Optical apertures and optical slits control the diameter of beams from light sources. An optical aperture may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place, or it may be a special element placed in the optical path deliberately to limit the light admitted by a system. An optical slit refers to a narrow gap, which is used to get a narrow beam of light. There are many types of apertures. Examples include circle, rectangle, ellipse, lhcscreen (a superposition of a circle and a rectangle), marguerite (two lhcscreens, one rotated by 90°), rectellipse (a superposition of an ellipse and a rectangle), and racetrack. Other optical apertures and optical slits are commonly available.
Optical apertures and optical slits are important elements in most optical designs. Optical apertures and optical slits reduce the amount of light that reaches the image plane, to prevent saturation of a detector or overexposure of film. The ‘aperture stop’ or simply the ‘stop’ is the limiting aperture of the system, which restricts the diameter of the cone or cylinder of light that can enter and pass through the system. The diameter of the ‘aperture stop’ is sometimes simply referred to as the aperture diameter of the system, especially when it is used in terms of cameras and telescopes. Aperture tolerance, a parameter closely connected to the aperture, is the sum of the mechanical and alignment tolerances. The mechanical tolerance is the maximal error margin of errors in the element body, which causes a decrease of aperture. The alignment tolerance is a misalignment of the element in the accelerator, which also causes a decrease of aperture. Spectrophotometers are photometers, which are devices for measuring light intensity. Spectrophotometers can measure intensity as a function of the color or more specifically the wavelength of light. There are many kinds of spectrophotometers. Among the most important distinctions used to classify spectrophotometers are the wavelengths they work with, the measurement techniques they use, how they acquire a spectrum, and the sources of intensity variation they are designed to measure. Other important features of spectrophotometers include the spectral bandwidth and linear range. An optical slit is specified according to material, thickness, flatness, tolerance, polish, optical density, and coating. The most commonly available optical slits are made of chromium coated soda lime glass. Optical apertures and optical slits are designed and manufactured to meet most industry specifications.
Optical apertures and optical slits are used in many applications. Examples include their use in the determination of depth of field of a system, limiting the effect of aberrations, determination of system’s field of view, and determining whether the image is vignetted. In imaging applications, the ‘aperture stop’ of a photographic lens can be adjusted to control the amount of light reaching the film or digital sensor (CCD or CMOS). In combination with variation of shutter speed and film speed, the aperture size will regulate the film's degree of exposure to light. Optical apertures and optical slits should adhere to standards specified by International Standards Organization (ISO).