Response curve of an LDR?
For the Advanced Photonix PDV-P9004, it seems like the illuminance is proportional to the conductance (1/resistance) of the photoresistor.  At some point it probably saturates.

This makes sense as the photons it receives and the applied electric field should be proportional to the electrons that flow.  It might not be purely linear between the field (voltage) and resistance though.