Retroreflector that shifts reflected rays by 4 degrees

  • Context: Optical 
  • Thread starter Thread starter ldanielrosa
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Discussion Overview

The discussion revolves around the feasibility of creating a retroreflector that reflects light at an angle of 4 degrees instead of directly back to the source. Participants explore the design modifications needed for a cubic corner array to achieve this deviation, as well as potential applications and methods for implementation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant proposes modifying a cubic corner array to create a retroreflector that deviates light by 4 degrees.
  • Another participant confirms that such a deviation is possible, referencing the lunar retroreflectors and suggesting that the reason for the angle could influence design choices.
  • A participant mentions the need for an autocollimator to measure the offset accurately if it is critical.
  • One participant explains that the 4-degree deviation is not critical and describes the application as a parking target for vehicles, emphasizing the ease of production and installation.
  • Alternative methods for achieving similar results are suggested, including using physical markers like dowels or chalk marks for parking guidance.
  • A participant discusses the geometric considerations for achieving the desired reflection angle, suggesting specific orientations for the retroreflector's faces.

Areas of Agreement / Disagreement

Participants generally agree that a retroreflector can be designed to reflect light at a 4-degree angle, but there are multiple approaches and methods discussed. The discussion remains unresolved regarding the best implementation strategy and the specifics of the design modifications.

Contextual Notes

Some participants express uncertainty about the exact methods and geometries needed to achieve the desired reflection angle, and there are references to external sources that may contain relevant information.

Who May Find This Useful

Individuals interested in optics, retroreflector design, vehicle parking aids, and practical applications of light reflection techniques may find this discussion useful.

ldanielrosa
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I've a bee in my bonnet and I want to know if it's feasable. I want a retroreflector that doesn't shine _right_ back at the source, but misses by 4 degrees or so in a direction determined by the orientation of the reflector. I hope this can by done by modifying a cubic corner array. I'd like to get what input I can before I begin.

I have an .stl file for a for a cubic corner array, and the rendering software does allow stretching the axes independently.

I intend to print this with resin, with resolution of 0.05mm. I think the surface roughness will be adequately treated with one or two squirts of chrome paint
 
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Welcome to PF.

Yes, it can be done. It was done with the retro-reflector placed on the Moon, because the laser and telescope on Earth move during the time it takes for the laser pulses to return.
https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiments

Maybe if you can tell us why you want the 4° angular return, we will be able to suggest ways to implement the deviation. There are alternative solutions.

You may need an autocollimator to measure the offset if it is critical.
https://en.wikipedia.org/wiki/Autocollimator
 
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Baluncore said:
Yes, it can be done. It was done with the retro-reflector placed on the Moon, because the laser and telescope on Earth move during the time it takes for the laser pulses to return.
https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiments
Great info, as always. :smile: Can you point me to the part of the article that describes how they did the small angular offset with the array? My skimming the article is failing me so far. Thanks.
 
The first Apollo retroreflector was adjusted for the EME return time. It was only a couple of arc seconds from orthogonal. I believe the work was done by a skilled astronomer in Melbourne, Australia. I am looking for a reference.

The problem of Velocity Aberration is an interesting one because it depends on the location of the retroreflector, the latitude of the Earth station, and many minor orientation changes. It is said that a miss, is as good as a mile, which is the magnitude of the correction required.

Extract from: Lunar Laser Ranging Retroreflectors: Velocity Aberration and Diffraction Pattern. The Planetary Science Journal, 4:89 (22pp), 2023 May.
Since the laser source is moving with respect to the CCR, as seen from the Moon the apparent positions of the Earth station at the transmit and receive times will be different. This aberration due to velocity v is 2v/c in radians or 412,530v/c in seconds of arc, where c is the speed of light. Velocity v is the component perpendicular to the line of sight. The orbital speed of the Moon is about 1.0 km s−1, and the equatorial rotational velocity of Earth is 465 m s−1.
 
"Maybe if you can tell us why you want the 4° angular return, we will be able to suggest ways to implement the deviation. There are alternative solutions.

You may need an autocollimator to measure the offset if it is critical.

Reference: https://www.physicsforums.com/threa...t-shifts-reflected-rays-by-4-degrees.1080353/
"

4 degrees is absolutely not critical. The application is a QCD parking target. Once I understand the method by which the return angle can be set, the generation of the next iteration will be trivial.
 
I don't understand, is QCD = quantum chromodynamics ?
 
"I don't understand, is QCD = quantum chromodynamics ?"

Quick, Cheap, Dirty

Hokay, so what's in my head needs to hit the paper. I wanted to keep this close to the vest because it may be marketable, and will be extremely cheap to mass produce.

I want something I can stick on any vertical surface without needing it precisely located, that will return a spot from whichever indicator lamp on my vehicle I choose to my face. Left turn signal for my truck and right turn signal for my car. These will allow me to pull in _just far enough_ that I can close the garage door but leave space in front.

Once I measure the relative positions of the lamps of interest and my face, and the combined distance to the wall or whatever surface I choose, I should be able to calculate the return angle. Then I distort the file for the retroreflector and print. Once printed and painted, I need only be aware of which axis the array must be oriented to put the return spot where I want it. The reflector doesn't even need to be directly in front of my vehicle so long as the lamp and operator can both "see" the bottom of the corner.
 
ldanielrosa said:
"I don't understand, is QCD = quantum chromodynamics ?"

Quick, Cheap, Dirty
LOL. o0)
 
ldanielrosa said:
These will allow me to pull in _just far enough_ that I can close the garage door but leave space in front.
Alternatives:
1. I hang a wooden dowel on a lightweight chain. When the front bumper hits the dowel, it bounces, so I stop. You can hang several dowels for different vehicles.
2. Place a chalk mark on the wall, so when you look sideways, the B-pillar of the vehicle lines up with the mark.
3. A shallow wooden block pinned to the floor. When you reach it, you drive over it and then stop against it.

I assumed you wanted to aim the return beam to the left or right by 4°, not up or down. Since the angle of incidence is equal to the angle of reflection, you will need to offset one mirror face by ½ * 4° = 2°.

Arrange the corner cube to place one face in a vertical plane on the left, with the two other faces meeting on the horizontal axis. Keep the face on the left in the vertical plane, while turning it to the left or right by 2°. You must now keep the retroreflector in that orientation to maintain the deviation in the horizontal plane.

Make a retroreflector from three identical mirrors, each face a 45° right triangle. Join two by their short edges, use the third to set the angle between the first pair, but tilt it to half the required reflection angle.
 
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"Make a retroreflector from three identical mirrors, each face a 45° right triangle. Join two by their short edges, use the third to set the angle between the first pair, but tilt it to half the required reflection angle."

Thank you so much, Baluncore. This is what I'm looking for.
 

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