See this page:

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

We convert Abs into Reflectance for our UV-vis measurements, not transmittance. Our IR measurements are in transmittance from the start.

In Vacuum absorption + transmission is normalised unit quantity. In general absorption+transmission+reflection is equal to one. But in limited wavelength range sometime people equate to one then you get a spurious result. In medium three quantities in entire wavelength range has to be unit. Except you know that at far end energy contribution is less than error of measurement.  Normally inversion from absorption to transmission may not have unique way. So experimental best choice to take data in transmission mode only. In our department many people do but that is unreliable in spite of many years of experimental expertised.

Dear Dr Pullar and Dr Jotania,

 thanks for reply. I have check link suggested by Dr Pullar. I understand that it is better to take in transmittance mode rather than converting Abs in to transmittance

thanks for kind suggestion and reply too 

Things are a little bit more complicated than presented in the link in answer 1. In answer 2 absorption is explained, but absorption is different from absorbance!

Overall all measurements are usually recorded either in transmittance or reflectance (except if you do photoacoustic spectroscopy). If you have absorbance data instead, it is very likely that a formula like A = -log10(T) was used for conversion which can give you easily errors on the order of 100% or more. Details are explained in the paper linked below, which was actually inspired by the many questions concerning this topic here in this forum.

Article Employing Theories Far beyond Their Limits-The Case of the (...

Dear Dr Thomas

thanks  for suggession and reply too. I will read the paper suggested by you

with best regards

FTIR spectra are in transmission or reflection modes. If one needs to quantify any structural changes, it is possible by estimating the optical density with other parameter such as concentration   kept invariant.

Dear Dr Guddo

thanks for information

regards

R B Jotania