I guess the main confusion of defining this lens is probably the setting of depth/frequency. b[`fQv$G
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The main fresnel lens types are: kr>H,%3~
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* Constant groove depth. &ub0t9R
* Contant groove pitch types. KkD&|&!Q7u
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So if your depth is fixed you will define +ve value in the cell and to specify the "height" or "depth" of each groove feature. If the frequency of the groove is fixed then you use -ve value. Say putting down - 5 this implies that there are 5 grooves with a unit length. bz\-%$^k
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Hope this helps!!
