ThermaNin is a ninhydrin derivative for developing fingerprints on thermal paper. Regular ninhydrin and DFO solutions are either based on polar solvents (like acetone or ether) or contain them. These turn the top (active) layer of thermal paper gray or black.
Japanese researchers have published that hemiketals of ninhydrin are soluble in petroleum ether (or other apolar solvents like pentane, heptane) without the need for addition of polar solvents. These solutions develop fingerprints on thermal paper without darkening of the surface.
A waiting number ticket treated with regular ninhydrin solution