An interesting scheme of picture reception is known as the pneumatic
light valve, the vibration of which causes a shadow band to oscillate
across a lens opening into the camera.
In a circular center opening in a magnetized iron diaphragm is suspended
an iron disc somewhat smaller than the opening. This small disc is
magnetically held by its edge to the inside edge of the opening in the
diaphragm, with its plane in the plane of the magnetic field of the
diaphragm.
Upon the disc is mounted a tiny mirror, and as the suspension of the
disc is in the magnetic field held there by the strength of the field
itself, it is extremely easily disturbed, so that a small beam of light
reflected from the mirror can be vibrated with a very little current
through great amplitude.
As the beam of light has a transverse shadow band therein of a width to
normally close the lens opening into the camera, the varying
amplification of the vibration of the mirror, and therefore, of the
shadow, admits a proportional amount of light.