Some SBIR blackbody controllers allow control of either absolute or differential temperature. Absolute temperature is the temperature of the blackbody surface. Differential temperature is the difference between the blackbody surface temperature and the temperature of some other surface.
For example, if you were controlling the absolute temperature of the blackbody and the setpoint was +20.000°C, then the temperature of the blackbody surface would be controlled at +20.000°C. If you were controlling the differential temperature of the blackbody, the blackbody surface would be held +20.000°C higher than the temperature of the blackbody’s reference temperature probe. As the temperature of the reference probe moved higher or lower, the temperature of the blackbody would move higher or lower to maintain the proper temperature difference.
Normally, the reference probe is mounted in a target in front of the blackbody surface. This allows control of a precise temperature difference between the blackbody and the target, creating a temperature contrast to be viewed by the unit under test.
SBIR instruments label the blackbody temperature as “T2”, and the reference probe temperature (target temperature) as “T1”. So when the instrument is in absolute control mode, it is controlling T2. When it is in differential control mode, it is controlling dT = T2 – T1.