Roughness Measurement Instrument Selection: Contact vs. Non-Contact

This article introduces the differences between contact and non-contact roughness measuring instruments during the selection process. Contact-type instruments use a stylus to slide across the surface, making them suitable for hard materials and high-precision reference surfaces. Non-contact instruments operate on optical principles and are ideal for soft or easily scratchable materials. The measurement standards for the two types differ, so their data cannot be directly combined. When selecting an instrument, factors such as material properties, measurement environment, required parameters (e.g., 2D or 3D), and budget should be considered. It is recommended to use both types together when necessary.

Measurement principle

Roughness measuring instruments are divided into contact type (stylus type) and non-contact (optical type) according to the contact mode between the sensor and the sample. The contact type slides along the surface through the diamond stylus and converts the vertical displacement into an electrical signal. The non-contact method uses optical interference, confocal or laser triangulation to collect surface topography. Before selection, it is necessary to clarify the properties of the material to be measured, the surface condition, and the measurement environment. For example, soft or scratchable materials should be prioritized for non-contact; High-precision datum or deep groove samples are more suitable for contact type.

Reference criteria and data comparability

Common domestic standards include GB/T 3505 (surface structure contour method) and GB/T 6062 (needle tracing method), and foreign references are ISO 4287 and ISO 25178. Contact measurement follows the contour method standard and outputs two-dimensional parameters such as Ra and Rz. Non-contact is based on regional standards (ISO 25178) and can output 3D parameters such as Sa and Sz. It should be noted that the data measured by the two types of instruments on the same sample have deviations, and it is not suitable to mix them directly. If it needs to be compared with historical data, the same principle instrument should be selected.

Formula example: The formula for calculating arithmetic average roughness is
Ra = (1/L) ∫₀^L |Z(x)| dx

Contact vs. non-contact comparison

Comparative dimensions	Brief description
Measurement principle	The stylus is in direct contact with the surface and records the mechanical displacement
Vertical resolution	Typically up to 0.5 nm~10 nm, limited by probe curvature
Applicable materials:	Metals, ceramics, hard plastics, and materials that are not resistant to scratches should be cautious
Measure efficiency	Line-by-line scanning, slower speed, suitable for local high precision
Environmental impact	It has strong anti-vibration and oil stain resistance, and is not sensitive to temperature changes
Data dimensions	The two-dimensional contour parameters are the main ones, and some can be spliced into three dimensions
Typical applications:	Calibration of machining surfaces, bearings, molds, and measuring tools
Main limitations	Cannot measure soft, viscous, steep sidewalls, and ultra-precise optical surfaces

Contactless	Brief description
Measurement principle	Based on the principles of light interference, confocal or laser scattering
Vertical resolution	Up to the 0.1 nm level, limited by optical diffraction limits
Applicable materials:	Suitable for almost all transparent, opaque and flexible materials
Measure efficiency	Fast surface scanning can obtain 3D topography data
Environmental impact	Sensitive to vibration, stray light, surface reflectivity
Data dimensions	Both 3D area parameters and contour parameters can be obtained
Typical applications:	Semiconductor wafers, optical components, flexible films, paper, coatings
Main limitations	Measurements on highly reflective or deep-hole surfaces are prone to distortion and costly

Key points of selection decision-making

First, evaluate whether the surface under test allows physical contact. If allowed and requires a high degree of repeatability and standard traceability, contact is a safe choice. If the sample is soft, light-transmitting or easily deformed, non-contact should be selected. Secondly, the measurement parameters are considered: only Ra/Rz is sufficient for contact; If 3D texture parameters (such as Sa, Ssk, Sku) are required, contactless is more reasonable. In addition, environmental factors also need to be weighed: there is oil pollution and vibration in the on-site production line environment, and the contact type is more reliable; The advantages of cleanroom optical method are obvious.

Comprehensive considerations and suggestions

Budget permitting, two principle instruments can be configured at the same time to handle different samples. When purchasing, the supplier is required to provide the measured comparison data on similar samples, and indicate the standard and filter conditions used. Attention should be paid to the calibration cycle after the contact stylus wear, as well as the periodic verification of the non-contact standard mirror. Choose an instrument that meets the actual measurement needs and standards to ensure data validity and long-term availability.

Reference source

1. Surface structure contour method standard, National Standards Technical Committee. 2. Surface texture area method measurement terminology standard. 3. Research report on contact and non-contact roughness measurement comparison, industry technical journal. 4. Major measuring instrument manufacturers publish technical descriptions.