You can say it's the same term, for a grating with period a, illuminated by incident coherent beam, then the diffracted light has the frequency cos(sita)/lambda, but a*cos(sita)=m*lambda according to grating theorem,so the frequency could also be represented as m/a, which is the fouriers series's each order's frequency in "image processing", so we can see in physical optics and image science, the two term is actually equivalent.