bolometer is an instrument that measures radiated power across all wavelengths by the temperature rise it produces in an absorber — in fusion, the diagnostic that measures the total electromagnetic power a plasma radiates away.
The device predates 📝fusion by decades: the American astronomer Samuel Pierpont Langley invented it in 1878 to measure the sun's infrared radiation, and bolometers remain standard instruments in astronomy today. The defining trait is spectral blindness, which is also the near-neighbor distinction against spectroscopy: where a spectrometer resolves radiation into individual wavelengths and lines, a bolometer deliberately integrates everything — visible light, ultraviolet, X-rays — into a single number, the total power falling on its absorbing foil.
In a magnetic confinement device that number anchors the power balance: heating power that does not leave the 📝plasma as radiation must exit as heat conducted to material surfaces, so bolometry tells operators how the exhaust burden is split. Machines field arrays of bolometer channels viewing the plasma along many chords, and tomographic inversion of those signals maps where the radiation comes from — revealing 📝impurity radiation concentrating in the core, or confirming that deliberately radiated power is doing its job in the exhaust during 📝divertor detachment. That makes the bolometer a central referee of the exhaust-handling strategies on which reactor-scale machines depend.