I was looking a varioos laser measuring devices (laser tape measures). Most of them seem to be accurate within 1/8th of an inch. I'm wondering what the limiting factor for accuracy is. Is it simply a calibration issue? the quality of the electronics?


-The way how the laser is used is the limiting factor. I list a few ways and their accuracy that I can think of.

  1. Angle of reflection: angle is measured with a line sensor. The location of the peak intensity is proportional to the distance. Accuracy is related to the line sensor pixel size and the algorithm. I have seen accuracy = 100 um sensors with range of 30 to 50 mm, so 0.05% of the distance.

  2. time of flight: Calculate how long does it take for pulse to travel to target and back. Accuracy related to sampling speed and measured distance... 1/8th of an inch requires 5 GHz sampling rate.

  3. interferometry: accuracy is the same order as wavelength of the laser (red 600 nm). This requires some mirrors and steady instrumentation (read: expensive), so not for all applications.

I think that your tape measure uses time of flight (TOF) measurement. There are specialized TOF chips, so you don't necessary need a fast microprocessor.

  • 4
    \$\begingroup\$ Time of flight is easy: light travels about 1 foot in 1 ns. To get 1/8" accuracy (about 0.01 foot), you need to measure time (round trip) with an accuracy and resolution of around 20 ps. There are ways to do this, but they aren't cheap. \$\endgroup\$
    – Dave Tweed
    Dec 8 '12 at 17:27
  • \$\begingroup\$ The "Angle of reflection" method is more commonly known as "structured light". en.wikipedia.org/wiki/Structured_light \$\endgroup\$
    – user3624
    Dec 9 '12 at 16:26

@Juha's answer is great (and correct). To contribute some additional thoughts...

Many of the cheap "laser" tape measures are not using lasers for the distance measurement. They consist of an ultrasonic distance measurement (sound), plus a cheap class-1 laser-pointer to help you aim it.

All of the medium price point laser distance sensors I have seen use @Juha's technique #1 (angle of arrival). For example, this popular unit from Stanley/Bosch

It's also very common in robotics due to its lower cost and lower power requirements. For even lower cost/power, you can use non-collimated light sources like in this infra-red LED-based sensor.


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