thank you for the kind reply. i am controlling the lamp using TRIAC phase angle control. secondly Integrator with propotional is a must to have a value at the ouput when the error goes to zero. the transducer used is a photovoltiac detector(solar cell). the controller will control any change in light intensity due to disturbances in the AC voltage supply and disturbances from any other light source. e.g doors and windows, light intensity usually flickers while watching tv due to light from tv. sometime in thunder storms when the windows are banging(open and close), in welding workshops these are all examples of some rapid disturbances, this will require a differentiator as well. this is all that i think maybe u have a better idea.