In this work, the authors report on the optical properties of carbonaceous nanoparticles produced in argon sputtering dc discharges. These particles are the result of a nucleation-condensation mechanism in gas phase and form porous particulates with a mean diameter greater than 20 nm. The resulting powderlike deposit, which can be collected in the chamber, is analyzed in terms of an effective medium approximation that allows the determination of their optical constants. These parameters are then used in a simple application of the Lorenz-Mie theory, in combination with a laser-extinction experiment, to provide the particle density in the plasma.