The forward voltage across the diode necessary to produce the output power (P). Vop has two components, the voltage across the p-n junction and the voltage due to device resistance. Vop is determined by: device structure material composition device fabrication and packaging
For electrical discharge lamps, it is a voltage measured across the discharge when the lamp is actually operating. It is governed by the contents of the chamber and in somewhat independent of the ballast and other external factors.