Ferrolens or Ferrocell is a type of superparamagnetic optical device which can display magnetostatic or dynamic magnetic fields in real time and in color. In other words, it is a quantum optical magnetic flux viewer. 
For its operation it is using a thin film of diluted ferrofluid encapsulated inside a flat lens loosely similar to a Hele-Shaw cell filled with ferrofluid however, there are some significant differences due the quantum magnetic optic properties of its thin film and lens structure. Usually the ferrolens is fitted with a LED strip light programmable source on its periphery emulating different lighting conditions. The superparamagnetic nanoparticles inside the ferrofluid are following the magnetic flux of an external field induced into the ferrolens (i.e. a permanent magnet placed on top or under the ferrolens). At the same time, the oriented and aligned with the magnetic field lines nanoparticles, are emitting back part of their incident light (Video) thus essentially allowing them to "paint" the magnetic field lines and therefore making them visible. Light intensity and color texture of the field lines shown can vary slightly accordingly to magnetic field strength and direction.
A ferrolens has advanced visualization capabilities in detail, spatial resolution, sensitivity, color information and can depict also depth of field information on an observed magnetic field essentially making it a 3D holographic nanomagnetic direct observation passive device for magnetic fields and related quantum effects. Also when the ferrolens is used with a LED light strip, it shows the wire-frame model of the individual magnetic flux lines of a static magnetic field.
Currently, it is the only device, reported by academia of this type which can efficiently visualize fast changing (i.e. dynamic) magnetic fields and its superparamagnetic properties is allowing it to display the Quantum Field of Magnets (QFM) or else called Quantum Magnet.
Further academic research is also carried out with the ferrolens for the polarization of light using magnetic fields. and study of dynamical systems by magneto-controlled diffraction of light and lasers.
Additionally, a ferrolens can be used effectively as a magnetically controlled Minkowski space–time emulator device for research on topological defects based on disordered ferrofluid such as magnetic monopoles , cosmic strings and the space–time cloak. It is imperative to notice here, that the device although controlled by magnetism is not magnetohydrodynamic since it is not electrically conductive but an insulator.
Ferrolenses are available today mainly for research applications and education in general, under the registered trademark Ferrocell as a patented product. Many Youtube videos are available with “do-it-yourself” instructions for making ferrolenses for personal use. Video
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