We started light this week in my physics class. I do a set of labs using CBLs (from TI) with the TI83/84 graphing calculators. We look at bulb wattage vs. brightness and distance vs. intensity (finding an inverse square relationship). I also have students see the fluctuations in a regular light bulb just caused by alternating current.
For those peope out there who don't have CBL's, LabPro's, or PASCO probeware all you need to do these labs is a cheap solar cell and a computer running Audacity and/or Visual Analyzer. You'll also need a 3.5 mm headphone jack. You can pick this up at Radioshack or you can go to the dollar store and pick up a cheap set of headphones.
Headphones will typically be stereo so there will be two leads, one for each ear. You only need one ear's worth of wire. When you strip the lead you will find two wires, solder one wire to each of the leads on the solar cell. Then simply plug it into the microphone jack on your computer and load up Visual Analyzer. Point the cell at a light and see the 60 Hz alternating current (shows up as the light turns on and off 120 times a second).
You can use this to determine relative intensity, but to get exact lumens or lux you'll have to find a way to calibrate your probe. You'll need a volt meter for this. You can find them for under $10 if you look around a bit (assuming you don't have any already).
The solar cell will also pick up infrared light. Just point a remote control at the cell and record in Audacity, zoom in to see how the pattern is different for different buttons and different remotes.
For under $5 you can have a very versitle light probe suitable for a number of great labs or demos.