Figure 2: Calibration of the system: (a) photon counts as a function of position for different exposure times: 0.4 second, 1.3 seconds, 5 seconds, (b) white-light image of the same area as (a), showing material degradation for the darkest (lowest count) square, (c) SHG image of 6-level gray scale obtained by varying the exposure time from 0 to 0.85 second. In all cases the laser power is fixed to 70 mW for writing and the readout time is 20 ms/pixel at 20 mW.