Also, pardon my ignorance, but what is the difference between a HID photocell and a LED photocell ?
One of the biggest difference is related to the anticipated character of the switched load.
HID uses to go more into the inductive character, so tend to arc when switching off, but the electronic LED ballasts tend to be more of a capacitor character, so high current spikes at turn ON. It requires a bit different approach to design the switching element. Yes, this is relevant mainly for higher loads, but even for lower ones e.g. the triac switches do not work well once capacitors are involved (the high di/dt when turning ON is killing them) in the load circuit and do not like lower currents in general (below 0.2..0.3A it becomes difficult to keep them switched ON when they are supposed to be), but LEDs tend to draw less. And on the opposite side, MOSFETs (became quite common for LED stuff) are not handling that well if the load becomes inductive (breaking the current causes an overvoltage spike).
Plus the LED can tolerate frequent switching pretty well and has nearly zero "restrike time", while the HID get worn off when switched too frequently and any switch off leads to many minutes dark time. So HID have to have something to prevent erratic switching (so have some form of filter for random upsets), LED ones can tolerate way more mess in that, so could be made simpler.