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A fiducial marker or fiducial is an object placed in the field of view of an imaging system which appears in the image produced, 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 of an optical instrument.
In high-resolution optical microscopy, fiducials can be used to actively stabilize the field of view. Stabilization to better than 0.1 nm is achievable (Carter et al. Applied Optics, (2007)).
In physics, 3D computer graphics, and photography, 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 reseau plates produce these reference marks (also called reseau crosses) and are commonly used by NASA. Such marks are closely related to the timing marks used in optical mark recognition.
Geographical Survey 
Airborne geophysical 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 This method of positioning has since been replaced by GPS, but the term "fiducial" continues to be used as the time reference for data measured during flights.
Markers may be used to make otherwise invisible or difficult to distinguish features of an image more visible. Markers can simplify computerized image processing applications such as motion capture, by providing an easy-to-track feature in images which follows the movements of the marked subject. Fiducial marks should intersect at principal points of the image. If an airplane flys perfectly horizontal, it should radiate in the perfect middle.
Virtual Reality 
In applications of augmented reality or virtual reality, fiducials are often manually applied to objects in a scene so that the objects can be recognized in images of the scene. For example, to track some object, a light-emitting diode can be applied to it. With knowledge of the color of the emitted light, the object can easily be identified in the picture.
The appearance of markers in images may act as a reference for image scaling, or may allow the image and physical object, or multiple independent images, to be correlated. By placing fiduciary 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, 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.
Medical Imaging 
Fiduciary markers are used in a wide range of medical imaging applications. Images of the same subject produced with two different imaging systems may be correlated by placing a fiduciary 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 or positron emission tomography can be related to anatomical information provided by magnetic resonance imaging (MRI). Similarly, fiducial points established during MRI can be correlated with brain images generated by magnetoencephalography to localize the source of brain activity. Such fiducial points or markers are often created in magnetic resonance imaging and computed tomography images by using the N-localizer.
In electrocardiography, 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 fiduciary 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.
Radio Therapy 
In radiotherapy and radiosurgical systems such as the CyberKnife, 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 and computed tomography images by using the N-localizer.
In printed circuit board (PCB) design, fiducial marks, also known as circuit pattern recognition marks or simply "fids," allow automated assembly equipment 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-stop coating (similar to clearcoat), in which a filled copper circle is placed. This center metallic disc can be solder-coated, gold-plated or otherwise treated, although bare copper is most common as it is not a current-carrying contact.
Most placement devices 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 three fids to allow placement robots to precisely determine the board's orientation. By measuring the location of the fids 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 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. Parts requiring a very high degree of placement precision, such as integrated circuit chip packages with many fine leads, may have subsidiary fiducial marks near the package placement area of the board to further fine-tune the targeting.
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 ersatz 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.
See also 
- CS 348C - Topics in Computer Graphics. Stanford University. Fiduciary markers in motion capture.
- Correlation of single photon emission CT with MR image data using fiduciary markers. BJ Erickson and CR Jack Jr., American Journal of Neuroradiology, Vol 14, Issue 3 713-720.
- Brown RA, Nelson JA (2012). "Invention of the N-localizer for stereotactic neurosurgery and its use in the Brown-Roberts-Wells stereotactic frame". Neurosurgery 70 (ONS Suppl 2): ons173–ons176. doi:10.1227/NEU.0b013e318246a4f7. PMID 22186842.
- Brown RA (2012). "The mathematics of the N-localizer for stereotactic neurosurgery". Cureus 4 (9): e56. doi:10.7759/cureus.56.
- Brown RA (June 1979). "A computerized tomography-computer graphics approach to stereotaxic localization". Journal of Neurosurgery 50 (6): 715–20. doi:10.3171/jns.1979.50.6.0715. PMID 374688.
- Brown RA (1979). "A stereotactic head frame for use with CT body scanners". Invest Radiol. 14 (4): 300–4. doi:10.1097/00004424-197907000-00006. PMID 385549.