Thermal sensors for calorimetry can include vanadium oxide, heavily p-doped amorphous silicon, or other materials with high temperature coefficients of resistivity. Such thermal sensors can have low noise equivalent temperature difference (NETD). For example, a thermal sensor with NETD no greater than 100 .mu.K over a bandwidth range of approximately 3 Hz or more can include a thermistor including vanadium oxide sputtered at room temperature under conditions that yield primarily V.sub.2O.sub.5; more specifically, the NETD can be no greater than 35 .mu.K, or even 10 .mu.K over a bandwidth range of approximately 3 Hz or more. If a low noise thermal sensor has NETD no greater than 50 .mu.K over such a bandwidth range, a low noise output circuitry connected to its thermistor can provide an electrical output signal that includes information about input thermal signal peaks with amplitude of approximately 100 .mu.K.