Purpose of knife-edge in Schlieren (optics)

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Discussion Overview

The discussion revolves around the purpose of the knife-edge in Schlieren optics, exploring its role in light intensity manipulation and the enhancement of visibility for specific light variations. The scope includes conceptual understanding and technical explanation related to optical systems.

Discussion Character

  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions whether the knife-edge's purpose is to reduce light intensity equally.
  • Another participant clarifies that the knife-edge reduces light intensity selectively, not equally.
  • A further explanation suggests that the knife-edge helps block "boring" light that passes through areas of small density variation, allowing for the enhancement of light that has passed through areas with interesting density variations.
  • It is proposed that instead of creating a small absorbing dot to block boring light, which is difficult to manufacture and position, a knife-edge can be used to effectively achieve this goal with less complexity.

Areas of Agreement / Disagreement

Participants generally agree on the selective nature of the knife-edge's function in Schlieren optics, though the initial question about equal intensity reduction indicates some uncertainty about its purpose.

Contextual Notes

There may be limitations in understanding the full implications of the knife-edge's placement and its effects on light manipulation, as well as the assumptions regarding the definitions of "boring" and "interesting" light.

Replusz
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I wanted to know what the purpose of the knife-edge in Schlieren optics is, but can't think of a suitable answer.
Is it only to reduce the intensity of light equally?
Thanks!
 
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Replusz said:
Is it only to reduce the intensity of light equally?
Not equally but selectively.
 
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Replusz said:
I wanted to know what the purpose of the knife-edge in Schlieren optics is, but can't think of a suitable answer.
Is it only to reduce the intensity of light equally?
Thanks!

Most of the light will pass through places where the density variations are small and will focus to a tiny diffraction limited spot at the focus. You are not interested in this light. It is boring. The light that passed through interesting density variations is perturbed. It no longer has a nice flat wave front and it won't make a tight little spot at the focus. You can greatly enhance the contribution of the interesting light if you block the boring light. What you need is a little absorbing dot about the size of the diffraction limited spot placed at the focus. However that is hard to make and hard to place in three dimensions. Instead, for the cost of about half of the interesting light you can much more easily block the boring light by inserting a knife edge at the focus. It only has to be placed correctly in two dimensions, and one of those, the focus dimension, is fairly forgiving.
 
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Yes, your answers are logical, well-thought and I think correct. Thank you very much, guys!
Replusz
 

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