Lesion size induced by laser photocoagulation is controlled in real time based on a two-dimensional reflectance image recorded by a CCD array during lesion formation. A feedback system using components of the reflectance image achieves uniform lesions by compensating for light absorption variability in biological media. Lesions are formed in a phantom by an argon laser to simulate retinal photocoagulation. The tissue model consists of a thin absorptive layer covered by a clear albumin protein layer. Results show a low variance in the sizes of the lesions (diameter or depth) produced in different irradiation conditions and the ability to produce lesions of a predefined size in varying illumination conditions.