  This paper shows analytically and numerically that a vortex plate coupled to
a neutral density filter can deliver a true optical derivative when placed at
the focal plane of a $2f$ lens pair. This technique turns spatial variations in
intensity into an intensity, which square root is the spatial derivative of the
initial intensity variation. More surprisingly, it also turns any spatial
variations in phase into an intensity, which square root is the spatial
derivative of the initial phase variation. Since the optical derivative drops
the DC component of the signal, it is possible to measure the full electron
plasma turbulence spectrum optically, without using any interferometer.
