Thanks for your replies...
So a kill-a-watt is an easy way to get the PF... but its kinda cheating and I'm not gonna learn much from doing it that way! As well as actually needing to know the PF, I also want to learn how to use my new scope, and this seems like an interesting thing to do... so maybe I'll try to measure it with a scope, and then use a kill-a-watt to see how accurate I was
IF you have a ( isolated ) current probe for the scope and a high voltage probe then measure phase difference between current and voltage.
power factor = cos ( phase difference)
When you say isolated current probe... do you mean a current clamp? I have an active current clamp probe, although I'm a bit concerned that the resolution will be too low as it can measure up to 600A, and a little 5W LED bulb isn't going to draw much current...
Can I make sure I've got the maths right: If I measure the time delay between the voltage and current in seconds (t), frequency = 50Hz (f), then find the phase angle (pa): 2 * pi * f * t. Then PF = cos (pa). Is that right?
If you are trying to be really accurate about power factor you should note that some CTs and other current sensors you might to sense the current signal can introduce a few degrees of delay.
If I compare the voltage and current of a linear load, like an incandesant bulb, I'm guessing that if my probe does introduce a few degrees of delay then I'd be able to see a phase difference? Then I can use correct my calculations by this offset?