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Glossary: F

Fall Time

Measurement of the interval during which a photodetector's signal and output current drops from 90 to 10 percent.

Fan Angle

The divergence angle of a laser of LED line generator. Used to calculate the length of the illumination line as a function of working distance.

Fiber Jacket

The outer material that surrounds and protects the buffered and unbuffered fibers in an optical cable.


An optical instrument consisting of an objective lens, a coherent fiber bundle, and an eyepiece to examine the output of the fiber bundle.

  See also Borescope


Interlaced imaging devices produce "live" video by scanning odd-numbered lines in the first pass, then even-numbered lines in the second pass, and so on. Two fields equal one frame.

Field Curvature

An optical aberration where focal position changes with field height in a spherical fashion, resulting in flat objects being imaged onto a curved surfaces, as the product of imaging through spherical lens elements. Results in increasing defocus when moving away from imaging axis. 

  See also Aberration

Field Efficiency

An imaging lens's ability to accommodate a large sensor (image), source (object), or angular field of view (in afocal systems).

Field Height

Half the image circle. 

Field Number

The diameter of the image area seen under an eyepiece or objective. The field number can be determined by the following formula:

Field Number (FN)= (Field of View, FOV) X (Objective Lens Magnification)*

*In case of Stereo Microscope: Objective Lens Magnification X Zoom Ratio

Field of View (FOV)

The imaged area of an object under inspection. This is the portion of the object that fills the camera's sensor.

  See also Doubler Tube , Magnification

Figure Error (Form Error)

The imaged area of an object under inspection. This is the portion of the object that fills the camera's sensor.This specification consists of low frequency or larger errors usually peaking one to three times across a part. Form errors are typically specified as peak- to- valley error in waves or fringes, but can also be specified as a linear deviation in microns or as an RMS deviation. These are typically the most common surface specification for aspheric lenses.


An optical component that allows certain wavelengths or frequencies of light to pass through while reflecting or absorbing all others. Types of filters include bandpass, longpass, shortpass, interference notch and rugate notch, color, dichroic, and neutral density.

Filter, Colored Glass

An absorptive filter created by doping glass materials with elements that selectively change their absorption and transmission spectra.

Fine Screw Drive

A type of ball bearing stage drive mechanism option. The 64 pitch screw is used for fine resolution positioning, but the head is not labeled for position readout.

Finite/Finite Conjugate

A conjugate distance relationship in which light is focused from a source located a finite distance away.

First Surface Mirror

A type of mirror featuring a high reflectivity coating, deposited on the front surface of the glass substrate. Light does not pass through any glass before it is reflected, but the coating is not protected and is more prone to scratching and oxidization.

  See also Dielectric Coating

Fixed Focal Length Lens

A lens with a fixed Angular Field of View (AFOV), also known as a conventional or entocentric lens.

Fixed Focus Lens

A lens which is used at a single, specific working distance and cannot be refocused for different distances. 


For ball bearing stages, it denotes error in the vertical plane (up and down movement in direction of travel).

Flint Glass

One of two types of optical glass used in the manufacturing of achromatic lenses.  Flint glass has higher dispersion and higher index of refraction than crown glass. 

  See also Crown Glass , Achromatic Lens

Float Glass

Glass manufactured by the float process, which involves floating glass on liquid tin as the glass cools. 

Fluorescence Microscopy

An optical microscopy technique that utilizes fluorescence, which is induced using fluorophores, as opposed to absorption, scatter, or refraction. 

Fluorescent Filter

A type of filter that absorbs light at short wavelengths (excitation wavelengths), typically in the ultraviolet (UV) spectrum, and emits light at long wavelengths (emission wavelengths) in the visible spectrum.


Also known as a Nikon-style F-mount bayonet, it is a camera and lens standard with a 46.5mm flange distance.

  See also CS-Mount , T-Mount , C-Mount

Focal Length Extender

A negative group of lenses that effectively changes the focal length (and therefore the field of view) of the lens being used, at a loss of brightness and image quality. 

Focusing Range

The distance range through which a laser diode module can be focused to achieve an ideal spot size. Typically specified from the face of the module to past collimation. Although a laser diode module can be used at distances shorter than the minimum focusing range, it's spot size will be larger than the focused spot at this minimum distance. It is important to note that the focused spot size will increase with distance from the module. At long distances, the optimum focus condition is a collimated beam with minimum divergence.


 Unit of illuminance equal to one lumen per square foot. One footcandle (fc) equals 10.764 lux.

  See also Lux , Minimum Sensitivity


A single full image in a sequence of video.

Frame Rate

The number of full frames (which may consist of two fields) composed in a second. In high-speed applications, it may be beneficial to choose a faster frame rate to acquire more "images" of the object as it moves.

  See also Frame


The number of wave crests that pass a fixed point in a given unit of time. In electro-optics, it is expressed in hertz or cycles per second.


Also known as second-harmonic generation, an input (pump) light wave can generate another wave with twice the original’s optical frequency in certain nonlinear materials.

Fresnel lens

A type of lens manufactured using a molding process that results in annular rings (or grooves) in thin plastic or acrylic. Each groove refracts the light as if it were a part of a whole conventional lens. The result is a lighter lens that can have a lower f/# than a conventional glass lens of the same focal length. The original design for this type of lens was made by Augustin-Jean Fresnel for use in lighthouses.

F-Theta Scanning Lens

A type of optical lens assembly designed to provide a flat field at the image plane of the scanning system. It is commonly used in laser marking, engraving, and cutting systems in conjunction with a beam expander and galvanometer.

Full Width at Half Maximum (FWHM)

A specific wavelength region of the bandwidth of a filter defined by the two points of the passband where transmittance is 50% of the peak.

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