BULB IS BARELY ON
A light pulse (top) of 1 msec just begins to turn the bulb on.
Note that the falling edge of this pulse must be perfectly in line with the falling edge of the zero-crossing trigger pulse (blue trace on the bottom). Otherwise the AC waveform will pass thru zero volts and not turn of at the next half cycle.
Timing is everything. The uP must create a light pulse exactly in phase (within microsec) for the dimming to work.