A Virtual Knife Edge

Adolf Cortel put a mask over his spectrograph sensor to create an inexpensive Low Pass (LP) Raman (spatial) filter. (dead link. Try the Wayback Machine)
How well does that work? 
The edge diffraction is wavelength dependent. A blue laser would provide
better results than a red laser. 

Look into how IC manufacturers deal with the diffraction limit.
Look into how the diffraction limit has been exceeded using Moiré or other methods.
How does slit geometry interact with the polarization/bireflectance? 
There are some modern experiments being done with extremely refined photon sources.
Can single photon sources minimize edge diffraction?


 
Glare
Blooming
Huygens–Fresnel principle
Overshoot (signal)
Ringing (signal)
Double-slit Experiment
Gibbs Phenomenon
Hawking Radiation
Bayesian Mechanics
Virtual Knife-edge
Quasiprobability distribution
Synthetic Aperture
Aperature Synthesis
Phased Array
Vortex Coronagraph
Coronagraphic Notch Filters for Raman Spectroscopy(NASA)
Quantum Interference between Light Sources Separated by 150 Million Kilometers
 Single Photon Cameras
  A superconducting-nanowire single-photon camera with 400,000 pixels