This invention provides a system and method that enables the use of incoherent light sources, such as light emitting diodes, to provide for the detection of gaseous species which exhibit broadband absorption features (e.g., nitrogen dioxide and the halogen gases). The light emitting diode (LED) is an ideal light source for such an arrangement in that it can be modulated at high frequencies (allowing for omission of external modulation equipment) and provides sufficient illumination within a reasonably narrow wavelength band as compared to, for instance, an incandescent light source. A further advantage of a LED as a light source compared to alternatives such as a gas discharge or arc lamp is that the light output of the LED is highly stable, limited by the stability of the current source used to drive it. Use of a confocal or near-confocal resonant optical cavity maximizes coupling of the light source to the cavity.