One way is via Photomultiplier http://en.wikipedia.org/wiki/Photomultiplier http://en.wikipedia.org/wiki/Photomultiplier essentially the photoelectric effect is

One way is via Photomultiplier http://en.wikipedia.org/wiki/Photomultiplier http://en.wikipedia.org/wiki/Photomultiplier essentially the photoelectric effect is used to create a cascade of electrons, the created current can be measured and related to the energy of the original photon (which is h * f, where h is Planck's constant and f = c/lambda is the light frequency.)