Just two summary patents. One from each of the key patent holders of the field: Kodak, and Ilford.

The first, from Ilford:

Photographic variable contrast material – European Patent Office – EP 0675400 A2

The first commercially available variable contrast material were produced by ILFORD Limited following 5 the methods set out in British Patents 494088 and 547883. In these variable contrast material is produced by combining two silver halide emulsions of which each emulsion has its own gamma value and is sensitised to a particular wavelength region of the light spectrum to which the other emulsion is not substantially sensitive. In practice one emulsion was sensitised to the blue green region of the visible spectrum and the other emulsion which is of soft contrast was not spectrally sensitised. When printing a io pale magenta filter was used to yield a low contrast print and a deep yellow filter was used to yield a high contrast print. Shortly after the introduction of the ILFORD variable contrast material the Defender Photo Supply Company of the US introduced another type of variable contrast material. This was described in British Patent 547060. In this system use is made of sensitising dyes which have a preferential action on some of the silver halide particles and such sensitising dyes are used in less quantity than to produce 75 maximum sensitivity. Thus the selected dyes impart to the emulsion a softer gradation when exposed to light within the spectral region to which the silver halide has been made sensitive than when exposed to light in the region of the spectrum to which the emulsion is naturally sensitive. But such material provides a

limited contrast range because some dyeing of all the silver halide crystals occurs.
Improvements have been made in the production of variable contrast material as described in British 20 Patent 547060 and in particular blends of emulsions having different levels of sensitisation have been employed. However in order to be able to achieve very low contrast from variable contrast material which comprises blended emulsions, when exposed to yellow or minus blue light, it is necessary for the spectral sensitivity of the emulsion component with the highest level of sensitising dye to be as high as possible in relation to the emulsion component which has not been spectrally sensitised. Unfortunately it is usually the 25 case that spectrally sensitised emulsions tend to have a contrast at least as high as their unsensitised precursors. Thus when a blend of emulsions is exposed under conditions adjusted to provide minimum contrast, that is to say when the sensitivities of the individual emulsions of the blend show maximum separation the contrast of the most heavily dyed component of the blend will be evident as a region of high contrast over the extreme low density end of the density/log exposure characteristic curve. This results in loss of highlight detail when printing high contrast negatives.
It is the object of the present invention to overcome this problem.

and this developer formula for Multigrade paper was provided:

EP0675400A2.pdf

And from Kodak:

When making prints from photographic negatives, it
is desirable to use photographic paper which has a contrast selected to achieve satisfactory tone-reproduction of the original image based upon the contrast of the negative. Papers having higher contrasts, for example,
are useful in printing negatives that themselves exhibit low contrasts, so that a satisfactory final print can be achieved. As such, photographic manufacturers offer several grades of photographic paper. In order to avoid the need for separate papers of different grades, “variable contrast’ papers possessing the ability to achieve
different, selected, contrasts depending upon the wave-length of exposing light have also been employed.
A useful element is described in commonly assigned copending applications of Henry et al. Ser. No. 774,392 filed Oct. 10, 1991 and Price et al. Ser. No. 774,440 filed Oct. 10, 1991. A problem with variable contrast systems employing such elements is that the contrast range that the emulsion is capable of producing has been extended
to such an extent that existing filters are not able to take full advantage of the capabilities of the light sensitive element.
Accordingly, it would be desirable to provide an element/filter system that can take greater advantage of the contrast range of the improved light sensitive elenet.

The yellow spectral absorption characteristics can be
provided by a filter like a Wratten 4 filter and the purple spectral absorption characteristics described above can
be provided by a filter like a Wratten 34 filter. To optimize the Wratten 4 and 34 filters for use in this system, their density can be adjusted so as to give transmittance values like that shown in FIGS. 1 and 2 so that the
exposure times with these filters are in the same approx imate exposure range as are used with existing filters for
light sensitive elements of this type.

Click to access US5445929.pdf

---

from my character of paper.