Building a half-mile long photogate and fighting diffraction

[Madysepanganahi]

I’ve got a laser that I want to use to measure a straight line (preferably within about a millimeter) over about 2000 ft. My first thought was to focus my laser better and bring up a card until it cut the beam completely, but focusing down to a millimeter point so far away feels pretty unachievable. Instead I put a tiny speck of retroreflective film up at the far end and raised a card through the beam till the shadow of the card stopped the light from hitting the reflector. In a world with ballistic light this would work perfectly, but unfortunately we live in a world with diffraction, so if you set the card partway through the beam and walk to the end to look at it, the “shadow” of the card is super fuzzy and has diffraction fringes around it, decreasing the accuracy of my measurement.

(unless that first order fuzziness is actually linear with distance to the target and I’m not actually losing accuracy at the card? It’s hard to tell)

I’m now imagining using a moderately narrow (1mm) slit instead of an edge and moving it up and down until the reflected light is brightEST, but I feel like there should be a better, more precise way. Does anybody have any thoughts?

Thanks so much!

 

[BvU]

Hello Madysepanganahi,

I’ve got a laser that I want to use to measure a straight line (preferably within about a millimeter) over about 2000 ft.

You want to measure that length, I suppose. What is the measuring method you have in miind ?

My first thought was to focus my laser better

experiment with that a little bit: no focusing versus focusing. Could well be that the laser beam is almost parallel light and therefore focusing with a single lens doesn't help much for what you want to do ...

unless that first order fuzziness is actually linear with distance to the target

might well be, but the width of the slit is also in the expression and to determine that with a relative accuracy of 10^-6 is extremely hard...

I’m now imagining using a moderately narrow (1mm) slit instead of an edge and moving it up and down until the reflected light is brightest

What information would that reveal ?

 

[Madysepanganahi]

Hello Madysepanganahi,


You want to measure that length, I suppose. What is the measuring method you have in miind ?

experiment with that a little bit: no focusing versus focusing. Could well be that the laser beam is almost parallel light and therefore focusing with a single lens doesn't help much for what you want to do ...Rufus[/color] 123Movies[/color] GoMovies[/color]


might well be, but the width of the slit is also in the expression and to determine that with a relative accuracy of 10^-6 is extremely hard...
What information would that reveal ?

thankyou so much! but my issue has resolved!

 

[Tom.G]

thankyou so much! but my issue has resolved!

Well, don't keep us in suspense! What did you come up with?