Techniques are disclosed for controlling the light output of a lamp, where lamp efficacy is estimated as a function of estimated lamp temperature and instantaneous input power, or as a function of estimated lamp temperature only. Whether efficacy is estimated as a function of temperature and power, or as a function of temperature only can depend on changes in the lamp operating scenario. The techniques estimate lamp temperature by tracking energy input to and losses from (losses such as radiation, conduction, emission) the lamp arc tube, and determine the corresponding instantaneous light producing ability. The techniques may further be implemented to deliver the appropriate power command to obtain a desired light output. The techniques can be applied towards a general control in which arbitrary or custom light output vs. time paths are produced, and may be implemented by a processor programmed or otherwise configured to execute the desired control scheme.