orifice

An orifice is generally a small hole through which fluid flows. In the case of electron microscopes, the orifice is defined to be a small aperture that allows to pass an electron beam but maintains the pressure difference between the high vacuum chamber and low vacuum chamber.
In a low-vacuum SEM (LVSEM), using the aperture, the electron gun chamber and the lens chamber above the objective lens must be kept at a high vacuum (a low pressure), and the specimen chamber below the objective lens must be kept at a low vacuum (a high pressure). The high vacuum gun chamber and lens chamber and low vacuum specimen chamber are evacuated independently by different evacuation systems (differential pumping), thereby the pressure difference of 4 to 5 orders of magnitudes being maintained.