In order to measure the gate profiles a fast laser diode (40ps, 820nm) is used along with a 7000 series Tektronix sampling oscilloscope and a slow photodiode.

The photodiode is used to integrate all the light from the phosphor during repetitive gate events. Note that the phosphor decay time is typically a millisecond.

The laser diode timing is slowly adjusted to probe different times in the gate of the image tube. The sampling system vertical amplifier already has such electronics and this is modified to make the signal available to trigger the laser diode at successive times through the gate drive waveform. The sampling timebase keeps the oscilloscope spot in time with the delayed trigger signal derived from the sampling unit. The integrated photodiode signal is dsplayed with a regular vertical amplifier.

By making the photodiode look at only a region of the output phosphor it is possible to measure the gating times and timings of different parts of the output screen. This can, for example, show irising effects when the cathode is heavily reverse biased.

Using this technique we have seen 65ps gating of 11mm tubes. When deconvolved from the 40ps laser diode light pulse source this indicates 50ps gating.

Note that gating at wavelengths other than 820 nm will give slightly different results, particularly for very fast gates as the photo - electron energy spread will be slightly higher for shorter wavelengths.