Surface roughness can be tested by contact or non-contact methods
contact or non-contact methods figure
Contact profilometer
The tip of the stylus touches the sample surface directly .
The stylus is mounted on the top of the detector and tracks the sample surface.It moves up and down for electronic detection.
The electronic signal is recorded after amplification and digital conversion.
To accurately measure fine shapes and roughness using a contact surface roughness meter, the contact pressure must be small and the radius of the stylus tip must be as small as possible. The stylus is made of sapphire or diamond. The tip radius is usually less than 10 um. The ideal shape of the stylus is a toroid with a spherical tip.
Tip radius: rtip = 2um.5 um, 10um
Cone angle: 60°, 90°
* Unless otherwise specified, the ideal cone angle for general measuring instruments is 60°.
Portable surface roughness tester
The working principle of a contact profilometer is to measure the Z displacement of a diamond stylus as it moves across the surface of a manufactured part. As the stylus moves along the surface of the product, typically up to 25mm in range, this displacement is converted into a digital value displayed on the profilometer screen. After display, the product designer or manufacturer then analyzes the measurement results and can gain a deeper understanding of the product's attributes.Roughness contact detection diagram
In a roughness contact instrument, the stylus tip is in direct contact with the sample surface. The detector tip is equipped with a stylus that traces the surface of the sample. The vertical movement of the stylus is recorded by an electrical signal amplified and digitally converted by a displacement sensor such as an LVDT. 
Disadvantages of contact measurement
Because the stylus may damage the surface of the product when it comes into contact with the surface during measurement, causing surface roughness changes. It is also slower than non-contact technology, and the measurement is limited by the radius of the stylus tip, so if adopted in large-scale production processes, it may slow down the assembly process. In addition, contact technology has difficulty locating and identifying fine measurement points, and the sample needs to be cut and processed for inspection.
2. Non-contact profilometer
Non-contact profilometers can be measured using a variety of techniques, including laser triangulation, confocal microscopy, and digital holography. The most common non-contact profilometer is an optical profilometer, which uses light instead of a physical probe.
The pinhole has a diameter of only several tens of micrometers, and its function is to cut off the reflected light when it is not in focus. When "in focus", the reflected light of both the normal optical system and the laser confocal optical system enters the light receiving element. When observing "out of focus", the reflected light of the normal optical system (out of focus light) enters the light receiving element, but the reflected light of the laser confocal optical system (out of focus light) is cut off by the pinhole. That is, the reflected light enters the light receiving element only when it is in focus, and this is the basis for forming a confocal optical system.
In optical measurement technology, light is directed onto the surface of a product. By obtaining reflections from a well-placed reference mirror, a camera can detect the surface in 3D.
Comparison of contact and non-contact measurement
Non-contact profilometers are very reliable, capable of measuring surface variations within microns, and can calculate surface roughness much faster. In addition, non-contact surface measurement tools can measure larger areas because they are not limited by the size of the stylus tip.
