Ronchi Grating
Ronchi grating is a specific grating with d=2b. d is the spacing of the grating and b is the width of the slit.
This grating is interesting because it can give us a square wave.
The Fourier Transformation describes this grating as
i.e. a0=1, am=0 (m=1,2,3,......), bm=2/(mπ) (m=1,3,5,......), bm=0 (m=2,4,6,......)
ω0=2π/d, which is named as spatial angular frequency.