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Other Sciences
Chemistry
Calculation of absorption spectra
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[QUOTE="roam, post: 6025607, member: 120460"] I am trying to plot the absorption curve of a given sample using some spectrophotometric data. The data that I've collected are transmittance ##T## and reflectance ##R## – the ratios of transmitted and reflected light power to incident light power. However, using two different methods I got two very different answers: [B](1)[/B] When radiation impinges on a material, we know that: $$R+A+T=1 \tag{1}$$ Therefore, if we have obtained ##R## and ##T## using a spectrophotometer, then we can simply solve for ##A## and plot the results. [B](2)[/B] Most textbooks define [I]absorbance[/I] (sometimes called [I]optical density, OD[/I]) as: $$A=\log_{10}\left(\frac{1}{T}\right) \tag{2}$$ If we know ##T## using a spectrophotometer then we can plot absorption using Eq. 2. Here are my results from both methods. The top curve is calculated using Eq. 1 (dashed/dotted lines are ##T## and ##R##). The bottom curve was calculated using Eq. 2 (but the answer had to be multiplied by 100 so that it appears as a percentage). [ATTACH=full]228011[/ATTACH] So, which is the correct method and why? :confused: Also, using the second approach we get negative values for absorption (on the ordinate of the graph). How is this usually solved in the post-processing? Any suggestions would be greatly appreciated. [/QUOTE]
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Calculation of absorption spectra
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