Dark halo

The mysterious "dark halo" around bright objects in an IO-captured image is based in the very fact that the IO relies on the splashing caused by highly energized electrons. When a very bright point of light (and therefore very strong electron stream emitted by the photosensitive plate) is captured, a great preponderance of electrons is ejected from the image target. So many are ejected that the corresponding point on the collection mesh can no longer soak them up, and thus they fall back to nearby spots on the target much as splashing water when a rock is thrown in forms a ring. Since the resultant splashed electrons do not contain sufficient energy to eject enough electrons where they land, they will instead neutralize any positive charge in that region. Since darker images result in less positive charge on the target, the excess electrons deposited by the splash will be read as a dark region by the scanning electron beam.

                    This effect was actually "cultivated" by tube manufacturers to a certain extent, as a small, carefully controlled amount of the dark halo has the effect of "crispening" the viewed image. (That is, giving the illusion of being more sharply focused than it actually is). The later Vidicon tube and its descendants (see below) do not exhibit this effect, and so could not be used for broadcast purposes until special "detail correction" circuitry could be developed.