I guess the main confusion of defining this lens is probably the setting of depth/frequency. *eJhd w*
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The main fresnel lens types are: DuTlYXM2^
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* Constant groove depth. USnD7I/b
* Contant groove pitch types. 8>% jZ%`a
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So if your depth is fixed you will define +ve value in the cell and to specify the "height" or "depth" of each groove feature. If the frequency of the groove is fixed then you use -ve value. Say putting down - 5 this implies that there are 5 grooves with a unit length. *pcbwd!/
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Hope this helps!!