Trying to obtain absorption coefficient from absorbance data. (HP 8453 Spectrometer). When a UV-VIS spectrometer measures absorbance as
A = log10(I0/I)
how does it determine that the loss in the intensity of light is only because of absorption?
Is absorbance a true measure of absorption?
What happens to the absorbance when a sample is mostly reflecting so most of the light is reflected back to the source rather than to go towards the sensor?
Do UV-VIS spectrometers have this reflectance correction?
When you measure the absorbance of for instance a dye dissolved in a liquid, measured in a cuvette, what you do in the simplest way is (i) to measure the cuvette with the neat solvent and take the intensity as I0 and (ii) measure the dye dissolved in the same liquid in the same cuvette and take it as I. Despite the fact that you have reflectances at each interfaces, these do not matter because you do the measurement two times and calculate the ratio.
However, when you do a measurement of a solid sample, e.g. trying to measure the pure absorbance of the cuvette itself, what would be done is soing the two measurements with and without cuvette. Here, you have the reflectances at the interface only in one measurement, so what you actually calculate is not true/total absorbance. However, you can correct the spectrum for reflectance if you assume normal incidence and you precisely know the refractive indey of your material, e.g. silica. The spectrometer does not correct for reflectances automatically.
The UV Vis spectrometer can be used only for absorption. The detector is placed 180 degrees straight line to the sample to receive the transmitted signals.Absorbance is measured by subtracting the transmitted light from incident light. We can't get reflectance information .For reflectance you need to replace sample holder in UV-Vis with DRS(Diffused reflectance Spectroscopy ) adapter
The spectrometer will typically give you log10(Io/It) plotted against wavelength.
It will not discriminate between losses due to absorption, scattering or absorption. To account for the latter effects then you had better consider using an integrating sphere.
UV-VIS do not have any refection correction for solids. However, highly reflecting samples can be measured using Photo-acoustic spectrometer .
In general, a spectrophotometer working in the solar radiation wavelength region measures either the transmittance or the reflectance of a sample. That is, the absorbance is not measured directly, but is calculated from the measured transmittance and reflectance (if both can be measured). When a spectrophotometer gives you the absorbance A’ on the logarithmic form, i.e. the optical density OD, which is a common output from many spectrophotometers (OD = A’ = log10(1/T), "10" supposed to be as a subscript here, i.e. the 10-log), it is in reality a calculation from the measured transmittance ignoring the reflectance. A spectrophotometer measures the radiation which is collected by the detector from either the transmitted or the reflected beam. Furthermore: T + A + R = 1 (100 %) where T = transmittance, A = absorbance and R = reflectance between 0 to 1 (or between 0 to 100 %). Note that the absorbance A in the above is not the absorbance A’ on the logarithmic form, i.e. not the optical density OD, which is a common output from many spectrophotometers (OD = A’ = log10(1/T), "10" supposed to be as a subscript here, i.e. the 10-log). You may see further details in e.g. chapter 3, 4, 7.6.3 and 7.6.7 in the following article: B. P. Jelle, ”Solar Radiation Glazing Factors for Window Panes, Glass Structures and Electrochromic Windows in Buildings - Measurement and Calculation”, Solar Energy Materials and Solar Cells, 116, 291-323, 2013.
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