Hi John,

Thanks for your question here on the forums!

The PLEN optimization operand computes the optical path length between the selected surfaces for a specific ray:

If you would like to compute the optical path length from the object to a specific surface then you can use the OPTH operand instead:

I hope this helps, but if you have further questions, please let us know and we will be happy to help!

Best,

Csilla

Hello again,

Were you able to see look at the example I posted as a clarification of what I'm trying to calculate?

I did notice the 'EFLA: Effective focal length in air in lens units.' but I wasn't able to make this work. Although, it wasn't clear to me how this merit function is intended to work.

Thanks,

John

I think the correct term for what I want is 'reduced thickness', and it does need to be divided by the index. Does this clarify?

Thank you, John

As you suggested, I ended up writing a ZPL macro to automate the equivalent thickness calculation. I’m posting here if anyone finds it useful.

!ZPL merit function marco to calculate equivalent thickness (t/n) between specified surfaces at the specified wave number. Ignores surfaces ignored in LDE. 

!John Hygelund, November 10, 2021

! ====== INPUTS ================

! Hx = first surface

! Hy = last surface

! Px = wavelength number

! ==============================

S1 = PVHX()

S2 = PVHY()

WAVE = PVPX()

CURRENT = PWAV()

PWAV WAVE

UPDATE 

LEN = S2-S1

DECLARE THI, DOUBLE, 1, LEN

FOR i, 1, LEN, 1

T = SPRO(S1-1+i, 16)

    IF (T == 1)

    THI(i) = 0

    ELSE

    THI(i) = THIC(S1-1+i)

    ENDIF

NEXT i

DECLARE IND, DOUBLE, 1, LEN

FOR i, 1, LEN, 1

IND(i) = INDX(S1-1+i)

NEXT i

DECLARE ETHI, DOUBLE, 1, LEN

FOR i, 1, LEN, 1

ETHI(i) = THI(i)/IND(i)

NEXT i

ETOT = 0

FOR i, 1, LEN, 1

ETOT = ETOT + THI(i)/IND(i)

NEXT i

OPTRETURN 0, ETOT

PWAV CURRENT

UPDATE