In Bolometers, the change in resistivity resulting from an increase in temperature is exploited to measure X-ray fluence and flux. Central to the Bolometer is a thin metallic foil that absorbs the incident X-rays. Prior to the X-ray pulse current is passed through the Bolometer foil, resulting in a steady state drop across the resistive foil. As the X-ray pulse is absorbed in the Bolometer foil, the foil temperature increases and the voltage across the Bolometer foil also increases because of the resistance increase. The increase in voltage is directly related to the absorbed energy. The voltage trace can also be differentiated to give the flux.

Bolometers are excellent fluence diagnostics. In comparison to calorimeters they generate a larger signal, making them easier to record. Also, the temporally resolved pulse provides additional performance information. Calorimeters on the other hand are simpler, less expensive devices that are easy to calibrate and use.

The spectral response functions (fraction absorbed) of aluminum, nickel, and gold foil bolometers.