Hazardous air pollutants monitor

Gases such as pollutants are discerned in a sample, preferably using a laser to emit an infrared light beam along a sight path for illuminating the gases. The beam is directed along the sight path and collects light from the gases. An optical tunable filter selects a particular optical wavelength or band, and the filtered wavelength is focused on a detector coupled to an analyzer. The analyzer can include a processor that can pulse the laser or coordinate collection of data from sample gases and from a reference cell containing known gases, e.g., using an optical chopper wheel. The processor analyzes the light levels as a function of wavelength to discriminate for the presence of selected gases by determining a characteristic pattern of light absorption and light emission by the gases. The tunable filter has an acousto-optical crystal of Tl3AsSe3, in which an RF acoustic wave is generated for varying diffraction of light by the filter, thereby selecting a wavelength. A nonlinear output crystal can in inserted controllably to selectively generate harmonics for increasing wavelength coverage, and can also be Tl3AsSe3. A broad band infrared illumination source can be used. When the infrared excitation beam is off, the filter acts on infrared emissions of the gases. Sensitivity to narrow emission lines is increased by modulating the RF drive to the tunable filter, producing derivatives of the spectra.