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WSI/PSI technical data

The white light interference signal has different characteristics from the monochromatic light interference signal. In the case of a laser representing monochromatic light, interference signals occur even in several kilometers, but interference signals using white light occur only within 3~4um (coherence area) of the optical path difference between the measuring point and the reference plane. This is because white light contains an infinite number of monochromatic light in the visible region. In other words, the white light interference signal can be assumed as the superposition of these monochromatic light interference signals, and constructive interference occurs when the distance difference between the measurement point and the reference plane is very small, but as the distance difference increases, destructive interference occurs and the interference signal disappears. If you look at a point P on the workpiece in the figure, if the measuring point P is outside the coherent area of white light (the position in the direction of the workpiece corresponding to the position of the reference mirror with the light splitter as the center), the interference signal as shown in the figure. Does not appear. In this case, only the image information of the measuring point P is acquired. Suppose that the point P gradually moves in the direction of the Z axis in the figure. When point P enters the coherent area, the interference signal begins to occur gradually, and when it comes to the center of this area, that is, the focus position, the interference signal appears strongest. Of course, when the point P goes out of the coherence area again, the interference signal disappears. The Z-axis position value at the strongest part of this interference signal is set as the height value of point P. In this way, the surface shape can be measured by calculating the height value for all points on the measuring surface. The position of the reference mirror may be different depending on the interference lens, but all can set the coherent area with the same concept. In a typical interference objective lens, this coherence area is calibrated to form the position where the image is best focused. Therefore, you can think of the objective lens at the most accurate focus position when the strongest interference signal occurs.Therefore, in white light scanning interferometry, you can think that the algorithm that detects the peak of the interference signal determines the performance of the measuring instrument.