Questions about Durst 138 S opal or halogen lamps and format masks

[Michael R]

It makes sense they would be the same or very similar in that Galerie was a graded paper - ie little need for dye sensitization other than to increase overall speed to incandescent light sources (which would be needed for a pure chloride emulsion but not the mixed chlorobromide or bromide emulsions of enlarging papers). The graph does more or less line up with the spectral sensitivity of a non-dye sensitized, mixed halide emulsion (see Haist, James etc.) or what we tend to refer to as “blue sensitive”.

You can find similar curves in tech docs for various bygone graded enlarging papers like Brovira etc.

[Mark J]

Interesting stuff. So, there's nothing intrinsically contrasty about a blue-sensitive emulsion.

[Michael R]

Correct.

The variable contrast of Ilford Multigrade papers has to do with the spectral sensitivities of the three emulsions. All three emulsions have the same contrast, and they all have the same sensitivities to “blue” and shorter wavelengths, but they have different sensitivities to longer “green” wavelengths. When you remove all the green light (as a #5 filter does), the three emulsions all behave the same and contribute equally (ie they have the same D/log-H curves). As you reduce the proportion of shorter “blue” wavelengths in the exposing light (lower the filter number), the blue-sensitive emulsion component becomes slower relative to the emulsion components with the broader spectral sensitivities. As a result, the D/log-H curves of the emulsion components are no longer superimposed. They all still individually have the same shape/slope, but are displaced from one another horizontally, and when you sum them up the “net” combined curve has a smaller slope (lower contrast).

You can prove this to yourself in a simplified way by plotting two hypothetical D/log H graphs each with two emulsions. Assume the two emulsions have identical curve shapes. On the first graph, assume they have the same speed, plot the two curves and the summed curve. On the second graph do the same but assume the two emulsions have different speeds, say a 1 stop difference. See what happens.

Ilford has a document about MG paper contrast control which essentially illustrates.

This is why for people DIY-ing “split grade” LED heads for B&W, and wanting to retain the maximum contrast range, I suggested thinking about violet LEDs in place of blue. By shifting the peak into shorter wavelengths you’re going to have less bleed into the wavelength range where the relative speeds of the three emulsion components start to differ (which is what results in reduced overall contrast)

Interesting stuff. So, there's nothing intrinsically contrasty about a blue-sensitive emulsion.