
A fiducial marker or fiducial is an object placed in the
field of view
The field of view (FOV) is the angle, angular extent of the observable world that is visual perception, seen at any given moment. In the case of optical instruments or sensors, it is a solid angle through which a detector is sensitive to elec ...
of an
image
An image or picture is a visual representation. An image can be Two-dimensional space, two-dimensional, such as a drawing, painting, or photograph, or Three-dimensional space, three-dimensional, such as a carving or sculpture. Images may be di ...
for use as a point of reference or a measure. It may be either something placed into or on the imaging subject, or a mark or set of marks in the
reticle
A reticle or reticule, also known as a graticule or crosshair, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the electronic v ...
of an optical instrument.
Applications
Microscopy
In high-resolution optical
microscopy
Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical mic ...
, fiducials can be used to actively stabilize the field of view. Stabilization to better than 0.1 nm is achievable.
Physics
In
physics
Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...
,
3D computer graphics
3D computer graphics, sometimes called Computer-generated imagery, CGI, 3D-CGI or three-dimensional Computer-generated imagery, computer graphics, are graphics that use a three-dimensional representation of geometric data (often Cartesian coor ...
, and
photography
Photography is the visual arts, art, application, and practice of creating images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is empl ...
, fiducials are reference points: fixed points or lines within a scene to which other objects can be related or against which objects can be measured.

Cameras outfitted with
Réseau plate
A Réseau plate is a transparent sheet of glass or plastic engraved with a grid of crosshatches called fiducial markers. It was commonly used in film cameras (before the advent of digital cameras) for scientific and technical photography. The p ...
s produce these reference marks (also called
Réseau crosses) and are commonly used by
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
. Such marks are closely related to the
timing marks used in
optical mark recognition
Optical mark recognition (OMR) collects data from people by identifying markings on a paper.
OMR enables the hourly processing of hundreds or even thousands of documents. A common application of this technology is used in exams, where students m ...
.
Geographical survey
Airborne
geophysical
Geophysics () is a subject of natural science concerned with the physical processes and properties of Earth and its surrounding space environment, and the use of quantitative methods for their analysis. Geophysicists conduct investigations acros ...
surveys also use the term "fiducial" as a sequential reference number in the measurement of various geophysical instruments during a survey flight. This application of the term evolved from
air photo frame numbers that were originally used to locate geophysical survey lines in the early days of airborne geophysical surveying. This method of positioning has since been replaced by
GPS, but the term "fiducial" continues to be used as the time reference for data measured during flights.
Augmented reality
In applications of
augmented reality
Augmented reality (AR), also known as mixed reality (MR), is a technology that overlays real-time 3D computer graphics, 3D-rendered computer graphics onto a portion of the real world through a display, such as a handheld device or head-mounted ...
, fiducials help resolve several problems of integration between the real world view and the synthetic images that augment it.
Fiducials of known pattern and size can serve as real world anchors of location, orientation and scale. They can establish the identity of the scene or objects within the scene. For example, a fiducial printed on one page of an augmented reality
popup book would identify the page to allow the system to select the augmentation content. It would also serve to moor the coordinates of the augmented content to the three dimensional location, orientation and scale of the open book, helping to create a stable and accurate fusion of real and synthetic imagery.
A slightly more complex example would be multiple fiducials, each attached to an individual piece in an augmented reality
board game
A board game is a type of tabletop game that involves small objects () that are placed and moved in particular ways on a specially designed patterned game board, potentially including other components, e.g. dice. The earliest known uses of the ...
.
Metrology
The appearance of markers in images may act as a reference for
image scaling
In computer graphics and digital imaging, image scaling refers to the resizing of a digital image. In video technology, the magnification of digital material is known as upscaling or resolution enhancement.
When scaling a vector graphic image ...
, or may allow the image and physical object, or multiple independent images, to be
correlated
In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistic ...
. By placing fiducial markers at known locations in a subject, the relative scale in the produced image may be determined by comparison of the locations of the markers in the image and subject. In applications such as
photogrammetry
Photogrammetry is the science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images and patterns of electromagnetic radiant ima ...
, the fiducial marks of a surveying camera may be set so that they define the principal point, in a process called "
collimation". This would be a creative use of how the term collimation is conventionally understood.
Fiducial marker sets

Some
barcode
A barcode or bar code is a method of representing data in a visual, Machine-readable data, machine-readable form. Initially, barcodes represented data by varying the widths, spacings and sizes of parallel lines. These barcodes, now commonly ref ...
readers can estimate the translation, orientation, and vertical depth of a known-size barcode relative to the barcode reader.
Some sets of fiducial markers are specifically designed to allow rapid, low-latency detection of 6D position estimation (3D location and 3D orientation) and identity of hundreds of unique fiducial markers.
For example, the ArUco markers, the WhyCon marker, WhyCode markers,
"amoeba" reacTIVision fiducials, the d-touch fiducials, or the TRIP circular barcode tags (ringcodes).
Medical imaging
Fiducial markers are used in a wide range of
medical imaging
Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to revea ...
applications. Images of the same subject produced with two different imaging systems may be correlated by placing a fiducial marker in the area imaged by both systems. In this case, a marker which is visible in the images produced by both imaging modalities must be used. By this method, functional information from
SPECT
Single-photon emission computed tomography (SPECT, or less commonly, SPET) is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera (that is, ...
or
positron emission tomography
Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in metabolic processes, and in other physiological activities including blood flow, r ...
can be related to anatomical information provided by
magnetic resonance imaging
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and ...
(MRI).
Similarly, fiducial points established during MRI can be correlated with brain images generated by
magnetoencephalography
Magnetoencephalography (MEG) is a functional neuroimaging technique for mapping brain activity by recording magnetic fields produced by electric current, electrical currents occurring naturally in the human brain, brain, using very sensitive magn ...
to localize the source of brain activity. Such fiducial points or markers are often created in tomographic images such as
computed tomography
A computed tomography scan (CT scan), formerly called computed axial tomography scan (CAT scan), is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers or ...
,
magnetic resonance and
positron emission tomography
Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in metabolic processes, and in other physiological activities including blood flow, r ...
images using devices such as the
N-localizer and Sturm-Pastyr localizer.
Electrocardiography
In
electrocardiography
Electrocardiography is the process of producing an electrocardiogram (ECG or EKG), a recording of the heart's electrical activity through repeated cardiac cycles.
It is an electrogram of the heart which is a graph of voltage versus time of t ...
(ECG), fiducial points are landmarks on the ECG complex such as the isoelectric line (PQ junction), and onset of individual waves such as PQRST.
Cell biology
In processes that involve following a labelled molecule as it is incorporated in some larger polymer, such markers can be used to follow the dynamics of growth/shrinkage of the polymer, as well as its movement. Commonly used fiducial markers are fluorescently labelled monomers of bio-polymers. The task of measuring and quantifying what happens to these is borrowed from methods in physics and computational imaging like
speckle imaging.
Social insect tracking

Automated behavioural tracking systems are used to study the organisation of social insect colonies, and the behaviour of individual colony members. These systems combine fiducial markers and machine vision to output the location and orientation of colony members multiple times per second and have, among other insights, revealed the social network structure of ant ''
Camponotus fellah''.
Radio therapy
In
radiotherapy
Radiation therapy or radiotherapy (RT, RTx, or XRT) is a treatment using ionizing radiation, generally provided as part of cancer therapy to either kill or control the growth of malignant cells. It is normally delivered by a linear particle ...
and radiosurgical systems, fiducial points are landmarks in the tumour to facilitate correct targets for treatment. In
neuronavigation, a "fiducial spatial coordinate system" is used as a reference, for use in neurosurgery, to describe the position of specific structures within the head or elsewhere in the body. Such fiducial points or landmarks are often created in
magnetic resonance imaging
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and ...
and
computed tomography
A computed tomography scan (CT scan), formerly called computed axial tomography scan (CAT scan), is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers or ...
images by using the
N-localizer or Sturm-Pastyr localizer.
Printed circuit boards

In
printed circuit board
A printed circuit board (PCB), also called printed wiring board (PWB), is a Lamination, laminated sandwich structure of electrical conduction, conductive and Insulator (electricity), insulating layers, each with a pattern of traces, planes ...
(PCB) manufacturing, fiducial marks, also known as circuit pattern recognition marks, allow
SMT placement equipment
Surface-mount technology (SMT) component placement systems, commonly called pick-and-place machines or P&Ps, are robotic machines which are used to place surface-mount devices (SMDs) onto a printed circuit board (PCB). They are used for hi ...
to accurately locate and place parts on boards. These devices locate the circuit pattern by providing common measurable points. They are usually made by leaving a circular area of the board bare from
solder-mask coating. Inside this area is a circle exposing the copper plating beneath. This center metallic disc can be solder-coated, gold-plated or otherwise treated, although bare copper is most common if not a current-carrying contact. Alternatively, it is possible to use clear solder-mask lacquer to cover the fiducials. In order to minimize
rounding error
In computing, a roundoff error, also called rounding error, is the difference between the result produced by a given algorithm using exact arithmetic and the result produced by the same algorithm using finite-precision, rounded arithmetic. Roun ...
s it was good practice to place fiducials in the same grid (or some multiple of it) that was used to place the parts, however, this isn't always possible on high-density boards nor is it a requirement any more with modern high-precision machines.

Most placement machines are fed boards for assembly by a rail
conveyor, with the board being clamped down in the assembly area of the machine. Each board will clamp slightly differently than the others, and the variance—which will generally be only tenths of a millimeter—is sufficient to ruin a board without proper calibration. Consequently, a typical
PCB will have multiple fiducials to allow placement robots to precisely determine the board's orientation. By measuring the location of the fiducials relative to the board plan stored in the machine's memory, the machine can reliably compute the degree to which parts must be moved relative to the plan, called ''offset'', to ensure accurate placement.
Using three fiducials enables the machine to determine PCB offset in both the X and Y axes, as well as to determine if the board has rotated during clamping, allowing the machine to rotate parts to be placed to match. Such fiducials are also called global fiducials.
Global fiducials are also used in conjunction with
stencil printing. Without them the printer would not print the solder paste in exact alignment with the pads.
Parts requiring a very high degree of placement precision, such as
ball grid array
A ball grid array (BGA) is a type of surface-mount packaging (a chip carrier) used for integrated circuits. BGA packages are used to permanently mount devices such as microprocessors. A BGA can provide more interconnection pins than can be pu ...
packages, may have additional local fiducials near the package placement area of the board to further fine-tune the targeting. Local fiducial, however, cannot be used in the stencil printing process.
Conversely, low end, low-precision boards may only have two fiducials, or use fiducials applied as part of the screen printing process applied to most circuit boards. Some very low-end boards may use the plated mounting screw holes as fiducials, although this yields very low accuracy.
For prototyping and small batch production runs, the use of a fiducial camera can greatly improve the process of board fabrication. By automatically locating fiducial markers, the camera automates board alignment. This helps with front to back and multilayer applications, eliminating the need for set pins.
Printing
In
color printing
Color printing or colour printing is the reproduction of an image or text in color (as opposed to simpler black and white
or monochrome printing).
History of color printing
Woodblock printing on textiles preceded printing on paper in both Ea ...
, fiducials—also called "
registration black"—are used at the edge of the cyan, magenta, yellow and black (
CMYK
The CMYK color model (also known as process color, or four color) is a subtractive color model, based on the CMY color model, used in color printing, and is also used to describe the printing process itself. The abbreviation ''CMYK'' refers ...
) printing plates so that they can be correctly aligned with each other.
"Banana for scale"

"Banana for scale" refers to an
Internet meme
An Internet meme, or meme (, Help:Pronunciation respelling key, ''MEEM''), is a cultural item (such as an idea, behavior, or style) that spreads across the Internet, primarily through Social media, social media platforms. Internet memes manif ...
involving using a
banana
A banana is an elongated, edible fruit – botanically a berry – produced by several kinds of large treelike herbaceous flowering plants in the genus '' Musa''. In some countries, cooking bananas are called plantains, distinguishing the ...
as a fiducial marker. The meme began in August 2010, where a man posted to
Facebook
Facebook is a social media and social networking service owned by the American technology conglomerate Meta Platforms, Meta. Created in 2004 by Mark Zuckerberg with four other Harvard College students and roommates, Eduardo Saverin, Andre ...
a picture of a
safe with a banana beside it. From there, the meme spread primarily via
Reddit
Reddit ( ) is an American Proprietary software, proprietary social news news aggregator, aggregation and Internet forum, forum Social media, social media platform. Registered users (commonly referred to as "redditors") submit content to the ...
; ''
The Daily Dot
''The Daily Dot'' is a digital media company covering the culture of the Internet and the World Wide Web. It was founded by Nicholas White in 2011, and is headquartered in Austin, Texas.
The site, conceived as the Internet's "hometown newsp ...
'' remarked that "
he memehad become a hilarious game with redditors try to one up each other." In 2024, the mobile Reddit app tracked the length of how much the user scrolled through content measured in number of banana lengths, including awarding users with achievements for scrolling various amounts of bananas.
[https://www.techradar.com/computing/social-media/reddit-recap-2024-is-live-heres-how-to-see-how-your-recap-and-find-out-how-many-bananas-you-scrolled-this-year]
It's a trend in the same vein as
planking.
See also
*
Secchi disk
*
Landmark point
References
{{Commons category
Medical imaging
Image processing
Computer graphics
3D computer graphics