/usr/share/radiance/clamp.cal is in radiance-materials 4R1+20120125-1.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | {
Map values from 0 to inf softly from 0 to 1
}
clamp(x) = x/2 * (sqrt(x*x+4) - x);
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