Dark spot in Fraunhofer diffraction

[thepopasmurf]

Hi, I've been doing Fraunhofer diffraction and I have been getting dark spots in the middle if my patterns, not predicted by the Fourier transform of the aperture. I don't think it's babinets principle and effect is stronger near the edges of the slits. Any ideas? Thanks

 

[Redbelly98]

Not sure what would cause that. What do you mean by "near the edges of the slits"? Near the slits would correspond to Fresnel (near field) diffraction, not Fraunhofer diffraction. What is your specific slit or aperture geometry?

 

[thepopasmurf]

What do you mean by "near the edges of the slits"?

I mean when incident beam is near the top or bottom of the slit aperture.
The aperture in question is a narrow slit, around 100 microns wide (i think, that's off the top of my head) and perhaps 2mm or 3mm high.

 

[Redbelly98]

Weird, not sure what would cause that.