Dear Cyril

Absorbance and transmittance are inversely proportional to one another. According to Beer’s law, a larger amount of light being transmitted through the sample corresponds to a smaller amount of light being absorbed by the sample

Absorbance and transmittance are parameters that indicate how much light is absorbed and transmitted for a specified material thickness. The two parameters are calculated for monochromatic light, as materials interact differently with different wavelengths of light.

A monochromatic source of radiant power (Po) is shined on a sample of thickness (t). If the beam of radiation leaving the sample has a radiative power of P, then the transmittance of the sample is calculated as: T= P/Po. The percentage transmittance, %T, is found by multiplying this value by 100.

Absorbance is found using the formula A =2-log(%T). As the transmittance increases, the percentage transmittance correspondingly increases, causing the absorbance to decrease.

Absorbance is also called optical density, as it is an indication of how well the material is able to stop light. Materials with low absorbance in a certain wavelength are considered transparent in that wavelength. Meanwhile, materials with high absorbance in a wavelength are opaque to that wavelength. Materials can be custom tailored to be absorbent in one wavelength and transparent in another. An example is greenhouse glass, which is transparent to visible light and opaque to infrared radiation.

Absorbance and transmittance are not  inversely proportional to each other, but exponentially dependent. The original Tauc plot is described for Absorbance spectra.

Thank you @Hugh J Byrne for the valuable information.

I hope the following links are helpful for you

https://www.researchgate.net/post/how_can_i_calculate_the_band_gap_with_absorbance_or_transmittance_data_using_uv-vis_spectroscopy

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

Agree with Prof. Hugh. I think literally there is no difference and you can calculate by using both way and check the band gap energies. Good Luck.

Tauc plots should be done in Absorbance - best to source the original work, referenced here - https://en.wikipedia.org/wiki/Tauc_plot 

@ Muhammad Sajjad : You have mentioned:

Absorbance is found using the formula A =2-log(%T). As the transmittance increases, the percentage transmittance correspondingly increases, causing the absorbance to decrease.

I think it will be absolute T (that's why a negative sign is there) not %T.

Muhammad Sajjad