Black and White digital filters in image processing...

I'm working up to a critique that I'll post at some point in the (hopefully) not too distant future.  To get there from here I want to cover Black and White digital filters in image processing.

The topic arose in my mind when musing over a digital recipe/filter that would accurately/correctly match the spectral response curve of early silver nitrate light sensitive materials, including but certainly not limited to wet-plate collodion.

I found a digital filter that I'm very happy with and for the sake of brevity I'll dispense with the steps I took to the final form.  If anyone wants the complete details, ask.

Here's the starting image.  Top and bottom are grayscale step wedges that I developed for the Digital Zone System I've worked on.  The center of the image is a simple color chip chart.   Examples were processed using RawTherapee.

Starting point -

 

Simple color desaturation - 

Relative RGB channel mix -

Luminance human perception modeling - 

Relative RGB channel mix "Ortho" filter -

Silver Nitrate relative RGB channel mix Red=0 Green=10 Blue=90 filter - 

Comments -

Short answer:  

Digital filters for Black and White color conversions seem to do what they're supposed to.

Long answer:  

Simple désaturation sucks. Colors don't translate to the tonality my eyes would expect to see.

Relative RGB channel mix is a minor improvement over simple désaturation. This is to be expected since all channels are set to 33 percent.

Luminance human perception modeling gives an accurate translation of colors into Black and White for the way I "see" tonality and luminance.   This is an outstanding foundation from which to build tonal separation in digital Black and White photography.  Further, in-camera Sony, Fuji GFX, and Panasonic Lumix S (the only system I've looked at) all appear to conform to luminance human perception modeling Black and White jpg generation.

Relative RGB channel mix with RawTherapee's "Ortho" filter seems to look very much like modern orthochromatique film response.  If I want early silver nitrate light sensitive material response, this is most definitely not what I'd look for.  Close-ish.  No cigar.

Silver Nitrate relative RGB channel set specifically to Red=0 Green=10 Blue=90 appears to hit the target.  The tonal response curve closely matches that of old silver nitrate light sensitive materials.  Goal!

A little more:

I could spend far too long looking/comparing/evaluating various combinations of channel mixture and digital filters and such and this and that.  Should I ever find myself in such a state I processed a number of images.  The collection of the Madness is found here, and scroll right.

In practice, I find the luminance formula in RawTherapee to be excellent for general Black and White conversions.  To explore the early pre-panchromatic Black and White photography "look" my little "Silver Nitrate" formula gives me pleasing results.