I have a question about recursive, backward raytracing.

When an incident ray (I) strikes an object at point P, first
the normal (N) is calculated. Light rays are calculated (L1 to Ln 
where n is the number of light sources) - these being the light
rays that do not intersect with anything. The reflected ray (R) and
the transmitted ray (T) is calculated from the formulae.

Calling the routine recursively on R and T will return the colours 
along the rays (R and T) as rCol and tCol. Each object has its own
colour oCol and each light source has liCol (1 <= i <= n).

The question is: 
  How do you combine rCol, tCol, oCol and all the liCol's to get
  the correct resulting colour to return along the I ray?

All colours are defined as strucures (records) having r, g, b components
between 0 and 1.

If anyone has done this before could you give me a few hints?

--
Adrian Secchia