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# distance measure

#### sml2010

##### New member
Hi to all

I need to measure some distance very accurate(0.0001 inch or 0.002milimeter).
Can you please help me to know about laser type that is very accurate for this request.
I want send a laser beam from a distance to a refrence point(or area) and in the refrence point i instal a head that be able to accept the laser beam and comprehension, speed and time or other factor of laser beam for measuring the distance between the lase beam source and head.

I want use it for measuring some distance between 0.01 inch(0.25 milimeter) to 200 inch(5000 milimeter)

Thank you

#### Sigurthr

##### Well-known member
There are two methods to measuring distance with a laser.

1) time based. You set a retroreflector at the point to be measured and determine how long it took the beam to reach that point and return to your laser distance measuring device, then divide by two to get the distance for one trip, and work out the distance from the speed of light in air at the ambient temperature. This is a very accurate method and is how it is done commercially, but it requires exceptionally precise equipment, and I haven't heard of it being replicated in the amateur world. *It can also be done with 1 laser and 1 receiver, both synchronised against a clock, instead of using a retroreflector, but synchronization is imperitive and exceptionally difficult. This is how the recent the experiment where neutrino speed was determined (using a set distance and requiring two coordinated synchronized detectors), and it took years to set up with billions of dollars investment. I don't think the synchronized Tx/Rx method is very viable outside of a lab.

2) divergence based. If you accurately measure the divergence of your laser you can determine how far away the spot is by accurately measuring the diameter of the beam or spot at the location to be measured. For example, if you had a 1mRad divergence on your laser, it's beam/spot would increase in diameter by 1mm every meter. The result is you shine your laser out on a certain location, go there, and measure the diameter of the spot. If it is 5mm in diameter more than the diameter of the beam at the aperture, your object/location is 5 meters away. This method is not as accurate, but is certainly easier to do.

I know you stated your target range is between 1/100 of an inch and 200 inches, but I fear even the time based method may be out of reach for this range (out of reach, not impossible!). The larger the distance to be determined, the more accurate the reading will be. Sure high grade industrial/scientific lab grade equipment can measure short distances that accurately, but you're talking about a piece of equipment that is way out of the budget of anyone who would come to this forum.

What is your application?

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