Are you trying to compensate for long term light loss due the ageing of the bulb or for short term light fluctuations due to input power fluctuations?
For short term a very stable power source would be simpler than adding feedback. If not possible to stabilise the source you could add, after the photo diode output, a comparator and a reference voltage. The output of the comparator controls the input voltage regulator. Make sure the reference does not fluctuate, it should have an independant supply source from the lamp.