It's not perfect, but it reflects 90% of the photons in photon missles. Also the stamp thing makes it's position a bit hard to guess so I worked hard, even though it has minor glitches.
Also the forcefield is supposed to protect cities or other stuff. Like, put it around a small city, then turn it on, and launch a few photon missles at it. Only a few photons get past the shield. I told you, not perfect.