I find the functioning of a laser rangefinder confusing. The explanation usually

Question

I find the functioning of a laser rangefinder confusing.

The explanation usually goes like this: "you shine a laser beam onto the object, the laser beam gets reflected and gets back to the device and time required for that is used to calculate the distance".

Okay. But the object surface can be uneven and not perpendicular to the laser beam so only a tiny fraction of beam energy is reflected back to the device. And there's plenty of other radiation around, sunlight included.

How does a rangefinder manage to "see" that very weak reflected signal in a reliable manner?

Explanation / Answer

so only a tiny fraction of beam energy is reflected back to the device.

This tiny fraction is enough. With respect to ambient light: One can modulate the laser beam, and filter the the voltage of the receiving photodiode for this modulation frequency and phase. Another precaution is to have a light filter in front of the receiving photodiode which only lets the wavelength of the laser pass. I think both precautions are used. And of course the receiving photodiode is focussed to a spot of some centimeters diameter around the laser spot. Try to point the range finder to a mirror, in that case the range finder should fail, exept the mirror happens to reflect precisely back to the range finder (which is rather unlikely). Reason is that from a (clean) mirror You don't get a stray reflection.

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