Quote Originally Posted by holmburgers View Post
I'm always curious about how we can say that the light is "perceived as being only half as bright". Any idea how these tests are conducted? ...
That goes into the study of human perception and you'll find more here:

http://en.wikipedia.org/wiki/Hermann_von_Helmholtz

and here:

http://en.wikipedia.org/wiki/Weber–Fechner_law