Small particles of optically encoded microporous Si, can detect organic vapors by measurement of the intensity of reflected light using a laser probe and a detector stationed 20 m away. The porous particles absorb condensable vapors, producing a change in refractive index and consequently a quantitative change in optical reflectivity at the wavelength of the laser. (M. J. Sailor et al., September 16, Adv. Mater. 2002, 14, 1270-1272)