Nigel, are you sure it's going to be noticed? Light bulbs run at 120hz already, no one even thinks that a standard incandescent bulb flickers. I can't test it though. I don't think I have any diodes floating around I can send through a desk lamp to find out if a person would be able to visually notice flicker in it. Anyone wanna try it? I'd bet you can get to 50% dimming using half phase drops without noticeable flicker, unless you were right up against a hard shadow of the bulb. I have seen circuits based on CDS cells that can detect the flicker produced by a light bulb, but I'd be curious what amount of dimming (number of dropped cycles) would be required to notice it with the human eye.
I can easily notice the flicker on my peripheral vision of my monitor when it's sest to 60hz. But I have never noticed any flicker on light bulbs at 120hz, and I have better vision than most. Depends on the 'modulation' depth that the dropped cycles creates in the light output. There is a BIG difference between 60 dropped cycles in seriers with 60 full cycles as opposed to 1 out of ever 2 cycles being dropped.