# Surface roughness cutoff length

Interpreting profile-filter settings, legacy sampling lengths, ISO 21920 nesting indices, and evaluation lengths without assuming a universal cutoff.

Reviewed: 2026-11-01

Tags: inspection, metrology, surface-texture

A profile cutoff is a horizontal wavelength, usually in millimetres. Ra and Rz are height parameters, usually in micrometres. The profile, filter and evaluation settings together determine the result. [NIST profile-filter explanation](https://physics.nist.gov/VSC/Help/Filters/Filters.jsp)

## Filtering is part of the measurement definition

**Illustrative filter calculation:** for a sinusoidal profile component with amplitude 0.20 µm exactly at a filter’s stated 50% transmission wavelength, the corresponding transmitted amplitude is **0.20 × 0.50 = 0.10 µm** in that filter response. This is not a prediction of the Ra of a real multiscale surface. It illustrates why “the cutoff removes everything above this wavelength” is an inadequate description. [Transmission definition](https://adobeassets.evidentscientific.com/content/dam/downloads/276828414/N8600858-022023_LEXT_Surface_Roughness_Guide_En_web.pdf)

In legacy profile terminology, the long-wavelength cutoff **λc** separates roughness from longer-scale waviness. The short-wavelength cutoff **λs** limits the other end of the roughness bandwidth. Form removal is another operation. KEYENCE’s JIS profile guide distinguishes the S-filter, which removes short-scale components, from the L-filter, which removes long-scale components, and from the F-operator for nominal form. Selecting λc alone does not completely define the processing chain. [KEYENCE profile filters](https://www.keyence.com/ss/products/microscope/roughness/line/filters.jsp)

A filter is not an ideal switch that preserves every shorter wavelength unchanged and deletes every longer one. Evident’s surface-roughness guide describes cutoff wavelength through a **50% amplitude-transmission** point. Its footer carries document code **N8600858-022023**; its profile terminology is tied to the legacy JIS references listed in that guide. Preserve filter type as well as its scale when comparing results. [Evident guide, profile glossary and footer](https://adobeassets.evidentscientific.com/content/dam/downloads/276828414/N8600858-022023_LEXT_Surface_Roughness_Guide_En_web.pdf)

## Distinguish the lengths

| Setting or length | Role | Do not substitute |
| --- | --- | --- |
| λc, or relevant nesting index | Scale of profile separation | Total stylus travel |
| Legacy sampling length lr | Interval used for legacy parameter assessment | Every modern parameter’s evaluation length |
| Section length lsc | Section used by section-based parameters | A claim that every parameter is averaged over five sections |
| Evaluation length ln or le | Profile length actually evaluated | Complete acquired traverse |
| Pre/post travel | Additional profile acquisition around the evaluated region | Extra evaluated surface |

ZEISS Academy’s ISO 21920 poster distinguishes **le**, section length **lsc**, section count **nsc**, and pre/post travel. Thus a 4 mm evaluation does not imply exactly 4 mm of available traverse. The extra travel depends on acquisition and processing; obtain it from the configured instrument/measurement procedure. [ZEISS ISO 21920 surface-parameter poster](https://dccprodblob.blob.core.windows.net/dccprodtrainingdocuments/en/knowledgeContent/Poster_Surface_Parameter.pdf)

## Legacy ISO 4287/4288 context

ISO 4287 supplied legacy profile definitions and parameters; ISO 4288 supplied assessment rules. Mitutoyo’s technical page explicitly labels its selection tables **JIS B 0633:2001 / ISO 4288:1996**. It lists separate tables for nonperiodic profiles using Ra-related or Rz-related criteria, and for periodic profiles using spacing RSm. These are different selection routes. A table headed by measured/estimated roughness must not be presented as a universal setting for every profile or drawing. [Mitutoyo legacy assessment tables](https://www.mitutoyo.co.jp/about-metrology/knowledge/roughness/)

For example, that page’s nonperiodic Ra table assigns **lr = 0.8 mm** and **ln = 4 mm** for **0.1 < Ra ≤ 2 µm**. The arithmetic is **5 × 0.8 = 4 mm**. This is a bounded legacy-table example, not a recommendation to set every machined surface to 0.8 mm. The page also provides an iterative selection procedure when sampling length is not specified. [Mitutoyo ISO 4288-based lengths and selection](https://www.mitutoyo.co.jp/about-metrology/knowledge/roughness/)

## What changes with ISO 21920

ISO lists **ISO 21920-3:2021**, first edition published December 2021, as the specification-operator standard replacing withdrawn ISO 4288:1996 and its corrigendum. A replacement in the standards catalogue does not by itself rewrite an existing drawing or agreed inspection plan: determine the requirement and edition governing the part before selecting the instrument’s standards mode. [ISO 21920-3 publication and lifecycle](https://www.iso.org/standard/72228.html)

Mahr explains that ISO 21920 introduces nesting indices **Nic** and **Nis**, the evaluation-length notation **le**, and removes the periodic/nonperiodic distinction for choosing the relevant nesting index. Its calibration guidance also shows why this is more than renaming: evaluation and filtering can differ for particular parameter families, including core-roughness parameters. Do not claim equivalent results merely because a legacy cutoff and a modern nesting index have the same numerical value. [Mahr ISO 21920 calibration changes](https://metrology.mahr.com/de-at/news/newsroom/artikel/mahr-bietet-dakks-akkreditierte-kalibrierung-nach-neuer-rauheitsnorm/)

ZEISS’s poster shows drawing-tolerance-based defaults for specified parameters and setting classes Sc1–Sc5. As a bounded **Sc3** example, it lists **Nic = 0.8 mm**, **le = 4 mm**, **Nis = 2.5 µm**, maximum point spacing **0.5 µm**, and maximum nominal stylus-tip radius **2 µm**. For section-based parameters it lists **lsc = 0.8 mm** and **nsc = 5**. Those values belong to the stated setting-class context; a specified override or a different parameter/tolerance case needs its applicable operator. The poster carries document code **EN_60_050_0038I** and print mark **CZ-VIII/2023**; it does not explicitly label a revision date. [ZEISS setting-class table](https://dccprodblob.blob.core.windows.net/dccprodtrainingdocuments/en/knowledgeContent/Poster_Surface_Parameter.pdf)

## Select and report a reproducible setup

Read the complete surface requirement: parameter, tolerance type/value, standard/edition, specified filter or nesting indices, evaluation length, and any acquisition or direction requirements. Confirm that the instrument can acquire the needed usable profile on the actual feature. A short land does not justify quietly selecting a shorter cutoff: that changes the assessed bandwidth and can undermine comparison with the requirement. NIST explains that a defined bandwidth is important when comparing results from different instruments. [NIST filtering and comparison](https://physics.nist.gov/VSC/Help/Filters/Filters.jsp)

When legacy and modern results disagree, retain the raw profile where available and compare the complete operators before attributing the difference to manufacturing. Mahr describes issuing surface-standard calibration results under both legacy and ISO 21920 frameworks to make the differences visible. Such side-by-side reporting preserves the basis of comparison; it does not establish a universal conversion between the results. [Mahr dual-framework calibration reporting](https://metrology.mahr.com/de-at/news/newsroom/artikel/mahr-bietet-dakks-akkreditierte-kalibrierung-nach-neuer-rauheitsnorm/)

A usable inspection record includes the location and direction, acquisition method, instrument/probe, tip and point-spacing settings, form removal, filter types/indices, section/evaluation lengths, standard mode, parameter result, and acceptance rule. Keep units explicit: **0.8 mm = 800 µm**, not 0.8 µm. A roughness number without its operator can be insufficient for either repeat measurement or acceptance review.

## Related reference

[Inspection and quality documents](../../sourcing/documents/index.md)