(1) A thin, opaque piece of material with a hole through which light can pass and placed in an optical system to keep unwanted light from reaching the image plane or detector e.g., to limit the field of view or to absorb internally scattered light. The stop is the theoretical counterpart of the diaphragm. A shutter is a diaphragm whose opening can be open or closed, usually in a specified interval of time. The mechanism that does the opening and closing is sometimes called the shutter mechanism but is also often considered, together with the diaphragm, to constitute the shutter. (2) The thin disc of glass on which lines forming a reticle are placed. However, the term reticle is often used to denote both the diaphragm and the lines.