Editing Absorption spectroscopy (section)

===Remote sensing===
One of the unique advantages of spectroscopy as an analytical technique is that measurements can be made without bringing the instrument and sample into contact. Radiation that travels between a sample and an instrument will contain the spectral information, so the measurement can be made [[remote sensing|remotely]]. Remote spectral sensing is valuable in many situations. For example, measurements can be made in toxic or hazardous environments without placing an operator or instrument at risk. Also, sample material does not have to be brought into contact with the instrument—preventing possible cross contamination.

Remote spectral measurements present several challenges compared to laboratory measurements. The space in between the sample of interest and the instrument may also have spectral absorptions. These absorptions can mask or confound the absorption spectrum of the sample. These background interferences may also vary over time. The source of radiation in remote measurements is often an environmental source, such as sunlight or the thermal radiation from a warm object, and this makes it necessary to distinguish spectral absorption from changes in the source spectrum.

To simplify these challenges, [[differential optical absorption spectroscopy]] has gained some popularity, as it focusses on differential absorption features and omits broad-band absorption such as aerosol extinction and extinction due to rayleigh scattering. This method is applied to ground-based, airborne, and satellite-based measurements. Some ground-based methods provide the possibility to retrieve tropospheric and stratospheric trace gas profiles.