The light dosimetry for photodynamic therapy (PDT)for superficial lesions is concerned with ensuring sufficient light in the superficial tissue layer to achieve efficacious PDT. The light dosimetry for bulky tumors is concerned with the maximum depth of effective treatment while expecting over exposure of the tumor surface. The light delivered to a tissue surface does not equal the light received by the superficial region of that tissue. Backscattered light from the underlying tissues augments the surface irradiance to yield a higher fluence rate (W/cm2)in the target superficial tissue layer. The effective irradiance of treatment light, E [W/cm2 ],seen by a superficial lesion is approximated: E =Eo(1 + 2R(1 +ri)/(1 -ri))=Eo(1 +6R)where Eo is the irradiance delivered to the surface, R is the fraction of incident light that escapes the tissue as observable reflectance (typically about 0.30-0.60),and ri is the total internal reflection of light attempting to escape at the air/tissue surface (about 0.50).Hence the factor (1 +6R)varies from 2.8-4.6.The backscattered light significantly affects the treatment light dose for superficial lesions. The light within a bulky tumor is approximated: E =Eo k exp(-z/delta),where z is the depth in the tissue, k is a backscatter term, k =3 +5.1R -2exp(- 9.7R)(eg.,k =4.1-5.9 for R =0.30-0.60),and delta is the 1/e optical penetration depth (typically about 0.3-0.5 cm).