For a coming project I'll need to know if a room has the lights on or off.
That's when a Light Dependent Resistor (LDR), or photocell comes in handy.
As usual, Ladyada provides an excellent tutorial on understanding and using electronics: Ladyada's 101 on LDR
I also found useful info here.
For the LDR that I have, I'm using this Datasheet. I'm not sure if this is the correct datasheet, but for this component I think it should be close enough.
The quantity of light can be measured in Lux, Lumen or Candela depending on several factors.
Here's some more info.
In this case, here's a table of some real world approximate Lux values, in order to understand the values we need to read:
Light source LUX
60W Bulb at 1m 50
1W Bulb at 0.1m 100
Fluorescent Lighting 500
Bright Sunlight 30,000
The resistance of a LDR can vary from +/-400 Ohm (for Bright Sunlight) to +/- 2 MOhm (in the Dark), depending also on the make of the component.
Here are some of the values I measured (from bright to dark):