I have a simple raytracer that only works back up to the first intersection. The scene looks OK with two different light sources, but when both lights are in the scene, there are dark shadows where the lit area from one ends, even if in the middle of a lit area from the other light source (particularly noticeable on the green ball). The transition from the 'area lit by both light sources' to the 'area lit by just one light source' seems to be slightly darker than the 'area lit by just one light source'.

The code where I'm adding the lighting effects is:

// trace lights
        for ( int l=0; l<primitives.count; l++) {

            Primitive* p = [primitives objectAtIndex:l];
            if (p.light) 
            {
                Sphere * lightSource = (Sphere *)p;

                // calculate diffuse shading
                Vector3 *light = [[Vector3 alloc] init];
                light.x = lightSource.centre.x - intersectionPoint.x;
                light.y = lightSource.centre.y - intersectionPoint.y;
                light.z = lightSource.centre.z - intersectionPoint.z;

                [light normalize];

                Vector3 * normal = [[primitiveThatWasHit getNormalAt:intersectionPoint] retain];
                if (primitiveThatWasHit.material.diffuse > 0)
                {
                    float illumination = DOT(normal, light);
                    if (illumination > 0)
                    {
                        float diff = illumination * primitiveThatWasHit.material.diffuse;
                        // add diffuse component to ray color
                        colour.red += diff * primitiveThatWasHit.material.colour.red * lightSource.material.colour.red;
                        colour.blue += diff * primitiveThatWasHit.material.colour.blue * lightSource.material.colour.blue;
                        colour.green += diff * primitiveThatWasHit.material.colour.green * lightSource.material.colour.green;
                    }
                }
                [normal release];
                [light release];
            }
        }

How can I make it look right?