# How does 3D Time-of-Flight (TOF) camera work?

This is my first post in this forum! So Point me if anything is not clear!

I want to know how TOF camera works? So I've done Google & read some nice explanation about How Phase difference between Transmitted & Received signals can be used to calculate the Depth of an Object from Camera with good example.

What i did understand:

1. Depth Can be measured by Calculating the Phase change of the Transmitted signal.
2. How a light source(IR) can be modulated & demodulated ? Answer
3. How to filter the reflected IR signal having the same modulation frequency? Answer

What i cannot understand:

1. How phase change of a continuous square wave signal can be used to Measure the depth?

Note: I'm talking about TI & Mesa Kind of TOF Camera's using IR Led

• My suggestions to increase your chances of receiving a good answer: * Don't use acronyms like TOF, unless they're very well known (like DSP). * Link to the references you have found and read. * Ask an specific question. The more specific, the better. Don't be afraid to open multiple questions on the different aspects of the camera you don't understand.
– MBaz
Dec 17 '14 at 17:54

The (unwrapped) phase between two constant same frequency signals is linearly related to any delay between the two signals.

Measuring phase of a fixed frequency periodic signal over many periods is sometimes more accurate than trying to detect delay locally (by zero crossing, etc.)

• thank you for your answer I'm basically new to this site. I couldn't understand what you are actually saying! Can you please explain a bit simpler? Dec 18 '14 at 5:28
• Draw 2 sine waves on a graph. Now redraw one of them shifted a little bit to the left or right. You will note that the sine's phase angle (arcsin(y)) with respect to some vertical line will change by the amount you shifted the waveform. Dec 18 '14 at 7:29

The method used by TI in their white paper uses some fancy processing to measure the phase difference, but the underlying principle is reasonably simple.

Each pulse of infrared travels at the speed of light. If you place an object right in front of the camera, then the pulses of light reflected back will be in phase with those transmitted - in other words, the leading edge of the received pulse will match the leading edge of the transmitted one, and the same for the trailing edges.

If the object is moved away from the camera, then the light will take a short time to reflect off it and get back to the camera. If you compare the edges of the reflected signal with those of the original, they will be time-shifted by a few nanoseconds. Since the speed of light in air is pretty much fixed, the time delay tells you how far away the object is.

The trick that TI are using is not to try to measure the exact timing of the edges of the signal (which is tricky on a weak signal), but to measure the degree of overlap between the reflected signal and a set of known square waves, that are synchronised to the outgoing pulses.