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Hi Daniel,
I would say simple wave optics, think Huygens principle.  Only in the focus you have constructive interference, if you move away from it you get destructive interference. Where depends on the angle of the incoming light. The emitted light is isotrop. But you only get fluorescence where you excite it.

Best wishes

Andreas

-----Original Message-----
From: "Daniel White" <[log in to unmask]>
Sent: ‎21/‎07/‎2017 18:52
To: "[log in to unmask]" <[log in to unmask]>
Subject: Fluorescence illumination NA vs. Z Resolution.

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Hi all

Geeky question :

What is the physical process by which illumination NA in a confocal or WF
fluorescence microscope affects the z resolution, the axial response (OTF)?

Since fluorescence lifetime is nanoseconds, emission is incoherent wrt
excitation. Assuming free dye rotation during fluorescence lifetime, how
can angle range of incident photon affect angle range of emitted photon?

What am I missing?

What's the quantum electrodynamics explanation here? What would Feynman
have said?

Cheers