[kurthr]

It's interesting, but there's no reason you can't do both reflection and diffraction. A refractive grating allows most light to pass through, but reflective gratings use smaller (sometimes less than half wavelength) steps/spacings to diffract different wavelengths.

What is interesting to me is that you should be able to stretch the grating (as well as rotate relative to perpendicular incidence) to change the effective periodicity/wavelength, and also what effect that would actually have (on angle and efficiency).

Perhaps one advantage of refracting diffraction is that blue light (with the largest momentum) might bend to larger angles? But that would also depend on the grating size/angle. A rotating grating with a blaze angle might allow the most flexibility.