I did a quick test on this and it seems diffuse reflection
from a surface is clipped to the diffuse value.

// output as 48bit and adjust histogram to compare

#declare M=1; // set levels max point to 255/51=5
//#declare M=51; 

global_settings{
  max_trace_level 6
  radiosity{
    pretrace_start 0.04
    pretrace_end 0.01
    count 800
    recursion_limit 5
    nearest_count 3
    error_bound 0.2
    max_sample 100
  }
}

camera{
  location <0,0,-39>
  direction 1.5*z
}


box{<-3,-3,-40>,<3,3,2>
  pigment{rgb 1}
  finish{diffuse 0.75 ambient 0}
}

light_source{
  <2.5,2.5,1>
  rgb 1/0.75
  spotlight radius 10 falloff 30 tightness 1 point_at<-2.5,-2.5,-1>
}


_____________
Kari Kivisalo

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