Effect of the continuous medium on the color of discontinuous substrates. IV. The effect of the refractive index of the continuous medium

It is known from experience that light scattering–absorbing substrates are darker when the continuous medium is water instead of air. This is due to the decrease in scattering efficiency of the scattering particles caused by the smaller value of the ratio n₁/n₂ of the refractive indices of the scattering particles (n₁ = 1.5 to 1.7) to that of water (n₂ = 1.33) as compared to that ratio when the continuous medium is air (n₂ = 1). Experimental evidence for this phenomenon is presented for polyester fabric viewed in air, water, and a concentrated sucrose solution. The wavelength range from 0.4 to 0.7 microns, and absolute “dry” reflectances from 0.02 to 0.7 were covered. The phenomenon cannot be adequately described with the Kubelka-Munk theory of the color of scattering–absorbing substrates.¹ Evidence is presented that at high reflectance values (0.4–0.7), the theory describes the color adequately and the results are consistent with a prediction based on a modified Mie equation for the scattering efficiency of “large” particles.^2,3 At low reflectance values (<0.05), the Kubelka-Munk theory breaks down completely. This is consistent with the observation that this theory is not capable of predicting precisely high dye concentrations on textiles but is quite adequate for low concentrations.