From the Greek episkot, to overshadow. Pronounced eh-PIS-ko-tis-ter.
An episcotister is a rotating wheel
with one or more openings in its surface. It may be rotated in front of a light source
, thereby altering the appearance
The aperture(s) may be variable in size, sometimes with shutters to allow for easy adjustment.
Episcotisters are often used in psychological experiments on perception of light and color. With a light source, an episcotister will produce a regular pattern of on/off flashes, and with an image, can produce the effect of transparency.
That is, an image viewed behind an episcotister that is rotating quickly enough will appear to be visible, as the eye does not register the interruptions.
Colored episcotisters are also used to influence color perception, and F. Metelli's episcotister model of transparency (which I don't really understand--anyone care to explain?) is often mentioned in articles on perceptional neuroscience.
Fun fact: a Google search on the word returns a page from the Nude Pics Of German Men Appreciation Organization.