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Optitape

cancel curvature loss using laser vectors in a modified tape measure
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I don't know if this is possible but here goes.

If an ordinary spring tape measure had a optic fibre embedded in it, a laser fired down the fibre would bounce the more the tape is bending. By putting reflection points down the fibre, the laser gives a set of length measurements. By using vector mathematics those measurements could be converted to a straight line distant.

A display could show the measurement or show an error value be to added to the measuring tape.

Long distances tend to introduce errors with a normal tape measure. This tape, although it needs batteries, hopefully will give more accuracy without using to much time and effort.

wjt, Dec 30 2014

optical time-domain reflectometer https://en.wikipedi...omain_reflectometer
[tatterdemalion, Dec 30 2014]

[link]






       [+] for innovation.
8th of 7, Dec 30 2014
  

       You could sell top end men's suits with such a tape measure.   

       Some would claim you get a better fit. And with the new suit you get the story to brag about your exclusive laser measured suit.
popbottle, Dec 30 2014
  

       The sag could be estimated based on the tension/tautness, angle, and length extended.
xaviergisz, Dec 30 2014
  

       It could become a form of optical network cable instead. Deployable, wearable, stickable... just need to find a reliable way to terminate the ends. Erp. I guess that's way off topic now.
spacer, Dec 30 2014
  

       If this idea is meant to work the way I think [wjt] means, it is a very cool idea.
MaxwellBuchanan, Dec 30 2014
  

       This is quite bakeable. Currently, in the construction of the new Crossrail tunnels, they are laying optical fibres in the concrete which will be used in this way to measure subsidence.
mitxela, Dec 30 2014
  

       You had me at "laser".
Voice, Dec 30 2014
  

       // reflection points down the fibre   

       Would need more elucidation on this aspect. I'm not sure how that jibes with total internal reflection.
tatterdemalion, Dec 30 2014
  

       I took it to mean that there were points along the fibre which would reflect a tiny fraction of the light. If these were at (say) 1cm intervals and if the tape were straight and you fired a very short pulse down it, you'd get a series of reflected pulses at 66 picosecond intervals. If the tape were bent between two of the partial reflectors, the timing between the corresponding pulses would be very slightly increased.
MaxwellBuchanan, Dec 30 2014
  

       OK, so there is a basis for that bit (see link). But how such a device would respond to bending using "reflection points" is not clear.   

       You could get it to work by using a continuous tape and break off the part you want to measure.
tatterdemalion, Dec 30 2014
  

       It would respond to bending like this:   

       If the light is passing along a straight section of fibre (and if we assume, for simplicity, that the light starts out travelling down the axis of the fibre), it will take some time T to travel a given distance D.   

       If the fibre is bent, the light will either (a) travel in a zigzag path, bouncing off a reflective coating or (b) in a graded-index fibre, the light will spend more time in the lower-index periphery of the fibre. In the first case, the overall speed of transmission will be lower; in the second case, it will be higher (faster). Either way, you should be able to work out the degree of curvature over that segment of the fibre.
MaxwellBuchanan, Dec 30 2014
  

       This is just on the periphery of my understanding of fibre optics, so I beg your patience. Within that fragile bit of knowledge, I had thought that // the overall speed of transmission will be lower // is a circumstance that does not occur in a bent fibre optic cable, and in fact is a feature, not a bug.
tatterdemalion, Dec 30 2014
  

       My knowledge isn't much greater. However, I do know that fibres work either   

       (a) by having a core of a high refractive index, and a sheath of low refractive index. The interface between the two acts as a mirror, so if the fibre is curved the light will repeatedly bounce off the "outside" of the bend. Thus, light will travel further if the fibre is bent, and the time will be longer.   

       OR   

       (b) by having a graded refractive index (high in the middle, grading to low at the periphery). A light ray entering the periphery is refracted back into the centre. At a bend, the light is continuously moving into the lower-index periphery (where it travels faster), and having to be refracted back to the higher-index (slower) core. Thus, light should travel faster around a bend (where it keeps dipping into the lower-index periphery) than along a straight bit.   

       However, please bear in mind that I might be completely wrong.
MaxwellBuchanan, Dec 30 2014
  

       The ray-diagram picture is pretty inaccurate for thin fibres. Solving Maxwell's equations in a fibre gives different modes of propagation (each with different rates of dispersion, hence the usefulness of having single-mode fibre). In some refractive index profiles, the bulk of the light may actually travel in the cladding, not the core.   

       But the point is that using optical fibres as strain sensors is a known technique.
mitxela, Dec 30 2014
  

       [bigsleep] I don't really want to carry another tool.   

       Maxwell has pretty well covered the outline sketch that was my idea. [Maxwell] Thanks.   

       My mind imagined the light take straight lines, until internally reflected by the fibre's sides. the more curve the more reflections. As Maxwell indicated, points along the fibre send back some light for partial calculation measurements.   

       I was thinking about a mechanical version with a two layers in the tape that take the curve differently. The show would be, vernier like, with differing marks between layers. But this didn't seem realistic.   

       But from what [mitxela] says, light is not that clear cut either.
wjt, Dec 31 2014
  

       I have just thought of a problem.   

       When I measure, sometimes I tend to extend the tape past the measurement so I can bend the tape into a corner to get the correct distance. This would give a false sag measurement.   

       Maybe the tape might allow the editing of how many laser vectors are used. A bit of fluffing about though.
wjt, Dec 31 2014
  
      
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