Fiduciary marker

Fiduciary marker

A fiduciary marker or fiducial is an object used 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 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.

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 This method of positioning has since been replaced by GPS, but the term "fiducial" continues to be used as the time reference for data measured during flights.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. [ [ CS 348C - Topics in Computer Graphics.] Stanford University. Fiduciary markers in motion capture.]

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".

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). [ [ 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.] Similarly, fiducial points established during MRI can be correlated with brain images generated by magnetoencephalography to localize the source of brain activity.

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.

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.

In printed circuit board (PCB) design, fiducial marks, also known as circuit pattern recognition marks, 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 spot of the board bare with a bare copper-, nickel-, or solder-coated dot inside.

In color printing, fiducials -- also called "registration black" -- are used at the edge of the cyan, magenta, yellow and black (CMYK) printing plates so that they can be correctly aligned with each other.


ee also

Landmark point

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