How an optical band gap is extracted from a UV–Vis absorption or transmittance spectrum — the conversions, the Tauc plot and the checks that make the number trustworthy.
UV–Vis spectroscopy measures how much light a sample absorbs across the spectrum. From that single measurement you can estimate the optical band gap: convert the raw column into absorption, convert the wavelength axis into photon energy, and run aTauc ploton the rising edge.
The route depends on what your instrument exported — absorbance, percentage transmittance, or a pre-computed absorption coefficient. All three converge on the same fit, and theUV-Vis band gap calculatorhandles each conversion automatically.
Powders and rough films scatter light, lifting the absorbance below the edge. A rising baseline inflates α in the tail and can bias the fit.
Above roughly A = 2–2.5 most spectrophotometers flatten out. Exclude that plateau from the fit window.
Fabry–Pérot fringes superimpose oscillations on the edge. Choose a fringe-free region or average before fitting.
Feeding %T into an absorbance-based workflow inverts the edge entirely. Always check the first rows of the file.
Yes. Absorbance data can be used for an absorbance-based Tauc analysis. If sample thickness and appropriate experimental assumptions are known, absorbance can also be converted to an absorption coefficient.
Yes. Transmittance can first be converted to absorbance using A = −log10(T), when T is expressed as a fraction rather than a percentage.
Yes. Convert %T to fractional transmittance first: T = %T / 100. Then calculate absorbance with A = −log10(T).
For wavelength in nanometres: E(eV) = 1240 / λ(nm), more precisely 1239.841984 / λ. This conversion is required because the Tauc plot x-axis is photon energy.
Use the absorbance-based approximation (Ahν)ⁿ instead of (αhν)ⁿ. It follows the same workflow and yields an intercept at the same photon energy, provided the path length is constant.
If the absorbance does not approach zero below the absorption edge, a baseline correction prevents a sloping background from distorting the linear region of the Tauc plot.
TXT, DAT and CSV files from common spectrophotometers are parsed automatically — absorbance or transmittance, wavelength or energy.