road wellington new zealand the beam passes through the modulator and a condenser lens 16 before striking the surface of a sample 7 the reflected light passes through an objective lens 8 a second polarizer 9 usually called the analyser and is focused onto the entrance slit of a spectrograph 20 which has an intensified charge coupled device iccd camera at the exit plane the iccd camera has a ccd 10 and a gate in front of the ccd which can be opened and closed in a time u02dc 5 ns depending upon the manufacturer the gate comprises an intensifier 11 and phosphor screen 12 the intensifier operates in a similar way to a photomultiplier and the gain of the intensifier 11 can be controlled via an input line 13 an example of a suitable iccd camera is the pi_max1024 manufactured by roper scientific 3660 quakerbridge road trenton n j 08619 in some iccd cameras the phosphor screen 12 and ccd 10 are coupled by optical fibre cables not shown in an alternative arrangement the birefringence modulator may be placed between the sample and the analyser 9 a gate controller 14 controls the gate and in turn is controlled by a computer 15 which also receives and processes data from the ccd 10 the gate controller 14 derives modulator coherent pulses each cycle from a positive going zero crossing of the primary modulation signal received from the gauge element 5 and opens and closes the gate at specified points during the modulator oscillation for instance it can be held open for one full period t of the modulator in which case we measure the ac terms averaging to zero other intervals for instance 0 t 2 0 5t 8 give functions.