The IRE unit is used in the measurement of composite video signals. Its name is derived from the initials of the Institute of Radio Engineers.^[1]

A value of 100 IRE is defined to be +714 mV in an analog NTSC video signal. A value of 0 IRE corresponds to the voltage value of 0 mV, the signal value during the blanking period. The sync pulse is normally 40 IRE below this 0 IRE value, so the total range covered from peak to trough of an all white signal would be 140 IRE.^[2]

Video signals use the "IRE" unit instead of DC voltages to describe levels and amplitudes. Based on a standard 1 V_pp NTSC composite-video signal that swings from -286 mV (sync tip) to +714 mV (peak video), a 140 IRE peak-to-peak convention is established. Thus, one NTSC IRE unit is 7.143 mV (1/140 V or 7+1/7 mV),^[3] where -40 IRE is equivalent to -285.7 mV, and +100 IRE is equivalent to +714.3 mV. 0 IRE is equivalent to 0 V. The black level is equivalent to 53.57 mV (7.5 IRE).^[4]

The PAL video signal is slightly different in that it swings from -300 mV to +700 mV, instead. Thus, one PAL IRE unit is 7 mV, where -43 IRE is equivalent to -300 mV at the sync tip, and +100 IRE is equivalent to +700 mV at the peak video level. Black level is the same as the blanking level 0 mV (0 IRE).^[5][6]

The reason IRE is a relative measurement (percent) is because a video signal may be any amplitude. This unit is used in the ITU recommendations BT.470 and BT.1700 which define PAL, NTSC and SECAM:^[7][8][9]