Have you considered photovoltaic (solar) cells, as opposed to photoresistive cells? They usually put out about 1/2 vdc per cell. Just put several in series and use a pot to calibrate the output in bright sunlight. (I used a battery symbol to represent the PV cells in the image). I don't have much experience with PV cells, but I believe they have a fairly linear output (Vout vs. Illumination). (Someone who has more experience with them, please correct me if necessary...) JB