I just did some little experiments and the diffuse lighting is as we'd expect in Poser, based on the simple Lambertian diffuse lighting model for directional lights.

Given
I: the light intensity, expressed as a unit number (0 to 1)
V: the "diffuse value" (also 0 to 1)
C: being the color value of the surface
A: the angle formed by the surface normal and the direction to the light (with 0 being the light is pointing straight at the surface)

then I find that the rendered pixels (as a unit number 0 to 1) is precisely:

I * V * C * cos(A)

This calculation is performed for each color channel, R, G, and B.

So, for example, assuming I is .8, V is .6, C is .902 and the angle is 45 degrees, I expect my pixel value to be:

255 * .8 * .6 * .902 * cos(45)

which comes to 78.06. When I set this situation up in Poser, I read out 78 for the LDRI value. Yay.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)