Metrology Glossary

Metrology Glossary

A metrology glossary consisting of common industry terminology used within the metrology community. Reference this glossary as you begin to educate yourself. 

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z


A

Accuracy
The closeness of a measured quantity to the actual quantity that was measured, the measurand. Sometimes thought of as a tolerance range on a measurement’s value. Often used colloquially as a synonym for uncertainty of measurement.

Alignment
The state or orientation of an object or feature with respect to a set of datums or the act of putting an object or feature into a desired state or orientation. Example alignments include: setting several bearings supporting a shaft in a line (or not quite a line); mounting aircraft sensors in a specific direction with respect to an aircraft’s navigation system; bore-sighting weapons mounted on aircraft; best-fitting point cloud data to a set of surfaces.

As-built
Or the As-built condition. Refers to the actual dimensions of a part or assembly as it was manufactured. Also sometimes called “As-found”. In reverse engineering, as-built models or drawings do not try to divine design intent. They try to exactly portray a part exactly as it is, including defects.



B

Ball-bar
A precision fixture that can be used as a standard of length.   Ball-bars are often built with precision nests to accept Sphere Mounted Reflectors or precision ball bearings to aid in checking or calibrating laser trackers and CMMs.

Boresighting
This term refers to an old method for setting sights on a firearm, in which the bolt or part of the action was removed and a target point was viewed alternately through the bore and through the sights while adjustments were made until the sights pointed at the target point.

The term Boresighting today has been expanded to include determining the primary axis of a directional antenna, and aircraft weapons harmonization.  Many techniques now exist including the use bore mounted lasers or laser trackers.



C

Calibration
“Calibration is the operation that, under specified conditions, establishes a relationship between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with measurement uncertainties.” (VIM)

Thus, calibration involves measurements and comparisons, not an actual adjustment. For metrology, the formal comparison is of measuring equipment against a standard of higher level (a national standard defined in the U.S. by NIST) under controlled and specified conditions to document the accuracy of the instrument being compared.

CMM
This acronym stands for Coordinate Measuring Machine. These machines measure parts through the acquisition of points.
Many types of CMMs exist, traditional 3 axis contact probing machines, articulated arm CMMs, and laser trackers are a few examples.  Laser scanners or “white light” scanners are other examples that don’t require contact with the measured part.  CMMs are often run with software that can compute features such as planes, cylinders, and lines from the measured points.



D

Datum
In metrology,  a datum is simply an idealized reference feature from which another features’ orientation, position, or other characteristic is defined.   Datums can be many types of features, planes, cylinders, points, centerlines,  constructions or offsets from other features, anything that can be measured or established for use in locating other features.

Although the plural form of datum is really data, many people, including the author, uses datums as the plural in this application.

Datum Feature
A datum feature is the measured or contacted feature which establishes a datum.  It is an actual point, line or surface on the part being measured.

Dimensional Inspection
The process of characterizing an object’s size and shape through measurements of points, lengths, and volumes.  Usually such an inspection results in a report comparing the measured object to another object, an idealized object, or a previous state of the object.

Dimensional Metrology
The study or practice of high precision measurements to quantify physical sizes, orientations,  and distances of objects and shapes.

Discrimination Threshold
“The largest change in a value of a quantity being measured that causes no detectable change in the corresponding indication. NOTE: Discrimination threshold may depend on, e.g. noise (internal or external) or friction. It can also depend on the value of the quantity being measured and how the change is applied.” (VIM)

Dimensional inspection
A method that involves measuring a part’s dimensions by way of a 3D laser tracker or scanner. There are several types of dimensional inspection services depending on the type of measurements that a project requires. Services offered by ECM: First Article Inspection, In-Line and In-Process Testing, As-Built Inspection as well as Calibration and Certification of machines, tooling and fixtures.



F

First Article Inspection (FAI)
As the name implies, first article inspection is usually performed on one or more of the first parts in production.  The inspection is usually exhaustive, covering every dimension on the drawing,  and provides a final verification of the manufacturing process.  After the first article is proved out,  subsequent parts are often inspected less exhaustively to save time and money.

Free-Form Shapes
In metrology, this refers to surfaces with unconventional or continuously varying shapes like bones, customized molds, boat hulls, or the sculptures of Henry Moore.



G

Gantry CMM
A gantry CMM has the measurement head mounted on a beam that overhangs and spans the measurement table.   The head can move both vertically and side to side along the beam, providing two axes of motion.  The third axis can be generated by moving either the towers that support the beam along a fixed table, or moving the measurement table between the towers.

Gage R&R (Gage Repeatability and Reproducibility)
A set of repeated measurements used to determine the fitness of a gage or other measuring instrument for a specific function.  The test tries to account for the effects of equipment, method, and operator in an estimate of precision and uncertainty by having a number of operators use the equipment to measure a reference standard or part.

GD&T
Geometric dimensioning and tolerancing is a system and symbolic language for describing the permissible limits, or tolerances, in a part’s physical dimensions and measured values. In the United States the standard describing GD&T is ASME Y14.5.   In Europe ISO has a series of standards that cover the same material.  The ASME and ISO standards use the same symbols, but interpretations are slightly different in some cases.



I

Influence Quantity
A quantity that, in a direct measurement, does not affect the quantity that is actually measured, but affects the relation between the indication and the measurement result.

An example in laser tracker work would be air temperature and humidity.

Instrumental Drift
Drift Change in a measured value, “related neither to a change in the quantity being measured nor to a change of any recognized influence quantity.” (VIM)

Instrumental Uncertanity
A component of measurement uncertainty arising from the measuring instrument or measuring system in use, and obtained by its calibration.



L

Laser Scanning
Using laser measurement systems to digitally capture the geometry of a product or part without contact between a probe and the part surface.  The laser scanner will measure and record thousands or even millions of data points from a part’s surface or a large area.  Laser scanning is done on objects ranging in size from several inches to over 100 feet.  These points, often called a point cloud, can then be  used for comparison to customers’ CAD models or  to reverse engineer 3D models.

Laser Tracker
One form of portable CMM, a laser tracker can measure large objects or spaces with a relatively high degree of accuracy.  The typical laser tracker uses two rotary encoders and a laser ranging system to make measurements at the center of an SMR which is placed on the project or at the point of interest.  Measurements are therefore limited by line of sight between tracker and SMR, and the tracker’s range.  Depending on the tracker and measurement conditions measurements can be made 100 yards or more from the tracker head.

A 3d laser tracker can be used in measuring, aligning and even calibrating automated measurement systems, large and small parts, complex surfaces and even immobile manufacturing machinery.



M

Maximum Permissible Error
There are a few synonyms for this term but all pertain to measurements and limits of error. The “extreme value of measurement error, with respect to a known reference quantity value, permitted by specifications or regulations for a given measurement, measuring instrument, or measuring system.


NOTE 1 Usually, the term “maximum permissible errors” or “limits of error” is used where there are two extreme values.
NOTE 2 The term “tolerance” should not be used to designate ‘maximum permissible error.’” (VIM)

Measurement Result
Any quantities attributed to a measurand from a measurement together with other relevant information.  Environmental conditions or measurement uncertainty are two examples of other relevant information.

Measurement Traceability
Also known as “metrological traceability.” This is simply how metrologists connect to the standard by which they measure. That standard in the U.S. is set and defined by NIST. Picture links in a chain. Each link represents a comparison that pertains to an instrument or tool’s measurements to a NIST defined standard or requirement. Because ECM calibrates to a NIST traceable standard, ECM can determine the precision and accuracy of a tool or instrument.

Measurement Uncertainty
An estimate of the uncertainty of a measurement.  Usually comprised of instrumental uncertainty, and a number of other factors such as procedural uncertainty, and environmental uncertainty.

Measuring System
A “set of one or more measuring instruments and often other devices, including any reagent and supply, assembled and adapted to give information used to generate measured quantity values within specified intervals for quantities of specified kinds.

NOTE 1 “A measuring system may consist of only one measuring instrument.” (VIM)

Metrology
The science of measurement and how measurements are used. Used to measure and verify an objects dimensional quality.

Measurand
Any quantity being measured.  The term applies to any type of measurement, mass, force, luminosity, etc.  in dimensional metrology, one could also call an object or a feature of an object being measured a measurand and be understood.

Mesh
1. A type of model consisting of small usually triangular planar surface patches that approximate an object’s shape.  It is much like an STL model.  This type of model is often produced from scanned data and may be used as an intermediate step to a Nurbs surface model.

2. Occasionally used to describe Nurbs surfaces because the mathematical model of a surface can be thought of as a series of intersecting curves that lie in the surface.

Micron or Micrometer (µm)
A unit in the metric system equal to one millionth of a meter or approximately 0.00003937 inches.  It is commonly used to describe the uncertainty of precision measuring machines.



N

Noise
At ECM, we use “Noise” to refer to incorrect data within a point cloud generated during a laser scanning. Reflections from dust in the air, errant reflections from corners, surface texture, or highly reflective surfaces are sources of noise.

Nominal Dimensions
In dimensional metrology Nominal Dimensions are the dimension values given on a drawing or in a computer model.  Measured values are compared to nominal to determine whether a part conforms to its design.

Non-Contact Measurement
This is simply the practices of taking an object’s measurements without making physical contact with it.   Often called scanning.  Non-contact measurements can be used to measure an object with a delicate surface or weak structure that could not otherwise stand up to contact measuring.

Non-Parametric Model
Also known as a “dumb model”, this is a 3D CAD model whose shape cannot usually be edited as easily as a parametric model.  Commonly available file formats for porting files between CAD programs like IGES or STEP typically produce dumb models.

NURBS
With a full name of Non Uniform Rational B-Spline, this refers to NURBS curves and surfaces.  The name comes from the mathematical technique used to model the curve or surface.  It is one of the more common ways to model freeform geometry.



O

Outlier
A point which lies outside the expected range of variation for data.  In scan data outliers are usually removed during post-processing of the point cloud using statistical algorithms.  Sometimes the scanning instrument and software remove them during the point cloud acquisition.  In data generated by CMM or laser trackers, outliers are considered for removal on a case by case basis, because the data is often too sparse for statistical methods and because the operator can usually tell by visual observation or confirming measurements if an outlier accurately represents the part or was acquired incorrectly, perhaps by probing debris on the surface instead of the surface itself.



P

Parametric Model
A parametric model is a CAD  model that, can be edited and changes will propagate through the model automatically preserving the relationships between features.  Typically these models can only be edited in the CAD program that created them.

Photogrammetry
Non-contact imagery that takes 3D coordinate measurements (XYZ) through photographs.

Point Cloud
A type of data consisting of many points in 3D space.  The number of points may run from hundreds or thousands in the case of a CMM or laser tracker, to hundreds of millions, or even more points in the case of a laser scanner.  Point cloud data can be compared to a CAD model or used to reverse engineer an object.

Polygonal Modeling
A CAD model using small planar surfaces to approximate the shape of a surface.  The surfaces are usually triangular and the model is quite similar to an STL model.  (See Mesh definition 1)

Precision
How close one measurement result will be to another result or set of results.  Precision should not be mistaken for accuracy.  A precise instrument could give a consistently erroneous result.  The term precision is often used as a synonym for an instrument’s repeatability.

Probe
A device attached to a CMM that is used in taking measurements. Typical CMMs have contact probes (called touch probes) but a probe can be non-contact like a portable CMM laser scanner.



R

Reference Model
Typically a CAD model, but occasionally a physical master, against which data on a measured part is compared.

Repeatability
A measurement system’s precision under a set of measurement conditions, the repeatability condition.

Reproducibility
Measurement precision under the Reproducibility Condition of Measurement.

Repeatability Condition
In contrast to the Repeatability Condition of Measurement, the Reproducibility Condition specifies different measuring systems, locations, operators, and environmental conditions for measurement.

Resolution
1.  (of a measuring instrument or system) Smallest change in a quantity being measured
that causes a perceptible change in the corresponding indication.  Resolution can be affected by internal or external factors like noise, friction, or temperature.  The value of the measurand may also affect resolution.  Not to be confused with definition 2.
2.  (of a displaying device) Smallest difference between indications that can be meaningfully
distinguished or the number of digits in a digital display.

Reverse Engineering
This process involves producing a drawing or 3D digital representation of a pre-existing, tangible object usually via CAD, CAE or CAM type software.  At ECM, the object is measured with a 3D laser scanner, the generated point cloud data is transferred into a NURBS surface or triangular mesh or a number of other options. From this point, the 3D digital CAD model is reconstructed to look like the original object that was laser scanned.



S

Scan Density
Laser scan density refers to the proximity between 3D coordinates in a given data point cloud.

Scan Speed
This refers to a laser scanner’s speed regarding the collection of 3D coordinates. The speed is measured in points per second or even millions of points per second.

Spatial Reference System (SRS)
This is another name for a coordinate reference systems (CRS) that captures geo-location entities. This would be considered a civil engineering term or a term used in the utility field more so than in dimensional metrology.



T

Temperature Compensation
This is something to consider when using a CMM. If using a CMM in less than ideal weather or within extreme temperatures, the CMM will swell or contract. To prevent in accuracy, this behavior needs to be considered when taking a 3D measurement unless the measurement system was built to prevent this and is highly accurate.

Tooling Ball
A close tolerance sphere, usually on a precision shouldered shank used in aligning or locating parts or assemblies.



U

Uncertainty
See Measurement Uncertainty.



V

Validation
Demonstration by test or analysis that an instrument, system, or procedure is fit for a certain task.

Verification
The proof, by means of objective evidence, that an item meets its specification requirements. In dimensional inspection the object’s dimensions would be measured and compared to the drawing or CAD model of the object.



References:

  • International Vocabulary of Basic and General Terms in Metrology (VIM) Third Edition, BIPM, 2006
  • Evaluation of Measurement Data – Guide to the Expression of Uncertainty (GUM) , BIPM, 2008

Important Organizations:

  • The American Association for Laboratory Accreditation (A2LA)
  • American National Standards Institute (ANSI)
  • American Society of Mechanical Engineers (ASME)
  • International Bureau of Weights and Measures (BIPM) – French organization: Bureau International des Poids et Mesures
  • International Laboratory Accreditation Cooperation (ILAC)
  • International Organization for Standardization (ISO)
  • International Organization for Standardization, Reference Materials Committee (ISO/REMCO)
  • International Society of Automation (ISA)
  • National Conference of Standards Laboratories International (NCSLI)
  • National Institute of Standards and Technology (NIST)
  • National Physical Laboratory (NPL)