you can see this discussion

https://www.researchgate.net/post/Is_there_any_free_software_that_can_calculate_optical_constants_from_thin_films_reflect_and_or_transmetance_spectra_data

You need the values of Tmin for wavelengths of the interference maxima and vice versa for the formula for calculation of the refractive index for Swanepoel method. To this aim you have to extract the values of the wavelengths and transmittance of the interference extremes. You can make quadratic polynomial fit of these points in order to find these values.

After then you can put these values in the equation for the refractive index. Since the equation for calcualtion of therefractive index is very sensitive to error of TM and Tm the calculated values of n are not very accurate. The actual refractive index can be derived through the basic interference equation: 2nd=m*(lambda) where m is the interference order. Since m is unknown, you have to initiate assigning values to the interference extrema – integers for the maxima and half integers for the minima, starting from the longer wavelengths. The real values of m can be found from plotting m’ versus n/λ and then extrapolating to n/λ → 0. For example you can see in R. Todorov, J. Tasseva, Tz. Babeva, K. Petkov, J. Phys. D: Appl. Phys., 43, 505103 (2010).

Hi,

To calculate the envelope plot I used a small software form McClain

http://gams.nist.gov/cgi-bin/serve.cgi/Package/ENVELOPE

It was some years ago, but I remember I wrote an email to the author to request a copy of the software and she gave it to me. Unfortunately, I don't have this software any more on my computer. If you have Origiin 8.x, you can also do the same thing.

Thank you Jerome for your kind suggestion, Thank you very much. Can you please give me email id of the same from where you got the software. Also can you please suggest me, how one can the same in Origin8.0. Or can you please give me the functions, using that you have did envelope plot, I will make a program is C language, once I know the functions, that is used in origin or any software for the envelope plot.

Its too late to answer this question but still may be useful for someone

To envelop a curve using Origin 8(Pro) follow this step

Analysis -> Signal Processing -> Envelope -> a dialog box will appear, in this dialog box you can choose the range of plot to envelop as well as option for Upper, Lower and Both envelope.