​@Ivair.Gontijo 

Without looking at your code, it might be difficult to say. But here are some elements of answer for you.

First, there’s a distinction to be made between UD, and FD. UD stands for User Data and it is what the DLL should compute (behavior you can alter) and return to OpticStudio. FD stands for Fixed Data, and it is available from within the DLL but cannot be changed.

How does Zemax “know” that it is a gradient index material? Line 47 of us_grin1.c (general Case 0):

case 2:
    /* ZEMAX wants to know if this surface is a gradient index media */
    /* it is. */
    strcpy(UD->string, "1");
    break;

Regarding 2. You are right. Once Zemax “knows” its a gradient index material, it won’t look at the Materials column anymore (instead it uses Case 6). Whether you type something in the Materials column or not shouldn’t make any difference (it is just ignored).

Regarding 3. It should be the refractive index value calculated in Case 6 and this is done at every step Delta T.

Does that help?

Take care,

David

Thank you ​@David.Nguyen for the reply above, I appreciate your kindness and help! Some confusion still remains regarding cases 5 and 6 in the example “us_grin1.c”:

• Note that in case 6 there is no function call to ‘Refract(...)’, the refraction function. In case 6 the UD->index and UD->dn/dx, dn/dy and dn/dz are calculated and they become available to Zemax, but it is not clear how/where they are used.

• Refraction only happens in case 5, where a real ray trace is done. The weird/confusing thing is that it does not use the new index calculated in case 6. It uses FD->n2 (see below) which, as you pointed out, is Fixed Data, that cannot be changed by the user.  I am guessing that FD->n2 used in refraction in case 5 normally comes from the material column. How is the GRIN refraction being calculated, if the material column is empty and UD->index is not used below?

• 

   

Yes ​@David.Nguyen, that is very helpful! In summary, FD->n2 is “Fixed Data” from the user’s perspective only. When Zemax is dealing with a GRIN surface, it will update this value by calling the DLL with case 6 to request the refractive index and making FD->n2 = UD->index. This in fact allows the software to use the same Refract(..) function in all cases, which is neat!

Thanks again!

​@Ivair.Gontijo 

That is an excellent summary. And it also prevents users from accidentally modifying FD->n1 and FD->2 during refraction.

Take care,

David