The ISO 3274 standard specifies that the primary profile should not contain the nominal form of the workpiece. The form component must be removed prior to any other metrological operation. With an areal measure, the production of a SF surface requires the application of an F Operator. When the form is just a line segment or a plane, the operation is called leveling. When the form is non planar, it is called form removal. This page introduces the different methods that can be applied and discusses their applications. 
Nominal formThe Nominal form is a geometric form intended by the designer of the mechanical workpiece. Usually it is a planar form or a rotary form (cylinder, cone, sphere). Specifications of surface texture are made on a flat surface, regardless of the original geometrical form, so the metrologist must apply an operation to flatten the profile or the surface before carrying on the metrological analysis. This operation consists in modelling the shape and associating with it to the cloud of measured points in order to subtract the form. In the case of a nonmanufactured object, there is no nominal form, but it can still be useful to remove the natural form using a polynomial fit or a filter. Surface texture parameters depend on a reference (mean line or mean plane) used in the detection of peaks and valleys, or on the calculation of mean values and moments. Form removal and leveling make it possible to define this reference. 
Above: profile measured on a sphere ; below, the profile, after a circular form removed (leastsquare circle, calculated radius of 9.53 mm) 
Internal reference of instrumentsOn a 2D contact profilometer, the scanning axis along X gives the reference and defines the origin of heights. This reference is usually provided by a high precision steel rod on which pads are sliding to move the sensor and the stylus during the scanning. Any straightness deviation from this reference is injected into the measured profile as a height bias. Some profilometers can calibrate this straightness deviation by measuring a glass flat and saving a straightness profile that is then subtracted from any subsequent measurement. Once the leveling or the form removal is done on the profile or on the surface, the user can analyze form deviation, waviness and/or roughness. Schematics of a 2D contact profilometer and its coordinate system. 
With a 3D optical profiler, the reference depends on the optical technology. When it uses a vertical scanning device, the alignment and linearity of this device influence the reference. The reference mirror of an interferometer also influences the height reference. In general, any disturbance is mapped during a calibration routine done in the factory, or sometimes by users. See also ISO 25178700. Skidded stylus Workshop profilometers usually do not have an internal reference but they use a skid that will follow the form and allow the stylus to record height variations with respect to the skid. Therefore the measured profile contains only the shortest wavelengths. Two examples of skids for workshop profilometers Profilometers equipped with skids cannot evaluate waviness, unless they use special wide skids; Such profilometers are usually dedicated to roughness parameters. 
Manual leveling and alignmentThe metrologist is supposed to align the workpiece with measurement axes as well as possible. For example, on a contact profilometer, the sample is positionned as horizontally as possible, aligned along the X axis which is itself aligned on the internal reference of the instrument. It guarantees that heights are correctly recorded along the Z axis of the instrument which is very close to the Z axis of the workpiece. The leveling operation then subtracts the remaining fractions of degrees of the residual slope. When measuring a surface, for example with a confocal microscope, the sample will be placed horizontally to ensure that the vertical scanning axis is as perpendicular as possible to the sample plane. Corrections carried out by the analysis software will be more efficient and precise when the sample is physically aligned and leveled on the instrument. Reference line and reference planeOn a profile, the reference line is defined by the leveling or the form removal, and therefore depends on the association method. By default, the least squares line is associated with the profile. It becomes the horizontal reference of the primary profile, and defines the origin of heights. Similarly on a surface, the mean plane is calculated by the F Operator. Local structures and geometrical features may affect this reference by deviating the associated plane. 
AssociationAssociation is an operation that configures a mathematical model in order to best fit a real workpiece, usually represented by a cloud of points. Models are geometrical forms (line, plane, portion of a sphere, of a cylinder, of a cone, polynomial, etc.). Association methods used in surface texture metrology are:
Leastsquares corresponds to the L2norm as they minimize quadratic deviations. Minimum zone corresponds to the L∞norm also called Chebychev. The L1norm is sometimes used by minimizing absolute values of heights or a simple average of heights. Constraints used in association may fix the radius of a sphere and allow the position of the center to be calculated, or may shift the leastsquares plane outside the material, i.e. shifted in contact with the highest point of the surface. Symbols used in association for leastsquares are LSLI (for a line) or LSPL (for a plane). The Minimum Zone method encloses the profile between two parallel lines, or the surface between two parallel planes, and tries to find the best orientation that minimizes the distance between the two lines (or two planes). Symboles for Minimum Zone association are MZLI and MZPL. 
Deviation along z or along the normalThe slope of a profile or a surface is usually very small, barely a few degrees, so it is common to subtract heights between the profile and the calculated line. But in the case of a larger slope, the subtraction introduces a significant error and may also deform the profile. Subtraction of the reference line along the Z axis. Leveled profile. The shape is modified by the subtraction due to the large slope. 
In the case of a large slope it is recommended to rotate the profile to bring it to the horizontal. It corresponds to subtracting heights with respect to the normal vector of the nominal form. The shape of the profile is not altered but the spacing of the points is not constant anymore. It implies a subsequent resampling which introduces a smoothing of the highest frequencies (noise). In some cases, overhangs may appear and are eliminated during the resampling. Or it may require the handling of irregularly spaced profiles, also called parametric profiles, (x, y) = f(t). Subtraction of the reference line along the normal. Leveled profile. The shape is respected but points are not equally spaced. 
Association methodsThe default association method for surface texture metrology is the leastsquares association. It is the best method when surface texture is globally random (stochastic), without structure or geometrical shapes. However, in the presence of periodic features or geometrical structures, the leastsquares method can be severely altered by the irregular distribution of heights. Above, the leastsquares line is disturbed by the shape of the profile: Profile on the top, because of the groove. Profile on the bottom, because of the distribution of heights, higher on the left and lower on the right. In these cases, a leveling using Minimum Zone is preferable as it provides the correct principal direction of the profile. On the top profile, it would also have been possible to exclude the groove portion from the calculation of the leastsquares line. 
Exclusion of structuresExclusion of structures can be manual or automatic, and allow the calculation to take into account only a subset of points of the profile or the surface when estimating the nominal form. In the case of this surface, the groove is not centered and it alters the calculation of the leastsquares plane, leading to an incorrect leveling. The solution consists in excluding the groove from the calculation by marking it with a selection tool in the dialog of the Leveling operator (or the Form removal) and to exclude this area from the calculation. The excluded area is marked in gray. 
Automatic exclusion of structuresIn other cases, structures can be automatically excluded. This surface of a DVD measured by an AFM contains too many dimples that would be inconvenient to exclude manually. Each cell influences the calculation of the leastsquares plane as their height is lower than the flat plane above, and their distribution is not homogeneous on the surface. 
The automatic exclusion option can be configured to exclude structures below or above the mean plane (or on both sides). Here, structures are excluded and the leastsquares plane is calculated correctly only from points on the flat plane above. 
Leveling of free formIn nonmechanical applications, for example, when measuring biological samples, or surfaces of archeological artefacts, it may be necessary to define a flat reference on sample that has a complex form. It is possible to approximate a polynomial shape, and optionally exclude a portion of the surface. Below, a wound measured on the skin of a patient; the form comes from the leg. Removing the form makes it possible to define a flat reference, calculate the volume of the wound and compare it to a later stage of healing. In any case, polynomials should be used with care because they usually tend to diverge on edges, especially with high order polynomials. 
Line by line levelingSome instruments that use lateral scanning, such as profilometers or AFMs, are occasionally affected by vertical shifts between lines. In the case of a profilometer, it is due to the straightness deviation of the Y axis. In the case of a scanning probe microscope, the metrological exploitation of measures cannot be done before such line by line leveling is carried out. The solution consists in leveling each line individually in order to align them on the same flat reference. It may be necessary to exclude some portions to avoid bias on the mean lines.

Multiplane levelingA surface having several planes at different heights may be leveled using the Partition and level operator that segments the surface using watersheds and pruning algorithms. Above: the image on the right shows the segmented surface. It allows the user to click on the areas that will be used to calculate the leastsquares plane. The whole surface will then be leveled with reference to that plane. 
An even more difficult case: when the different planes are affected by a general curvature that needs to be removed, for example in order to evaluate the step height between the planes. It is then necessary to use a special function, multiplane leveling, where the algorithm will assess the curvature ignoring edges and heights. 
Normative referencesIn addition to surface texture standards, it may be useful to consult the following documents:

Next page: Areal functional parameters
© 20062017 by François Blateyron  Digital Surf