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Measuring with extreme precision

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Measuring with extreme precision - below the wavelength of light

Written by Jørgen Læssøe

 

The measurement accuracy of a vision system is often perceived as the pixel size. However, there are many ways to improve the precision, and under certain conditions it is realistic to improve the precision by a factor 500 compared to the pixel size.

JLI has a range of systems for measuring minute dimensions like the inner diameter of glass tubes. The latest JLI system, developed to measure the inner diameter of a glass tube, has revealed a precision of 5 nm. This is approximately 80 times less than the wavelength of the light used to illuminate the tube.

The pixel resolution of the system is 70 nm and the depth of field is only 2 µm. If the working distance (the distance between lens and focus area on tube end) is more than 5 µm off, the image is blurred – and the tube end isn’t visible. This can only be accomplished if the vision system controls the part positioning through complex algorithms and perform accurate positioning stages.

To achieve the precision integration techniques are used. The inner diameter is approx. 12 µm. It is measured by taking several exposures and integrating the measurements. The measurement precision is improved by the square root of the number of measurements, but you have to master all the compensations. The distance from the camera to the object, illumination, temperature, vibrations, and viewing angle. All these parameters must be understood, controlled or compensated.

The end result is a vision system measuring to a precision of 5 nano meters - far below the wavelength of light. The inspection system is integrated on a high precision machine delivered by Solomatic A/S in Silkeborg, Denmark.

Illustrations of the 2 camera setup can be seen below.

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