I'd assume LI and LO to be just that - Read it either as Line In / Line Out or as LIne / LOad

AC power source means swaping the 2 input wires to the entire circuit, or 2 wires to some 2 wire component, does not have any effect. But when you are swapping 2 out of 3 wires, then things can go malfunction or be destroyed, whether in AC or DC. Imagine for example, that you wire the photocell so its contact shorts the supply when it closes - You will get a bang and the contact will be welded closed forever

And I will definitely make sure to not do that.

If you dont know for sure which terminal is which, there is a chance of doing just that. This is why you use the high power lamp in series with the test, so if this happens. it will power the lamp at full brightness and not make a short circuit. Similar connection in series with lamp is usefull when testing switching PS's after repairs of blown primary, when not yet sure if its not going to blow again

So I will now assume:
Li/1: Connect to live of light source.
LO/o: Connect to AC live wire.
N: Connect to neutral.
Any other technicalities?

LI would be the line and LO the lamp. But how you know which is which ? Is it written on the photocell base ? If no, you have to either test it as described by Medved or take apart the cell to find out

LI would be the line and LO the lamp. But how you know which is which ? Is it written on the photocell base ? If no, you have to either test it as described by Medved or take apart the cell to find out

It clearly labels the pins, so I know which is which. I just needed to decipher them.

I have another question: How can I easily test higher voltage (208-277) PCs without messing around with more dangerous wiring?
Also, are the 208-277 volt PC receptacles the same as the 120 volt ones?

Mess with the wiring safely, and then you dont have to care what the voltage is

I guess you can get 240V, this might work for testing a 277V cell but a thermal cell will react at wrong light levels. For a 208V cell it is too much (probably won't burn in the moment you connect it, but i dont think it's good for the cell for longer time, especially if your 240V is relatively "high") and will react at wrong light levels too. For 208..277V rated cell 240V is fine

If you got ballast with taps for those voltages, you can use it as autotransformer to get them

If you got a transformer of 120V to couple 10's V with few VA or higher rating (not an "AC to DC adapter" but a transformer that puts out AC straight from the coil), then you can use it to add or subtract the transformer's output voltage from 240V and get close to 277V or 208V this way

The problem with the outlet is that there are practically none of them. Also, how would I test 480 volt PCs?

Got any ballast with 480V tap or 2 ballasts with 120/240 taps ?

I don't have any ballasts with multiple taps.

Li = AC supply hot
Lo = hot to light
N = neutrals from supply and light

To test higher voltage PCs, you will need to produce a higher voltage somehow (transformer, ballast taps, etc.) and you will also need a load that can handle the high voltage (like several incandescent lamps in series).