Reduction in camera to camera interference in depth measurements using spread spectrum
Reduction in interference between different time of flight (ToF) cameras used for depth measurements and operating in the same application environment is achieved using a spread spectrum technique in which the cyclical operations of a pulsed light source such as a laser or light emitting diode (LED) and gated image sensor are varied in a pseudo-random manner in each camera. In an alternative embodiment, spread spectrum logic is applied in a ToF camera that employs phase modulation techniques.
1. A time of flight (ToF) camera configured for optical depth sensing supporting reduced camera-to-camera interference in a multi-camera environment, comprising:
a source configured for emitting light pulses to illuminate a surveillance area;
an image sensor configured for collecting light pulses reflected from the surveillance area and including a gate that is operated with a time difference with respect to the emission from the source; and
a controller operatively coupled to the light source and the gated image sensor, the controller including a timing module applying spread spectrum logic, the controller configured for
cyclically operating the source to emit light pulses and the gated image sensor to collect light pulses, wherein the operating is convolved so that light pulses are collected for a defined range of distances between the ToF camera and the surveillance area, and
utilizing a pseudo-random sequence of periods for the cyclical operation wherein the pseudo-random sequence of periods spans a range of time differences associated with the defined range of distances.
2. The ToF camera of claim 1 in which the cyclical operating of the source and gated image sensor is performed synchronously to generate a depth map of portions of the surveillance area corresponding to the defined range of distances.
3. The ToF camera of claim 2 further including a memory for storing the depth map.
4. The ToF camera of claim 2 further including reading out depth data from the gated image sensor.
5. The ToF camera of claim 4 further including pseudo-randomly varying the time difference between successive operations of light pulse generation using a predetermined number of time differences.
6. The ToF camera of claim 5 further including performing the cyclical operating using a count that exceeds the predetermined number of time differences by at least an order of magnitude.
7. The ToF camera of claim 1 further including optical components selected from one of lens or filter.
8. The ToF camera of claim 1 further including matching a contrast ratio function of the gate to a spectrum of the source so as to maximize the convolution.
9. The ToF camera of claim 8 in which maximization of the convolution substantially centers the spectrum within an operating band of the gate.
10. The ToF camera of claim 1 in which the defined distance range comprises a predetermined slice of the surveillance area.
11. A method for reducing interference among multiple instances of time of flight (ToF) cameras used to render a depth image of a scene, comprising:
illuminating the scene using light pulses from a source that are generated in a cyclical sequence;
capturing light at a gated image sensor in a ToF camera in each cycle of the sequence, the image sensor having a plurality of light sensitive pixels each of which provides an output responsive to light captured thereon; and
configuring a controller to operate the source to generate light pulses and operate a gate of the image sensor, the operating being responsive to convolved timing signals so that depth is measured over a predetermined range of distances, the timing signals causing the sequence of light pulses to have a pseudo-randomly selected period for each cycle; and
generating a period of variable length by pseudo-randomly varying a time delay between successive light pulses, in which the pseudo-randomly varying time delay exceeds a duty cycle length of a ToF camera.
12. The method of claim 11 further including distributing the pseudo-random time delays over the cyclical sequence by using a non-constant period for each cycle.
13. The method of claim 11 further including generating the light pulses with the pseudo-random periods so that each ToF camera operates in an asynchronous manner with respect to other camera instances.
14. The method of claim 11 further including integrating the time delays of light pulses received from other ToF cameras over multiple operating cycles so that the time delays are substantially evenly distributed across the predetermined range of distances to thereby mimic an effect of ambient light.
15. The method of claim 11 in which the time delays received at an image sensor are integrated into a signal that is unrelated to depth.