A method for treating a condition in a tissue, includes the steps: (1) providing a PS within the tissue (2) irradiating the tissue containing the PS with a first light of a first wavelength and (3) irradiating the tissue containing the PS with a second light of a second wavelength so as to treat the condition in the tissue, wherein: (a) the PS absorbs light at the first wavelength and the second wavelength and (b) the second light is more strongly absorbed by the tissue than the first light or vice versa, so as to achieve a predetermined absorbed photon density gradient. An apparatus for conducting the method includes first and second light sources, a power supply, a focusing device, and a controller which adjusts light emission such that I(d)=I(λ1 at d=0)×exp (μeff (λ1)×d)+I(λ2 at d=0)×exp (μeff(λ2)χd).
