Abstract: A narrow-linewidth, variable-wavelength, active, absorbing LIDAR (Light Detection and Ranging) system and method. The linewidth of the source and detector is sufficiently narrow to produce a reasonable signal-to-noise ratio for the system. The central wavelength of the source and detector can be synchronously changed. The system comprises a tunable light-emitting source and uses the absorption of light to determine the chemical composition of the item being detected. A wavelength tunable filter synchronized to the tunable source reduces or eliminates background noise and thus increases the level of detection for low concentrations of the substance to be detected.