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An IRE is a unit used in the measurement of composite video signals. Its name is derived from the initials of the Institute of Radio Engineers.
A value of 100 IRE was originally defined to be the range from black to white in a video signal. A value of 0 IRE corresponds to the zero voltage value during the blanking period. The sync pulse is normally 40 IRE below the zero value, so, peak to peak, an all white signal should be equal to 140 IRE.
The reason IRE is a relative measurement (percent) is because a video signal may be any amplitude. This unit is used in the ITU-R BT.470 which defines PAL, NTSC and SECAM.
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