What can justify the decrease of the electrical conductivity with the increase of light intensity? I have currently been working with a sample that "appears to" decrease its resistance when I cover it and protect it from light.
Basically it presents the opposite behaviour of a photoresistor.
What kind of phenomenon may cause this? Or it has to be a measurement error?
EDIT: I am talking about typical room light from fluorescent lamps on the ceiling.
Experimental Setup:
Sample: A dense, sintered, ceramic pellet (with a disc shape, 1 cm in diamater and ~3mm thick). The faces of the disc were carefully painted with conductive silver ink to make the electrical contacts. An Agilent 4294A impedance analyzer was used to measure the electrical properties with a 0.5 V AC signal. This was done at air, in a room with the ceiling lights turned on. Approaching my hands to the sample, blocking the light, the resistance increases.
 A: As you measure impedance in AC it may be related to change in capacitance of the system when you came close with your hand. This principle is used for capacitive sensors of presence. 
Do you see the same effect when you turn off the lights rather than blocking it with you hand? You can just stay by the light switch and turn it on and off, without changing your position.
So far you did not prove that is the effect of the light and not of your hand.  
A: We know that at lower temperatures conductors achieve superconductivity. So maybe your samples are heating in presense of light, and upon cooling in absense of light going towards superconductivity.
Although once when I had been working on building a circuit, I was experiencing similar results from my photoresistor. Alas it turned out to be some sort of error which fixed itself when I rebuilt the circuit, so I would suggest you to retry a few more times.
A: It can be just the dependence of the material with temperature due to the heating from light.
