Shiny metal surfaces are one of the hardest cases for 3D machine vision. Ground steel, aluminium, stainless steel and chrome-plated parts reflect light like a mirror, so the 3D scanner gets an incomplete or incorrect image. The usual laboratory fix is to coat the part with matting spray. In production, that often does not work. We scan shiny surfaces without any spray.
Why a shiny surface is a problem
Most 3D technologies - structured light, laser triangulation, time-of-flight (ToF) cameras - rely on the surface scattering light in all directions so that some of it returns to the camera. A matte surface behaves exactly like that. A shiny surface reflects light in almost a single direction, which leads to three typical problems.
1. Glare
Where the reflection falls straight into the camera, the image saturates. The projected pattern or laser line can no longer be distinguished at that spot, so depth cannot be calculated.
2. Missing points
Elsewhere, the reflected light does not return to the camera at all. The point cloud is left with holes - sometimes exactly where a measurement is needed or where the robot has to grip the part.
3. False points
The most dangerous case. Light reflects off one surface, then off another, and only then reaches the camera. The system assigns the point to the wrong place, so "ghost" surfaces that do not actually exist appear in the point cloud. This is particularly common in internal corners, grooves and concave parts. If the robot trusts such a point, the pick or the measurement will be wrong.
Why matting spray does not suit production
Matting spray (anti-reflective spray) covers the surface with a thin matte layer, and the scanner sees the part as matte. In a metrology lab or for one-off scanning, this works. On a production line, however, several problems arise:
- An extra step. Spraying, drying and later cleaning take time and lengthen the cycle.
- Contamination. The spray layer stays on the part and in the surroundings. For many products this is unacceptable, and cleaning is yet another operation.
- Not suitable for a line. When every part has to be inspected or picked by a robot in real time, spraying each one is simply not possible.
- The surface itself is changed. You are measuring a coated surface, not the part itself.
In production, therefore, the 3D system has to see a shiny surface reliably, exactly as it is.
How we scan shiny surfaces without spray
There is no single magic setting. The result comes from a combination of measures that we select for the specific part:
- Multiple exposures (HDR). The same scene is scanned at several exposures: short for bright, glaring areas, long for dark ones. Reliable points are taken from each and combined.
- Scanner angle and position. We choose a scanner position at which direct reflections do not fall into the camera in the critical areas. When the scanner is mounted on a robot, the robot itself points it at the right angle.
- Filtering of reflection artefacts. We detect and remove points caused by reflections in software, so that the robot does not rely on "ghosts".
- Merging scans from several poses. We scan the part from several poses and merge the data into one point cloud. What glares or disappears in one pose is clearly visible in another.
For scans from different poses to line up, accurate calibration is essential, and when the scanner is carried by a robot, hand-eye calibration: the exact position of the scanner relative to the robot is known in every pose.
Where this matters
The shiny surface problem arises wherever 3D vision works with metal: when a robot picks machined metal parts from a bin or pallet, when the shape or dimensions of a part are inspected, or when the exact position of a part has to be found before the next operation. In all these cases, holes or "ghosts" in the point cloud mean a wrong decision, so reliable data matters here more than anywhere else.
Suitable equipment
We work with high-accuracy Photoneo PhoXi structured light scanners and, for moving objects, with MotionCam-3D. We describe them in more detail in Photoneo PhoXi and MotionCam-3D. ToF 3D cameras are also sensitive to reflections, so we choose them for large scenes where lower accuracy is acceptable. For a general overview of the technologies, see 3D machine vision in industry.
How to find out whether this will work for your parts
With shiny surfaces, the result depends heavily on the specific shape, finish and material, so we do not promise general figures in advance. The right approach is a trial:
- you send or bring a few real parts - ideally the most difficult ones;
- we scan them without spray and show you the point cloud;
- together we assess whether the data is sufficient for your task - measurement, inspection or robot picking.
We always start vision projects with a trial like this, so that you see the result before any larger investment. More under machine vision services; to discuss the details, use our contact page.