An optical head assembly for use in a spectrometer is provided that is configured to characterize one or more constituents within a sample gas. The assembly includes a thermoelectric cooler (TEC) having a cold side on one end and a hot side on an opposite end, a cold plate in thermal communication with the cold side of the TEC, a hot block in thermal communication with the hot side of the TEC, a light source in thermal communication with the cold plate such that a change in temperature of the TEC causes one or more properties of the light source (e.g., wavelength, etc.) to change, and an optical element in thermal communication with the cold plate positioned to collimate light emitted by the light source through the sample gas (such that properties of the optical element vary based on a change in temperature of the TEC).