Differential spectroscopy is spectroscopy of the difference between two similar spectra, the two from slightly different circumstances, to gain information about this difference. Essentially, the values obtained from one spectrograph are subtracted from those of the other. Transit spectroscopy basically refers to differential spectroscopy between a spectrograph of electromagnetic radiation from some source versus EMR which also has passed through some particular gas, e.g., for the analysis of extra-solar planet atmospheres through analysis of the spectral energy distribution (SED) of the host star and the SED of the star at the beginning and end of a transit when some of the EMR is passing through the atmosphere. The term also applies to spectroscopy near versus during the time of the secondary eclipse to compare the SED of the host star with that when some light reflected from the planet is added to it. In both these cases, the technique produces results even if the star and planet are not resolved.
Various forms of differential spectroscopy for other (e.g., non-astronomical) uses may use a single instrument that carries out the comparison during observation, e.g., analysis of material in Earth atmosphere such as pollutants.