spectroscopy is the analysis of radiation resolved by wavelength — in fusion, the diagnostic family that reads a plasma's composition, temperature, and rotation from the light it emits.
The technique long predates 📝fusion: dispersing light into its component wavelengths and reading the bright and dark lines is how nineteenth-century science identified the elements in the sun, and the same principle applies unchanged to a laboratory 📝plasma. Every ion species emits its own characteristic set of spectral lines, so line identification names the impurities present in the discharge and their concentrations, which set the plasma's effective charge, 📝Zeff (effective charge). The lines carry motion as well as identity: Doppler broadening measures the emitting ions' temperature, and the Doppler shift of a line measures 📝plasma rotation.
The near-neighbor distinction is against the 📝bolometer, which views the same radiation but deliberately integrates it: a bolometer answers how much total power the plasma radiates, while spectroscopy answers who is radiating and in what state. One refinement matters enough to name: a fully stripped core ion emits no lines at all, so charge exchange recombination spectroscopy fires a neutral beam into the core, letting bare ions briefly capture electrons and radiate — the standard route to core 📝ion temperature and rotation profiles in tokamaks.