/ˈɛp.ɪ.skoʊp/

Episcope

noun · optics

What it is

An episcope is the optical assembly that makes reflective projection possible. A bright lamp illuminates a solid, non-transparent original; a mirror and lens then collect the light bouncing off its surface and cast an enlarged image onto a screen or wall. It is, in essence, the working core of the opaque projector — the term names the optical principle rather than the boxed consumer product built around it.

The distinction that matters is what the device can accept as a source. An episcope handles opaque originals only — a photo, a page, a drawing. An epidiascope combines two light paths so it can project either an opaque original (by reflection) or a transparent one such as a slide (by transmission). The transmissive-only cousin, which projects only transparencies, is the diascope. For an artist tracing a small reference up to full size, the episcope's reflective path is the one that removes the need to first make a slide.

Etymology

"Episcope" is built from the Greek epi-, "upon," and skopein, "to look" — literally an instrument for looking upon a surface, fitting for a device that reads light off the face of an opaque object. The companion term "epidiascope" adds dia-, "through," to signal the second, transmissive light path that lets it project transparent originals as well. Both words entered technical English through late-19th and early-20th-century optics, the era when reflective and combined projectors became standard classroom and studio equipment.

Examples in use

A lecturer of that era slotted a book page under the episcope to throw a diagram onto the wall for the whole room. An artist used the same reflective path to enlarge a small study before tracing it. Museum and teaching catalogues distinguished an "episcope" from an "epidiascope" precisely by whether the machine could also take slides. Today the same enlarge-and-trace task is handled without optics at all by the AR tracing app, which registers the reference over the real surface on a phone screen, while the light box covers the same-size, thin-paper case the episcope never addressed.