The property of two or more electromagnetic waves when they are in fixed phase relationship over time. If the crests and troughs of the waves meet at the same time and place they are said to be in phase.
Fr.: zone de cohérence
Of an electromagnetic wave, the area of a surface perpendicular to the direction of propagation, over which the electromagnetic wave maintains a specified degree of coherence.
Fr.: longueur de cohérence
The distance over which an electromagnetic wave train maintains a specified degree of coherence.
Fr.: temps de cohérence
The time over which a propagating electromagnetic wave may be considered coherent. The coherence time of an interferometer is the interval during which the fringe phase remains stable.
Fr.: cohérence spatiale
In a wave train, a correlation between the phases of waves at points separated in space at a given time.