Your title is not entirely correct and does not actually mean anything. We are not "measuring light". What do you want to actually measure? We can measure the radiation spectrum of some atoms (or anti-atoms) or the properties of some radiation. This is slightly different from measuring light.

Now, in fact, light emitted by antimatter has already been seen for many years, like for instance in electron-positron collisions where the positron emits some photons before colliding. What has been measured by the ALPHA collaboration for the first time is the radiation spectrum of antimatter atoms.

Good post otherwise. I guess that you may get a few extra upvotes but tagging it appropriately :)