Your SICK ID gives you access to our extensive range of services. This includes direct online orders, price and availability checks, and access to our digital services.
Wrong measurement values when measuring on a high reflective surface
Article No: KA-10357
Version: 1.0
Subject to change without notice
Related Products
OD Mini
DT20 Hi
OD2000
OD Value
Profiler
OD Max
OD5000
OD Precision
OD200
OD1000
Table of Contents
Introduction
Some applications require to measure on high reflective/ specular objects.
A typical example application is "Measuring the distance to a high specular silicon wafer".
Such measuring objects are very challenging for optical sensors, that's why it is important to understand the sensors limitations and how to set up the sensor correctly.
In this article we will show you what to consider when using a laser triangulation sensor to measure on a high reflective object.
Why is it challenging to measure on a high specular object?
To gain a better understanding of the situation, it is important to consider the two ways in which light is reflected:
Specular reflection
Diffuse reflection
When pointing a laser at an object, you usually obtain a mix of diffuse and specular reflections. Because diffuse reflection scatters light in all directions, most applications rely primarily on the diffuse component. The advantage is that the mounting angle between the sensor and the object becomes less critical. In addition, even contoured surfaces can be measured reliably.
However, when measuring highly specular surfaces—for example, a polished wafer—there is almost no diffuse reflection. In such cases, the sensor is unable to generate a stable measurement value using only the diffuse component.
There are two main reasons why measurements on these surfaces are difficult:
Almost all light emitted onto the specular surface is reflected without diffuse scattering. This is why the laser spot is often not visible on highly reflective materials. Consequently, the sensor struggles to detect a stable laser signal.
Nearly all of the reflected light is directed into a single direction, similar to a mirror. This means that unless the sensor is positioned exactly in the direction of reflection, it will not receive sufficient signal.
Diffuse objects Laser light is scattered in all directions by the diffuse surface. The receiver element of the sensor captures a portion of this reflected light, allowing the sensor to process the signal and calculate a measurement value..
Specular objects
Nearly all laser light is reflected in a single direction. If the reflected light does not reach the sensor’s receiver element, no measurement value can be calculated.
In other words, if an object has a highly reflective surface, you will typically obtain a very weak measurement signal due to the very low amount of diffuse reflection.
How do I get the best signal on a high specular surface?
In the case of specular objects, the direction of the reflected light depends on the sensor’s angle and the angle (or structure) of the surface. If there is no change in angle between the sensor’s laser beam and the surface, you can choose a mounting setup in which the reflected laser light precisely hits the sensor’s receiver element. Some sensors, such as the OD5000, provide specific modes (“Specular mode”) that help you set up the sensor correctly in such situations.
Limitations
Tilting the sensor or the surface to work with specular reflection only works with short‑range displacement sensors (e.g., the OD5000‑C85x20 with a measuring range up to 110 mm, or shorter‑range models).
The mounting angle must be set very precisely.
When tilting the sensor, a recalibration should be considered, as the angled mounting introduces an angular error into the measurement result.
Ghost measurements
In some situations, a laser triangulation sensor may output incorrect measurement values when measuring highly reflective surfaces. This can occur if the laser light is reflected from the specular object back onto the sensor’s front screen, or onto another surrounding object, which then reflects a portion of the light into the receiver element.
A highly specular object reflects the laser light back toward the sensor’s front window. Even small tilts of the sensor, the front window, or the object can cause the laser light to bounce multiple times between the sensor’s front window and the object until some portion of the light eventually reaches the receiver element.
A highly specular object can reflect the laser light onto a diffuse, bright surface. This diffuse surface will then reflect a portion of the light back toward the sensor’s receiver element.
In both cases, the receiver element does not evaluate the direct signal from the target surface but instead detects random reflections. These random reflections lead to unstable or incorrect measurement values generated by the sensor.
How to troubleshoot?
If such “ghost measurements” occur, slightly tilt the sensor so that these random reflections no longer reach the receiver element.
Keywords: reflection, ghost measurements, high reflective surface
