How is the phase shift of light measured? This may seem like a simple question, but I cant seem to make any headway. 
Consider the following; I have two beams of light, a reference beam $(A=\cos(wt))$ and phase shifted beam $(B=\cos(wt+\phi))$.
What is the normal way (say in a lab) that $\phi$ is detected & measured with respect to the reference beam?
I believe an interferometer is used, but I dont understand how this yields a measurement.
Thanks for any insight.
EDIT: I don't know if I've explained this very well. I think better in pictures, so here is a VERY crude diagram of my thought experiment.

 A: First remove the "process" step from your diagram so that you are comparing two beams of light with $\cos(\omega t)$. Let's say, for the sake of simplicity, that they can be considered plane waves, and your interferometer combines them at a small angle, so that you see a series of stripes (bright and dark fringes) on your screen.
Now add the "process" step, being careful not to disturb any other part of the interferometer. This causes a phase difference $\phi$ between the beams. The positions of the bright and dark fringes will shift. Say $d$ is the distance between two crests or troughs (brightest parts of two bright fringes or darkest parts of two dark fringes), and $\Delta x$ is the distance by which the fringes shifted when you introduced the phase difference.
Then,
$$ \phi\bmod{2\pi} = 2\pi\Delta x / d$$
Note that you won't be able to tell if $\phi > 2\pi$.
A: Briefly, one technique for observing phase shifts in light is Phase Contrast microscopy which garnered Fritz Zernike the 1953 Nobel prize in physics.
