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Glossary: E


The difference in alignment between the optical axis and the mechanical axis of a component, typically specified with a mirror.

Edge Thickness (ET)

A calculated value which depends on radii, diameter, and center thickness of a lens. It is thus used as a reference to indicate physical limitations for mounting considerations.

Effective Focal Length (EFL)

An optical measurement given as the distance from the rear principal plane of an optical system to its imaging plane, with an object located at infinity. Determines angular field of view of a lens.

Electronic Shutter

Controls the amount of time a camera collects light or charge during each frame, also known as the exposure time. Standard signal formats employ approximately 60 frames per second (fps), indicating a collecting charge of 1/60 seconds for each frame. As light levels increase the sensor overfills with charge, resulting in blooming or streaking. Electronic shuttering circuits prevent these effects by allowing the sensor to collect charge for a shorter period during each frame.

Electrostatic Discharge (ESD)

Built up charge through the action of static electricity that discharges into a person or device after contact. While ESD voltages frequently exceed 3,000 volts, the current associated with ESD is low making it safe, though at times unpleasant, for humans.


The quality of asymmetrical intensity distribution in a laser beam, as opposed to a circular distribution.


Electromechanical device that creates discrete electrical pulses directly related to the angular position of the input device (shaft). This allows for high resolution feedback data on position. 

Entrance Pupil

The image of the aperture stop through the lens elements preceding it into object space. 

  See also Marginal Ray , Exit Pupil , Chief Ray

Equilateral (Dispersing) Prism

A dispersion prism used to separate white light into its spectral components.  It has three equal 60° angles.

  See also Dispersion

Excitation Filter

A filter used in fluorescence microscopy and spectroscopy to selectively pass a band of wavelengths from a light source.

Exit Pupil

The image of the aperture stop through the lens elements following it into image space.

Extinction Ratio

The relationship between the intensity of incident light to intensity of transmitted light passing through an optic.  It is used to categorize a polarizer or laser's ability to pass a given polarization state relative to another.

  See also Polarization


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