Gather the N nearest photons using the nearest neighbor search function on the photon map. Let S be the sphere that contains these N photons. For each photon, divide the amount of flux (real photons) that the photon represents by the area of S and multiply by the BRDF applied to that photon. The sum of those results for each photon represents total surface radiance returned by the surface intersection in the direction of the ray that struck it.
Gather the N nearest photons usin,g the nearest neighbor search function on the photon map. Let S be the sphere that contains these N photons. For each photon divide the amount of flux (real photons) that the photon represents by the area of S and multiply by the BRDF applied to that photon. The sum of those results for each photon represents total surface radiance returned by the surface intersection in the direction of the ray that struck it.