Verdet's coherence condition

The Verdet coherence condition (according to Marcel Émile Verdet (1824–1866)) is a term from optics which is used for diffraction at a single or double slit when (spatially) coherent light is to be generated with an illumination slit . This stipulates the maximum width an illumination gap may have in order to generate sufficiently coherent light so that a diffraction pattern can be generated with the diffraction gap illuminated with it.

${\displaystyle D\cdot {\frac {d}{f}}=D\cdot \sin(\vartheta )\leq \lambda }$

With

• ${\displaystyle \lambda }$: Wavelength
• ${\displaystyle D}$: Width of the lighting gap
• ${\displaystyle d}$: Width of the diffraction slit (or slit distance for a double slit)
• ${\displaystyle f}$: Focal length of the lens between the illumination and diffraction slit
• ${\displaystyle \vartheta }$: half the effective opening angle between the illumination and diffraction slit