Its characteristics make it ideal for its application in automatic surface inspection systems.2.?Materials and MethodsThe interferometer is built around a Savart plate, which consists of two beam splitting plates made of birefringent material with their optic axes at 45�� to the surface normal, rotated 90�� with respect to each other. Light entering the Savart plate is divided in two components (ordinary and extraordinary) that exit parallel but with a small displacement (or shear) between them. An analyzer is used to bring both beams to the same state of polarization so they can interfere in the detection plane.From any light reflecting point from a surface at a given distance, a spherical wave emerges, and the interference pattern carries information of the radius of this wavefront, or the distance to the surface.
2.1. Sensor DescriptionFigure 1 shows the setup of the prototype. The system uses an ImperX IPX-2M30H-G charge-coupled device (CCD) camera with an active image area of 14.21 �� 8 mm, a density of pixels of 1, 920 �� 1, 080 (pixel size is 7.4 ��m), and 75 mm focal length lens. This provides an angle of view around 10��. With a typical working distance of 100 mm, we can obtain around 19 mm long profiles with a lateral resolution around 10 ��m per pixel.Figure 1.General setup of the prototype system.The sample is mounted over a Thorlabs NanoMax? stage, controlled by the computer to perform surface scans.The profile to be inspected is illuminated by projecting a laser line with a wavelength of 685 nm over the sample.
The line thickness is 280�C290 ��m when focused at 100 mm, and it is projected in the direction of the longer side of the CCD to maximize lateral resolution. A SpeckleFree? module manufactured by Dyoptyka is used to minimize speckle noise while keeping a high maximum frequency of acquisition (up to 500 kHZ).In the detection branch a 50.8 mm focal length cylindrical lens is used to expand light in the direction perpendicular to the laser line. A calcite Savart plate with a total thickness of 7 mm sandwiched between two cross polarizers is placed between the camera lens and the CCD. This plate generates a shear of 0.53 mm for normal incidence.2.1.1. Brefeldin_A Fringe FormationThe formal derivation of fringe formation has been developed in depth in [23]. Here we will only outline the main results needed to follow the working principle of the sensor and the processing method.In the direction of the profile (x-axis in Figure 1) the cylindrical lens has no effect, and the camera lens focuses the light onto the CCD. Therefore each column of the CCD contains the information of a small section of the profile (lateral resolution). We will consider this as one point of the profile.