Aperture

In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are, which is of great importance for the appearance at the image plane. If an aperture is narrow, then highly collimated rays are admitted, resulting in a sharp focus at the image plane. If an aperture is wide, then uncollimated rays are admitted, resulting in a sharp focus only for rays with a certain focal length. This means that a wide aperture results in an image that is sharp for things at the correct distance. The aperture also determines how many of the incoming rays are actually admitted and thus how much light reaches the image plane (the narrower the aperture, the darker the image for a given exposure time). In the human eye, the pupil is the aperture.

An optical system typically has many openings, or structures that limit the ray bundles (ray bundles are also known as pencils of light). These structures may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place, or may be a special element such as a diaphragm placed in the optical path to limit the light admitted by the system. In general, these structures are called stops, and the aperture stop is the stop that determines the ray cone angle, or equivalently the brightness, at an image point.

Counter (typography)

In typography, a counter is the area of a letter that is entirely or partially enclosed by a letter form or a symbol (the counter-space/the hole of). Letters containing closed counters include A, B, D, O, P, Q, R, a, b, d, e, g, o, p, and q. Letters containing open counters include c, f, h, i, s etc. The digits 0, 4, 6, 8, and 9 also possess a counter. An aperture is the opening between an open counter and the outside of the letter.

The lowercase 'g' has two typographic variants: the single-story '' has one closed counter and one open counter (and hence one aperture); the double-story '' has two closed counters.

Open and closed apertures

Different typeface styles have different tendencies to use open or more closed apertures. This design decision is particularly important for sans-serif typefaces, which can have very wide strokes making the apertures very narrow indeed.

Fonts designed for legibility often have very open apertures, keeping the strokes widely separated from one another to reduce ambiguity. This may be especially important in situations such as signs to be viewed at a distance, materials intended to be viewed by people with vision problems, or small print, especially on poor-quality paper. Fonts with open apertures include Lucida Grande, Trebuchet MS, Corbel and Droid Sans, all designed for use on low-resolution displays, and Frutiger, FF Meta and others designed for print use. This design trend has become increasingly common with the spread of humanist sans-serif designs since the 1980s and the 1990s and the use of computers requiring new fonts which are legible on-screen.

Antenna aperture

In electromagnetics and antenna theory, antenna aperture or effective area is a measure of how effective an antenna is at receiving the power of radio waves. The aperture is defined as the area, oriented perpendicular to the direction of an incoming radio wave, which would intercept the same amount of power from that wave as is produced by the antenna receiving it. At any point, a beam of radio waves has an irradiance or power flux density (PFD) which is the amount of radio power passing through a unit area of one square meter. If an antenna delivers an output power of P_o watts to the load connected to its output terminals when irradiated by a uniform field of power density PFD watts per square metre, the antenna's aperture A_eff in square metres is given by:

So the power output of an antenna in watts is equal to the power density of the radio waves in watts per square metre, multiplied by its aperture in square metres. The larger an antenna's aperture is, the more power it can collect from a given field of radio waves. To actually obtain the predicted power available P_o, the polarization of the incoming waves must match the polarization of the antenna, and the load (receiver) must be impedance matched to the antenna's feedpoint impedance.