In long - Why it should not? What is the absorption of e.g. water and air at 633 nm? Can you differentiate them using SPR? The sensed value in SPR is a complex value (complex dielectric premittivity or refractive index respectively). Absorption is mainly represented by its imaginary part, in most cases it has a negligible influence. Actually, only materials with very high absorption (e.g. dyes, metalls etc) can be used to experimentally demonstrate the influence of absorption on SPR signal.