Lamp rerating is modelling the predicted properties of a filament lamp when running the lamp at a voltage other than its specified rating, using a power law function of voltage. The following equations can be used to estimate the new operating point. The exact value of the exponent parameters will typically vary slightly with the particular lamp design.
Va = Applied voltage
Vd = Design voltage
Rerated current = (Va/Vd)0.55 × current at design voltage