newt thanks for the reply, but that's not giving the effect that I'm looking for.
I found this post, but I don't know if it's helpful since I don't know anything about code:
The effect is sometimes described as: "Linear Light uses a combination of the Linear Dodge Blending on lighter pixels and a Linear Burn on darker pixels."
Here is an example PSD with a layer using linear light
I'm not sure how much effort it is to make shaders. Should I post this in the C3 feature requests site? But it's also the kind of thing that I assume a community member could make, in that case should I offer money?