Can reflections create a focused beam of light from a point source?

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Discussion Overview

The discussion revolves around the feasibility of using reflections to create a focused beam of light from a point source, specifically examining the principles of collimation and beam divergence in optical systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether reflections can direct light from a small luminous sphere into a thin beam that can focus to infinity.
  • Another participant asserts that radiance is conserved in a lossless optical system, indicating that while collimation is possible, the beam diameter will exceed the size of the source.
  • A participant suggests that a point source could be focused to infinity using a parabolic mirror and lenses, referencing the concept of collimation.
  • It is noted that beam divergence is influenced by the ratio of the wavelength to the mirror's diameter, though some argue it is more related to the source size and the lens's numerical aperture.
  • A participant shares their practical experience attempting to collimate a UV beam, expressing disappointment over the inability to maintain a constant spot diameter and observing increased diffusion at greater distances from the lens.
  • Responses to the practical experience indicate that the observed behavior may be normal.

Areas of Agreement / Disagreement

Participants express differing views on the principles of collimation and beam divergence, with no consensus reached on the effectiveness of reflections in achieving a focused beam from a point source.

Contextual Notes

Participants discuss various assumptions regarding the conservation of radiance, the effects of lens characteristics, and practical challenges in achieving desired beam properties, but these assumptions remain unresolved.

Who May Find This Useful

This discussion may be of interest to individuals exploring optical engineering, particularly those involved in beam collimation and the practical applications of lenses and mirrors in directing light.

hl_world
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Is it possible to use a system of reflections to direct most (if not all) of the light from a small luminous sphere to produce a thin beam that can focus to infinity?
 
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No. For one thing, the radiance is conserved- the size of the object multiplied by the divergence of the light- is constant in a lossless optical system. You can make a well-collimated beam from a small source, but the beam diameter will be much larger than the source.
 
Thanks. Could a point source be focused to infinity with the help of a parabolic mirror and maybe some lenses?
 
Sure- that's called 'collimation'. Put the source at the rear focal point.
 
You'll always have some divergence in the beam, which is going to go like ratio of wavelength to mirror's diameter.
 
K^2 said:
You'll always have some divergence in the beam, which is going to go like ratio of wavelength to mirror's diameter.

Actually, the beam divergence is more related to the size of the source with respect to the numerical aperture of the lens, than the wavelength (assuming a perfect lens).
 
Thanks everyone.
 
With your permission, may I revive for a short while this topic?
So, I tried to collimate the UV beam from Deuterium lamp with glowing area of 0.5 mm diameter, using plano-convex fused silica lens having 75 mm focal length and about 12 mm clear aperture.
The lens plane surface was facing the lamp.
Also, I had a possibility to vary the distance between the lens and the lamp.
The UV beam diameter was evaluated by viewing the phosphorescent spot size on a special phosphor screen.
At first sight everything looks OK, doesn't it?
Nevertheless, I am a little disappointed because I still could not achieve a more or less constant spot diameter (at least up to 10%) within the range of several centimeters from the lens. Moreover, with increasing the distance from the lens the spot edges became more diffused.
Is that normal? Please, comment. Many thanks in advance.
 
Tougbaev said:
<snip>
Is that normal? Please, comment. Many thanks in advance.

Yes.
 
  • #10
Thanks a lot, Andy
 

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