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Distance measurement using lasers and photodiode

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ikatz23

New Member
Hi,

I am new in a project where we have to measure the distance from a laser to an object. My workmates have told me that a laser is conducted to an optical fiber ended with a high numerical aperture lens. The light is reflected by the surface of the measured object, and conducted again by the optical fiber to a photodiode. The signal from the photodiode is conducted to an A/D conversor and after that we receive the info in a PC.

It appears that to get the distance to the object we have to process the intensity of the signal received from the photodiode, but I don't know what could we do with the intensity to get the distance. I have studied Optical Communications at the career and don't remember anything similar, does anybody know how could we get the distance to the object??

Thanks in advance!
 

MikeMl

Well-Known Member
Most Helpful Member
"Inverse square law" ring a bell?
 

ikatz23

New Member
I have searched on the Internet and people say that the inverse square law doesn't apply to lasers, so are they wrong or that is only when you work with "ideal" lasers?
 

BrownOut

Banned
I'm no expert in this field, but I would think that intensity is a poor way to measure distance, since it is affected by other things besides distance ( unless all other conditions are very carefully controlled). Back in the 80's I worked on ranging RADAR, and we used a frequency modulation technique for ranging. That would not be affected by any other conditions besides distance.
 

Noggin

Member
If you had a lens to make the laser spread slightly, that would lower the intensity over a distance.
 

Mr RB

Well-Known Member
I think the system you are talking about uses 2 receiver lenses, and compares the intensity returned from each, which varies in angle based on distance. Some industrial lasers used for measuring close distances (like the thickness of sheet metal) use this system.
 
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