Inline sensors of the GLAST series are mainly used for the inspection of highly reflective surfaces (such as stainless steel panels, aluminum sheets, painted surfaces and plastic foils) and transparent objects (foils, glass sheets, Plexiglas sheets).
A projection lens projects a line grid onto a line sensor using the object to be inspected, which acts as a more or less optimal mirror. Transparent objects are placed close to the line grid, which is then projected again through a projection lens onto a line sensor. In both cases, the quality of the object determines the quality of the video signal on the line sensor.
The reflection method operates at an angle of 30° (45°, respectively), with the optical axes of the transmitter and receiver components diverging from the vertical axis. In the beam-through method, the object to be inspected is perpendicular to the optical axis of the transmitter-receiver arrangement. In both cases, the distance between transmitter and receiver is approximately 135 mm.
Three different line gratings are available in both cases: 0.5 m/0.5 mm, 1.0 mm/1.0 mm and 2.0 mm/2.0 mm.
The GLAST (GLoss And STructure) sensor is designed, among other things, to detect the haze behavior of objects to be inspected. For this purpose, a line grid is placed in front of a homogeneously illuminated surface. A projection lens projects this line grid onto the line sensor, with the object to be inspected practically acting as a mirror (reflector). The video signal available at the receiving end then provides information about the quality of the "mirror," or reflection properties of the object surface.
As the blur ratio increases, the video signal "flattens," which also results in a change in the spatial frequency spectrum of the line grating projected onto the line sensor. This means that the higher frequencies are less present and the amplitudes generally decrease.
The GLAST sensor is basically a line camera equipped with an adjustable diffuse light unit (16 white LEDs + diffuser + line grating). The detection range (line) is 20 mm at a distance of 85 mm from the sensor housing (see dimensions) and runs in the longitudinal direction of the sensor. The line-shaped portion of the surface to be inspected (approx. 20 mm x 0.5 mm) is projected onto the line sensor by the projection lens (receiver optics). The video signal generated by the line sensor is converted into a spatial frequency spectrum via suitable algorithms in the sensor's internal controller. This means that a signal is available that provides information about the blur ratio of the surface.
Up to 5 spatial frequency spectra and up to 31 states can be stored in the sensor. During the inspection process, the current spatial frequency spectrum is compared with the spectra stored in memory, looking for the most similar spectrum. Amplitudes, frequencies and harmonics are also compared.
Using the surface to be inspected (object surface), the line grid is reflected in the direction of the projection lens and projected onto the line sensor through the projection optics. The video signal from the line sensor - and thus the spatial frequency spectrum - changes depending on the quality of the "mirror," or reflection properties of the object surface.
