How does the laser rangefinder work?

[vlaiv]

Not the fancy big one - like lunar ranger laser thing, but rather those pocket units that you can use to measure your house or distances in your back yard?

I recently contemplated distances for artificial star to minimize spherical aberration in telescope testing and then I realized - it is not easy to measure say 60 or more meters precisely on rough terrain. For that reason I searched the net and found out that these hand held laser units are quite precise (about 2mm or so) and rather affordable - there are models for less than £50 that have ranges 80+ meters.

Then it hit me - I have no clue how they operate. I do have good understanding of how they could possibly work:

- pulse measurement (lunar ranger experiment)

- intereference of light

- amplitude modulation interference

First one is of course very difficult for small distances with hand held unit. Speed of light is 300,000 km/s or 300 km in millisecond, or 300 meters in micro second or 30cm in nano second - see, we are approaching atomic clock speeds here, do you know a clock that can measure nano seconds with precision - not to mention the need to measure less than 1% of that to get 0.3cm accuracy.

Second is good for very short distances because wavelength of light is in nano meters - less than one micron. It is perfect for measuring precision curve on telescope mirror and such - but not good on 80m+ distances.

Third one seems like reasonable option - except, how strong does laser need to be and how good sensor needs to be to measure enough photons per part of second needed to get down to two millimeters of precision.

Good thing about amplitude modulation interference is that one can vary frequency of modulating wave and that gives us rather good way to "switch" between distance units. We can use one frequency to measure meters with some precision and then use other frequency to measure 1/10th of meter and then another again for 1/100th of meter and then fourth frequency for 1/1000th of meter.

However, we need something like 1mm wavelength for 2mm precision. Speed of light is again 300,000km/s and that converted in millimeters per second is again very short amount of time. What kind of sensor can do exposure that fast, and how many photons can we hope to capture to get decent SNR and figure out where on amplitude curve we are currently?

Can anyone shine some (laser) light on this?

[andrew s]

Try this How they work Regards Andrew 

[johninderby]

Interesting thread on the subject.

https://physics.stackexchange.com/questions/255947/how-do-laser-tape-measures-work

[vlaiv]

3 minutes ago, johninderby said:

Interesting thread on the subject.

https://physics.stackexchange.com/questions/255947/how-do-laser-tape-measures-work

This is basically the third method I've mentioned but with some fancy electronics to make it work