I notice that the author's comparison between happiness and vision testing is flawed, for a couple of reasons. First of all, he's comparing absolutes and relatives. A person can say, "This is clearer than that." He cannot say, "These glasses provide me with 36 oculars worth of visual clarity." The study seems to be relying on the latter type of classifications for its data.
Secondly, and more importantly, when you're being shown all these different sheets full of Es and Os, you're being told to make a comparison between two events only a few seconds apart. If you were given a pair of glasses then taken them off, and then donned a second pair a week later, it would be impossible to accurately compare them unless the differences were pretty stark.
The latter problem, in particular, is going to bite these researchers in the butt. Sure, you can deduce with reasonable certainty that someone derived more happiness from his wedding than from getting his toe ripped off in an accident. But was he happier at his wedding or at the birth of his child? Was he made more unhappy by the passing of his father, or by the passing of his mother? It's going to be a tough call.
And this is coupled, ironically, with research that has already been done showing that people are absolutely horrible at accurately recalling their own emotional experiences. Memories tend to get overwritten by popular notions of what people feel they should be (a crappy wedding may be replaced with a memory of something that was "the happiest day of my life", because, well, that's what it should have been), extremely negative experiences are made less negative as a coping method, and so on.
All of this isn't to say that its impossible to study happiness, of course. But we need to be more careful than these researchers appear to be, and everything should be taken with a grain of salt. Psychology is a far cry from physics.