Doppler time-of-flight imaging
Systems and methods for imaging object velocity are provided. In an embodiment, at least one Time-of-Flight camera is used to capture a signal representative of an object in motion over an exposure time. Illumination and modulation frequency of the captured motion are coded within the exposure time. A change of illumination frequency is mapped to measured pixel intensities of the captured motion within the exposure time, and information about a Doppler shift in the illumination frequency is extracted to obtain a measurement of instantaneous per pixel velocity of the object in motion. The radial velocity information of the object in motion can be simultaneously captured for each pixel captured within the exposure time. In one or more aspects, the illumination frequency can be coded orthogonal to the modulation frequency of the captured motion. The change of illumination frequency can correspond to radial object velocity.
1. A method for imaging an object's velocity, the method comprising the steps of:
illuminating an object in motion with an amplitude-modulated signal g having an illumination frequency ω g ;
capturing, using a Time-of-Flight camera, first and second signals s representative of the object in motion, each over an exposure time T;
modulating each of the first and second signals s with a modulation frequency ω f within the exposure time T, at a sensor of the Time-of-Flight camera, to obtain corresponding first and second modulated signals ĩ ψ wherein the first modulated signal ĩ ψ corresponds to a heterodyne measurement, and the second modulated signal ĩ ψ corresponds a homodyne measurement;
and
extracting information about a Doppler shift from the first and second modulated signals ĩ ψ to obtain a measurement of an instantaneous per pixel radial velocity of the object in motion,
wherein for the heterodyne measurement, the illumination frequency ω g is different from the modulation frequency ω f , and for the homodyne measurement, the illumination frequency ω g is equal to the modulation frequency ω f .
2. The method of claim 1 , wherein the radial velocity of the object in motion is simultaneously captured for each pixel captured within the exposure time.
3. The method of claim 1 , wherein the illumination frequency is coded orthogonal to the modulation frequency so that the exposure time T is an integer multiple of periods corresponding to the illumination frequency and the modulation frequency.
4. The method of claim 1 , wherein the instantaneous per pixel radial velocity of the object in motion is given by a ratio of (1) the first modulated signal ĩ ψ corresponding to the heterodyne measurement, and (2) the second modulated signal ĩ ψ corresponding to the homodyne measurement.
5. The method of claim 1 , wherein the heterodyne measurement is simultaneously captured with the homodyne measurement on corresponding Time-of-Flight cameras.
6. The method of claim 1 , wherein the exposure time is longer than the wavelength of a modulated captured signal.
7. The method of claim 1 , wherein the heterodyne measurement and the homodyne measurement are alternately captured with the Time-of-Flight camera.
8. The method of claim 1 , further including the step of: simultaneously capturing color, depth and velocity information concerning the object in motion during the exposure time.
9. The method of claim 8 , wherein an optical flow of the object in motion is computed on red, green and blue (RGB) frames within a measured change in the illumination frequency.
10. The method of claim 9 , including estimating a 3D velocity field for the object in motion.
11. The method of claim 1 , wherein depth and velocity imaging are combined either using the Time-of-Flight camera by alternating modulation frequencies between successive video frames over the exposure time or using at least two Time-of-Flight cameras.
12. A system for imaging an object's velocity, the system comprising:
at least one device for illuminating an object in motion with an amplitude-modulated signal q having an illumination frequency ω g and for capturing first and second signals s representative of the object in motion over an exposure time T;
at least one computing device comprising a processor and a memory; and
an application executable on the at least one computing device, the application comprising machine readable instructions stored in the memory that, when executed by the processor, cause the computing device to at least:
modulating each of the first and second signals s with a modulation frequency ω f within the exposure time T, to obtain corresponding first and second modulated signals ĩ ψ wherein the first modulated signal ĩ ψ corresponds to a heterodyne measurement, and the second modulated signal ĩ ψ corresponds a homodyne measurement;
and
extract information about a Doppler shift from the first and second modulated signals ĩ ψ to obtain a measurement of an instantaneous per pixel radial velocity of the object in motion,
wherein for the heterodyne measurement, the illumination frequency ω g is different from the modulation frequency ω f , and for the homodyne measurement, the illumination frequency ω g is equal to the modulation frequency ω f .
13. The system of claim 12 , wherein the device is at least one Time-of-Flight camera.
14. The system of claim 12 , wherein the radial velocity of the object in motion is simultaneously captured for each pixel captured within the exposure time.
15. The system of claim 12 , wherein the illumination frequency is coded orthogonal to the modulation frequency so that the exposure time T is an integer multiple of periods corresponding to the illumination frequency and the modulation frequency.
16. The system of claim 12 , wherein the instantaneous per pixel radial velocity of the object in motion is given by a ratio of (1) the first modulated signal ĩ ψ corresponding to the heterodyne measurement, and (2) the second modulated signal ĩ ψ corresponding to the homodyne measurement.
17. The system of claim 12 , wherein the heterodyne measurement is simultaneously captured with the homodyne measurement on corresponding Time-of-Flight cameras.
18. The system of claim 12 , wherein the logic captures color, depth and velocity information concerning the object in motion during the exposure time.
19. A non-transitory computer readable medium employing an executable application in at least one computing device, the executable application comprising machine readable instructions stored in the medium that:
controls illuminating an object in motion with an amplitude-modulated signal g having an illumination frequency ω g ;
captures first and second signals s representative of the object in motion over an exposure time T;
modulates each of the first and second signals s with a modulation frequency ω f within the exposure time T at a sensor of the Time-of-Flight camera, to obtain corresponding first and second modulated signals ĩ ψ wherein the first modulated signal ĩ ψ corresponds to a heterodyne measurement, and the second modulated signal ĩ ψ corresponds a homodyne measurement;
and
extracts information about a Doppler shift from the first and second modulated signals ĩ ψ to obtain a measurement of an instantaneous per pixel radial velocity of the object in motion,
wherein for the heterodyne measurement, the illumination frequency ω g is different from the modulation frequency ω f , and for the homodyne measurement, the illumination frequency ω g is equal to the modulation frequency ω f .
20. The non-transitory computer readable medium of claim 19 , wherein the first and second signals s are captured using at least one Time-of-Flight camera.