BS: beam splitters; CL: collimating lenses ; DC: diffusion cell; FG&C: frame grabber and computer; L: laser; M: mirrors; NDF: neutral density filter; OL: object lens; OS: opal screen;SF: spatial filters. To evaluate the diffusion coefficient, one does not need to know the wavelength, length of the cell, and initial concentration. The sensitivity is proportional to d, proportionality constantk, and inverse of wavelength.