# Laser TOF distance measurement

Discussion in 'Off-Topic' started by jpanhalt, Jul 16, 2010.

1. ### jpanhalt Thread Starter Expert

Jan 18, 2008
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This is off topic, because, well, there are no current topics related to it. As frequent visitors to this forum will know, the subject of object detection and ranging comes up with some frequency.

When light is mentioned, comments to the effect that the time differences are too small to be done at low cost. I often respond that my Stanley FatMax works pretty good. Technology marches on, if you can believe the specifications: accuracy ±1/16" (< 2mm); range 195 feet; cost <<\$100.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=120596525141&ssPageName=STRK:MEWAX:IT

John

2. ### beenthere Retired Moderator

Apr 20, 2004
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I ran rod on a survey party as summer jobs. Holding a steel tape against 40 pounds force to work to an accuracy of 1/10 inch in 100 feet. The laser goodie sounds amazing to me.

3. ### hgmjr Retired Moderator

Jan 28, 2005
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John,

Do you happen to know how this particular unit manages to measure the small times that are associated with speed of light? I believe the estimate of light speed is around 1 foot per nanosecond. That would mean that the max distance (195 Feet) would have a time of flight on the order of 400 nanoseconds.

hgmjr

4. ### jpanhalt Thread Starter Expert

Jan 18, 2008
7,183
1,584
I suspect it is a small improvement on my FatMax that is ±1/8 inch. I was skeptical at first that it was a laser TOF instrument rather than triangulation. After some effort, I confirmed from an engineer at Stanley that it was, in fact, TOF. At home, the FatMax can easily tell a 1/8" change in distance for a wall about 15 ft away. You can easily discern the refractive index difference between water and air with it (i.e, you can determine the depth of water in a bucket based on the apparent increase in distance to the bottom of the bucket). I am just not curious enough to pursue Ryobi technical service to the same extent.

Its short term accuracy means that it times to one part in 5X10E-12 second. Put in human terms and extrapolated over a longer time interval, that is just a little more than 300 mS since Antony and Cleopatra were an item (if my math is correct).

I would be curious to learn when the NBS (now NIST) first attained that level of precision with its atomic clocks.

John

5. ### hgmjr Retired Moderator

Jan 28, 2005
9,029
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I would not be surprised to find that the Ryobi technique is based on the "Vernier Method".

hgmjr

6. ### Wendy Moderator

Mar 24, 2008
21,648
2,987
When I was going to college surveying was one of my electives. I remember going to a field looking for a stand of apples trees that had been planted around 130 years earlier (using a very old surveying document) and a rock that had been inscribed. The other members thought they found it, I was never too sure about it myself.

My teacher had a very expensive, very large laser rangefinder. We started with a steel tape measure to show us why the laser was a big deal. Both halves (transmitter and receiver) were both large and cumbersome, but they worked. This was in 1976.