Is a Lambertian reflector illuminated by a smaller fraction of the incident radiation when it's tilted? In reading about Lambertian reflectance on Wikipedia I found the following phrase (in bold) which doesn't sound right to me:


  In computer graphics, Lambertian reflection is often used as a model for diffuse reflection. This technique causes all closed polygons (such as a triangle within a 3D mesh) to reflect light equally in all directions when rendered. In effect, a point rotated around its normal vector will not change the way it reflects light. However, the point will change the way it reflects light if it is tilted away from its initial normal vector since the area is illuminated by a smaller fraction of the incident radiation.


The way I picture the situation described in the paragraph, only tilting away from the light source would cause  less light to be incident in a given area. In general, tilting away from the initial normal vector could lead to either an increase or a decrease in incident light per area, as this says nothing about the location of the light source.

Have I misunderstood the context, or is this something that should be rewritten on Wikipedia?
I see some problems in the quote you posted.


  In effect, a point rotated around its normal vector will not change
  the way it reflects light.


This is true, because a Lambertian reflector will never change the way it reflects light. The underlying principle stays the same. Also, Lambertian surfaces are isotropic, so the amount of reflected light won't change either (which is probably what this sentence is aiming for).


  However, the point will change the way it reflects light if it is tilted away from its initial normal vector since the area is illuminated by a smaller fraction of the incident radiation.


Again not true, because the principle doesn't change. The amount may change, except for the special case that the cosine is &lt;= 0 before and after the tilting. The amount does not necessarily grow, except if we define that the cosine equals 1 before, i.e. that the normal points directly towards the light source.

This whole paragraph should probably be rewritten to be less ambiguous. Including isotropy could make it more complete.