The formation of soot in high-pressure sprays of n-dodecane is visualized and quantified under conditions relevant to heavy-duty diesel engines. Sprays are injected from a single-hole diesel injector belonging to the family of Engine Combustion Network (ECN) Spray D injectors. Soot optical thickness (KL) and the total soot mass are quantified using a high-speed extinction imaging diagnostic with incident light wavelengths of 632 nm and 850 nm. The total soot mass is derived from KL using a non-dimensional extinction coefficient based on the Rayleigh-Debye-Gans approximation for fractal aggregates (RDG-FA). Previous work showed consistency between the total soot mass measured with incident wavelengths of 406 nm, 520 nm, and 632 nm; however, the present work demonstrates a significant difference for measurements with incident light in the infrared. Large differences in optical properties at the two incident wavelengths are observed in the early transient stages of soot formation based on the ratio of the non-dimensional extinction coefficients (ke ratio). During the quasi-steady period of the spray combustion event, the ke ratio (ke,632-nm/ke,850-nm) is 1.5 +/- 0.5. When computing the ke ratio via RDG-FA, the use of a smaller imaginary component in the complex refractive index leads to better consistency with the experimental data.