First, we consider the effect of powder particles on the spectrum of light reflected from the skin. Figure 4(a) and 4(b) show the reflection spectra for skin covered with various sizes of particles that are surrounded by air and gel, respectively. The angle of observation is 0 deg through 10 deg. For the suspension medium of air, the shape of the spectrum changes dramatically from that of the bare skin for particles of size 0.25 and 0.3 μm diameters because of the complex wavelength dependence of Mie scattering for particles with a diameter comparable to the wavelength of light. Another reason for this is that the average scattering probability is relatively high for these particle diameters [see Fig. 5(a)]: 27.3% for 0.25 μm and 20.8% for 0.3 μm, larger than those for other diameters (4.4, 9.3, 4.4, and 0.8% for 0.1, 0.5, 1.0, and 5.0 μm, respectively). Conversely, the spectral shape is almost preserved for 1.0 and 5.0 μm size particles. For these particles, the average anisotropy factor is 0.42 and 0.56, for diameters 1.0 and 5.0 μm [see Fig. 3(d)], respectively, which are larger than those of other diameters (0.15, 0.25, 0.22, and 0.27 for 0.1, 0.25, 0.3, and 0.5 μm, respectively). The small scattering probability and the large anisotropy factor make application of large particles on skin less effective. The spectrum of 0.1 μm particles differs from that of bare skin only in the short wavelength region (). In this region of the spectrum, the scattering probability increases as the wavelength becomes shorter for these small particles (probability less than 8% for wavelengths larger than 460 nm compared to 19.5% for 400 nm). The incident photons are then scattered away in other directions at short wavelengths, reducing the reflected intensity in the direction around 0-deg.