[(λ 1 , λ 2 , . . . λ N )); (1+k cos(λ 1 +ξ L ), (1+k cos(λ 2 +ξ L ), . . . , (1+k cos(λ N +ξ L ))], or
[(λ 1 , λ 2 , . . . λ N ) (1+cos λ 1 cos ξ L ), (1+cos λ 2 cos ξ L ), . . . , (1+cos λ N cos ξ L )].