Page "Photon mapping" Paragraph 19

* To avoid emitting unneeded photons, the initial direction of the outgoing photons is often constrained.

Instead of simply sending out photons in random directions, they are sent in the direction of a known object that is a desired photon manipulator to either focus or diffuse the light.

There are many other refinements that can be made to the algorithm: for example, choosing the number of photons to send, and where and in what pattern to send them.

It would seem that emitting more photons in a specific direction would cause a higher density of photons to be stored in the photon map around the position where the photons hit, and thus measuring this density would give an inaccurate value for irradiance.

This is true ; however, the algorithm used to compute radiance does not depend on irradiance estimates.