Variable Contrast Paper: One Emulsion in Five Parts Dyed 30, 15, 6, 2 and 0 mg per Litre (Schwarz, Bell & Howell, US 2,620,272)

Variable contrast paper lets one box of paper print hard or soft: the colour of the printing light chooses. Schwarz split one emulsion into parts, dyed each with a halving dose of a green-sensitising dye, and blended them back. Blue light then works on every grain and gives hard prints; green light reaches only the dyed grains and gives soft ones, with smooth steps between. Result: variable contrast paper, hard through a blue or magenta filter, softer through yellow, softest through orange or light red. Source: Schwarz, US 2,620,272 (Bell & Howell, 1952), Example 3. Unknown: the emulsion and the dye, as the patent gives no recipe for either. Not given: dye solvent, coating thickness, drying. Picture drawn by Youblob.