DoN. Nichols wrote...

What adding the slit will do is decrease intensity at *all*
points in that function, and reduce in a narrower part of it being above
the threshold of detectability -- which is probably a function of the
ambient illumination. So -- it gives the *user* a narrower apparent
beam.

Possibly. In my crude setup, distinct diffraction lines were visible on
the squares next to the slit. The brightness of the line was
significantly reduced, but it was still quite visible (much better than
in the photo). The apparent width of the line before the slit was about
3/16". That after was approximately equal to the slit width: just a tad
more than 1/32" over the two or three foot distance that I checked.

Jim