Light on light off
Here is a photo of two of my SL students doing some very serious scientific research. The technique is actually quite difficult. They are trying to measure the seperation of spectral lines of a Helium lamp, the problem is if you have the light on you can't see the lines but if you turn it off you can't see the ends of the caliper. I'd definitely go for the light off option it might mean that I can't see the scale but it would also mean that no one could take a photograph of me.