How to Get the Optimum Measurements Using Your Confocal Sensor

The distinctive confocalDT measurement system from Micro-Epsilon is based on the optical measuring principle that particularly benefits from the so-called chromatic aberration light. This refers to the different light refraction in relation to its wavelengths. Distance is measured by means of wavelength (spectrometry).

Confocal chromatic measuring systems are now being used to determine fast distance and thickness.  The variety of sensor and controller models open flexible fields of application, e.g. in the glass or semiconductor industries, medical engineering and production of plastics.

Decisive Advantages

Confocal chromatic sensors offer numerous advantages as compared with laser triangulation sensors. These become more evident where conventional laser sensors come up alongside their limits. When reflecting and shiny targets are being measured, confocal chromatic sensors provide high accuracy compared to the laser sensors that are overdriven due to the high light intensity.

Measuring Principle

The principle of confocal chromatic measuring is based on polychromatic light (white light) divided into separate spectral colors done by focusing a multi-lens optical system in the sensor at different distances from it. Short-wave, blue light (400 nm) is refracted more than long-wave, red light (700 nm). Measuring range is usually started with blue light and ends with red light. A factory calibration of the controller assigns each wavelength to a specific distance to the target.

The wavelength which is accurately focused on the target is the only one used for the measurement in the sensor system. The light reflected from this point is imaged by an optical arrangement onto a light sensitive sensor element.  This is where associated spectral color is detected and evaluated. Several distance points are evaluated in the case of thickness measurements.

Distance Measurement on any Surface

Confocal chromatic sensors are almost independent of the light reflected by the surface in comparison with the laser-optical sensors. Therefore, it is possible to measure the distance from dark and mat surfaces and from shiny and reflecting surfaces as well.

Micro-Epsilon Technology Advantages

Due to the nature of the factory calibration of Micro-Epsilon confocal systems, each light wavelength or color is being assigned with a certain distance to the measurement object.  The surface properties do not have any influence with the measurement accuracy. This distinctive measuring principle allows high precision distance and displacement measurement for diffused and reflecting surfaces even on mirror and liquid.

Extreme resolution: Surface scan of a wafer

Extreme resolution: Surface scan of a wafer

The measurement spot is smaller than a few microns and remains constant even with varying measurement distances. During a scan, providing microscopic topography results can help attain extremely high lateral resolution.  By utilizing a high-powered light source and spectrometer with high speed, measurement rates up to 70,000 Hz is usually reachable. The system is naturally immune to EMI and suitable for EX-proof environments.

High Performance

  • Distance and thickness measurement with high resolution and measuring rate
  • Very small and constant spot size
  • Nanometer resolution
  • Almost surface-independent, also suitable for mirrored and glass surfaces

Submicron Resolution

Confocal chromatic sensors give an extremely high resolution. Resolutions in the submicron range are made possible by individual spectral colors that are being used for the measurement. Confocal controllers from Micro-Epsilon can give a resolution of 1 nm. It is, however, lessened by optical effects of the lens arrangement in such a way that a max. resolution of 10 nm is achievable.

High Measuring Rates for Dynamic Measurement Tasks

The confocal measuring principle enables high speed measurements.  The controllers of different manufacturers provide different measuring rates depending on what models and components used. Controllers from Micro-Epsilon can reach up to 70 kHz, the highest rate of measurement in the world.

It must be noted that adapting the exposure to the respective surface is very important. Thus, Micro-Epsilon controllers operate based on an exposure control feature that uses the previous measuring cycle offering the fastest surface adaptation on hand.

Ready for Vacuum

Unlike other measuring systems, confocal chromatic sensors are basically suitable for use in vacuum, as the sensor consists of passive components. Hence, the sensor does not emit any heat. But all components must be fit for use in vacuum. In a vacuum, particularly those in a high vacuum (HV) and an ultra-high vacuum (UHV), conventional materials that might contaminate the environment by outgassing must not be used. Micro-Epsilon offers confocal chromatic sensors, cables and accessories which can be utilized based on their respective specification designed mainly for vacuum applications use.

One-Sided Thickness Measurement of Transparent Materials

Measuring thickness of transparent materials such as glass is made possible by the confocal chromatic measuring principle. It is detected to micrometer accuracy using just one single sensor that employs the reflections of the front and back side of the material. These reflections generate peaks on the CCD array based on the corresponding distance and thickness being calculated. Therefore, there must be knowledge of what is the refraction index of the transparent material.

Surface Adaption Based on Control and Exposure

The intensity of the reflected light is dependent on the quantity of reflection of the target. If the reflectivity changes while it is being measured e.g. shift from bright to dark materials or from reflecting to mat surfaces, the intensity on the receiver element in the controller alters. Stable measurements, however, require an adequate high level of intensity.

Due to this, confocalDT controllers from Micro-Epsilon are set with high-speed exposure control that regulates the existing exposure cycle of the CCD line. Each value that was detected serves as a basis regardless of the set measuring rate. The exposure control feature guarantees instant adjustment to different reflection characteristics, e.g. of glass and metal.

The advantages of this CCD line regulation consist of high dynamics and fast adaption. In comparison to other methods e.g. the light source regulation depending on the surface reflection, the surface adjustment from Micro-Epsilon offers the worldwide fastest control feature and makes sure that measurement results are reliable.

Cavity Inspection

The confocal chromatic sensors laser beam path is compact and concentric. Compared to laser triangulation sensors, its sensors avoid shadowing effects, allowing measurements even in sleeves and recesses. Moreover, Micro-Epsilon offers confocal chromatic sensors with a 90° beam path that permits geometric characters to be measured inside holes and recesses.

Ease of Use via Web Interface

Due to its user-friendly web interface, the whole configuration process of controller and sensors is done without using extra software. It can be accessed through the use of Ethernet and provides options for set up and configuration. An expandable database of materials is provided to store all the materials. Data output is via Ethernet, EtherCAT, RS422 or analog output.

This information has been sourced, reviewed and adapted from materials provided by Micro Epsilon.

For more information on this source, please visit Micro Epsilon.

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