Purpose of knife-edge in Schlieren (optics)

[Replusz]

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!

 

[A.T.]

Is it only to reduce the intensity of light equally?

Not equally but selectively.

 

[Cutter Ketch]

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.

 

[Replusz]

Yes, your answers are logical, well-thought and I think correct. Thank you very much, guys!
Replusz