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How does Laser Scanning work?

Time of Flight

The simplest way to think about “time of flight” laser scanning is to look approach it as nothing more than a laser range finder.  I think we all understand how a laser range finder works.  It basically measures a distance by shooting a laser beam out to an object and measuring how long it takes that laser beam to bounce back.  By knowing the speed of light (laser beam) and measuring the time, I think we can all figure out that the distance is computed by multiplying the time divided by two and multiplying it by the constant for the speed of light.  Anyone that understands surveying realizes that to compute an x,y,z position, you need to know the distance to the object, the bearing (horizontal angle from a known line) to the object and the vertical angle (angle from gravity) to the object.


Phase and Time of Flight Comparison

Therefore a time of flight scanner does nothing more than measure the distance and horizontal and vertical angle for every position it is at. This is accomplished by the scanner moving in a grid fashion in a 360° horizontal plane and approximately 330° vertical plane.  This is why “time of flight” scanning is substantially slower than “phase based” scanning.  The scanner head must actually occupy all of the grid positions for each x,y,z position.  Time of flight scanning has been known for longer range, but a slower acquisition time.  Acquisition speed has always hovered right around 50,000 points per second, but with a range of 300 meters.  Just recently a million points per second “time of flight” scanner was brought to market, but the range is only 120 meters.  More in line with the phase based scanners.  However the range was decreased to 120 meters, down from 300+ meters.   It will be interesting to see which technology wins out, “time of flight” or “phased based”.

Phase Based

Phase based scanning utilizes a constant beam of laser energy that is emitted from the scanner.  The scanner then measures the phase shift of the returning laser energy to calculate distances.  Everything else is the same as the “time of flight” scanner.  Phase based scanners can acquire data at a rate upwards of a million points per second.  However their range is limited to around 80 meters.  Phase based scanners are mostly suited for industrial plant work and building interiors.

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