Building a half-mile long photogate and fighting diffraction

In summary, the person tried two methods to measure the distance to a target - without focusing and with focusing. Without focusing, the laser beam was almost parallel and it was difficult to measure the distance accurately. With focusing, the laser beam was focused to a point and it was easier to measure the distance accurately.
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Madysepanganahi
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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!
 
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Hello Madysepanganahi, :welcome:


Madysepanganahi said:
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 ?
 
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BvU said:
Hello Madysepanganahi, :welcome:


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 123Movies GoMovies


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!
 
  • #4
Madysepanganahi said:
thankyou so much! but my issue has resolved!
Well, don't keep us in suspense! What did you come up with?
 

1. What is a photogate and how does it work?

A photogate is a device used to measure the velocity of an object by detecting when the object passes through a beam of light. It consists of a transmitter and receiver, with the receiver sending a signal to a computer or data logger when the beam is broken by the object. This allows for precise measurements of time and distance, which can then be used to calculate velocity.

2. Why is diffraction a problem when building a half-mile long photogate?

Diffraction is the bending of light around an obstacle, which can cause the beam of light in a photogate to spread out and become less focused. This can lead to inaccurate measurements, especially over long distances. In a half-mile long photogate, diffraction can be a significant issue as the beam of light has a greater distance to travel and can be affected by any imperfections in the environment.

3. How can diffraction be minimized when building a half-mile long photogate?

There are several ways to minimize diffraction in a half-mile long photogate. One method is to use a laser with a shorter wavelength, as shorter wavelengths experience less diffraction. Another approach is to use a collimated laser, which produces a parallel beam of light that is less susceptible to diffraction. Additionally, careful alignment and calibration of the photogate components can help reduce any diffraction effects.

4. What are some potential challenges in building a half-mile long photogate?

Aside from diffraction, there are several other challenges that may arise when building a half-mile long photogate. These include finding a suitable location with minimal environmental disturbances, ensuring a stable power source for the photogate components, and designing a sturdy support structure to hold the long beam of light in place. Additionally, the length of the photogate may require specialized equipment and precise measurements to ensure accuracy.

5. What are some potential applications for a half-mile long photogate?

A half-mile long photogate could be used in a variety of scientific experiments and research studies. For example, it could be used to track the velocity and acceleration of objects over a long distance, such as in a projectile motion experiment. It could also be used in sports science to measure the speed of athletes over a long distance. Additionally, a half-mile long photogate could be used in engineering projects to test the performance of vehicles or structures over a large distance.

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