Abstract
Wave phenomena are mathematically described in similar terms, even though they are of different nature, as is the case for light and sound. Yet, the basic parameters such as velocity and frequency are so different that some of the concepts defined in optics are no longer relevant in acoustics, in particular the notion of spatial and temporal coherence. We briefly present the historical evolution that led to a clear definition of coherence in optics, then we show that it is not adequate for pulse-echo ultrasound and we propose an alternative definition. This leads us to a modified and extended version of an essential theorem in the optical theory of coherence: the Van Cittert-Zernike theorem.
Keywords

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