Correlation spectroscopy is an attractive technique for sensing and analysis applications because it combines the attributes of mechanical and optical simplicity, high selectivity, and good sensitivity. In correlation spectroscopy, the degree of correlation between the transmission of an unknown sample and a reference cell containing a known target compound, or chemical species, is determined over a fixed spectral range.
Correlation spectrometers measure trace amounts of a chemical in the presence of many other chemicals by comparing the light transmission of a sample to a known reference. A correlation spectrometer determines the concentration of a target compound in a sample. A new instrument was developed for measuring trace amounts of chemicals like methane or carbon monoxide.
The instrument comprises an entrance aperture for allowing sample spectrum light from the sample to enter the spectrometer; a collimating section for collimating the sample spectrum light from the entrance aperture onto a diffraction grating, in which the diffraction grating spectrally disperses the sample spectrum light; a telecentric imaging section for focusing the spectrally dispersed light onto a mask wheel, which comprises a region of coded masks to provide spectral filtering of the sample spectrum light; and a broadband photodetector for recording the spectrally filtered sample spectrum light.
The spectrometer encodes dozens of reference signals onto a compact disk, alleviating the need to port around reference gas samples or run samples to a laboratory for gas chromatography. The disk-encoded correlation spectrometer enables rapid, high-confidence, point-of-service measurements of trace gases. The spectrometer can measure the transmission spectrum from a given sample of gas using infrared light, and can detect the transmission or emission spectra in any system where multiple species are present in a known volume.
In contrast to many conventional spectrometers, the new instrument is simple and can be miniaturized. Eigen-spectra encoded on a rotating mask enables lock-in detection of gas concentration.