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Focus variation sensors

Non-contact measurement of complex surface topographies

In the simplest version of the focus variation sensor, the distance between the object and the sensor is determined as the measurement point. The same hardware components are used for this as for image processing.

Autofocus and image processing in one

When the sensor is moved along the optical axis, a sharp image is only produced in one position for an image section (window). If the sensor is defocussed, blurry images are produced. Contrast can be used as a characteristic for the sharpness of an image. If the sensor is moved in the orientation of its optical axis in a department within which the object plane lies, the contrast value will reach its maximum when the focal plane coincides with the object plane. The position of the point on the surface can be determined from this sensor position (Fig. 18). This point can then be focused on by positioning (autofocus).

Autofocus and image processing in one
<p>Fig. 18: Focus point determination by sensor travel in the Department z1 to z2 and evaluation of the resulting contrast curve</p>

Large aperture angle: more precise focus

The sensitivity of the method described is primarily influenced by how large the department along the optical axis is that appears to be in sharp focus from the lenses used. This department, also known as the depth of field, depends directly on the resolution or numerical aperture of the lenses used. The larger the numerical aperture, the smaller the depth of field and the more accurate the measurement realised with the autofocus. With conventional lenses, the most favourable results are obtained with high magnifications. In order to achieve a high level of accuracy, many points on the focus curve must be recorded. Several seconds are required per measurement point. This leads to high total measurement times when measuring many points. The autofocus function is used to determine the measuring planes for image processing and to measure height levels. The fields of application are therefore the same as for image processing.

3D patch: measure many points simultaneously

Alternatively, the focus method described above can be carried out with a moving camera for several groups of image points (several windows) or for each image point of the camera simultaneously. By moving once through the desired measuring range along the optical axis, a large number of measurement points are obtained as a point cloud within a few seconds. This method (Werth 3D Patch) enables a particularly simple and fast three-dimensional capture of surface topographies (Fig. 19a). Larger surface areas can be measured by automatically placing several patches next to each other. The method can be used to measure topographies of small surface areas of workpieces made of a wide variety of materials or, for example, of fillets on tool cutting edges.

3D patch: measure many points simultaneously
<p>Fig. 19: Multidimensional distance sensors: a) Focus variation: Werth 3D Patch or confocal area sensor (NFP), b) Laser line sensor, c) Pattern projection sensor, d) Photogrammetry sensor</p>

Werth 3D Patch

Focus variation sensor