We continuously see new patents for diffractive optics lenses, something Canon continues to work and that we'll see in a new EF 600mm f/4 DO IS sometime in the future (Photokina 2018 perhaps?).
This latest patent selectively uses aluminum to reduce light scattering or flare in diffractive lens elements.
Keith at Northlight Images, who is far better at reading patents than I explains:
Aluminium is deposited onto the diffractive element at an angle, so as to build up a denser layer on the grating walls (blue) than the front surface (yellow). The aluminium layer is then partially oxidised, converting the front surface coating to a transparent layer of alumina (Al2O3)
Light going (‘the wrong way’) through the grating walls is a major contributor to flare, and in the patent, a test example reduced this by 8 times.
While we can only confirm that an EF 600mm f/4 DO IS is coming, thanks to Canon showing off an early prototype at Canon EXPO 2015, we have had suggestions that multiple DO lenses may be announced in the coming years, and not just in supertelephoto applications.
image credit // focus numerique