Angle of incidence is varied by moving the light source. The system is designed so that the beams of light coming through the aperture at various angles are all parallel in the vicinity of the beamsplitter. The picture on this page http://eckop.com/optical-scatter-2/optical-scatter/how-conoscopic-scatterometers-work/ color codes the various incident angles. Blue is on-axis, cyan is at the edge of the field and so on. If the light source is moved from the position shown toward the right, it will illuminate the sample from 25 degrees when it gets to the position of the green rays, 40 degrees for the red rays and so on.
There is also a linear correspondence between the position of the beam in the vicinity of the beamsplitter and the angle of light through the aperture. For example, a 0 degree beam passes through the center of the beamsplitter, an 80 degree beam passes through the beamsplitter 40mm from the center and a 40 degree beam passes through the beamsplitter 20mm from the center. This means that one only needs to translate the light source in order to change the angle of the incident beam on the sample.
Please note that the above description discusses moving the light source in the plane of the page. We actually move the light source in the out-of-page direction. This is shown in the new photo on this page: http://eckop.com/optical-instruments/scatterometers/ We've sold a couple scatterometers, upgraded ouir light source and added light source translation to the 45 degree scatterometer (2nd from right), but the picture still gives the idea.
Practical and theoretical implementation discussion.
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