[janoskiss (Steve H)]

I use the Gimp mostly, but PS should be the same. There is no "blur vertical" as such, I just go to Filters->Blur->Gaussian Blur. Set "Vertical" to something very large, say 1000 (this effectively averages all pixels values along the vertical) and "Horizontal" to 0 (no blurring along horizontal). Motion blur is another good one, and you can adjust the blur angle if the spectral lines are not quite vertical.

Actually I just tried it and motion blur gives sharper lines in the Gimp. Needs to be applied repeatedly (max length is 256 pixels) until all detail along the vertical is completely blurred out. See attached pic.

Edit: replaced pic. First one wasn't cropped properly before blurring and some of the comparison inset got mixed in.