Each goggle lens is made of crossed polarizers sandwiching thin sheets of PLZ (lanthanum-modisied lead zirconate titanate). A photodetector, mounted directly behind each lens, senses a high intensity light flash (trigger event) and triggers the control circuit that applies a high voltage to the PLZT lens. With the high voltage applied, the lenses are in the "open" (transparent) state. If, at any time, the light level doubles in less than 100 μs, the control circuit will discharge the high voltage from the PLZT lenses, reducing the optical transmission to approximately 0.1%, corresponding to an optical density (OD) of 3.0. This rapid reduction of transmission is referred to as the switching response, and it occurs in less than 160 μs, with a resulting final transmission corresponding to OD 4.1, which is called the fully "cloced" (opaque) state. The pretrigger light level is stored for future reference, and the lenses remain in the fully closed state for about 200 ms. The system then enters the servo mode, lasting approximately 25 s. During the servo mode, the control circuit increases or degreases the transmittance as necessary, maintaining a constant pretrigger light level throughput.