An apparatus includes an emitter, the emitter comprising an ultraviolet light emitting diode (UV-LED) and being disposed on a first end of a bounded volume suitable for holding a gas. The bounded volume can be a chamber, a room, or the like. The apparatus includes a detector, the detector comprising an ultraviolet light sensor (UV sensor) and being disposed on a second end of the bounded volume, the second end being opposite the first end, wherein the UV-LED comprises a point source, and wherein the emitter generates a parallel beam of light.