absorbance or reflectance? When should each of them be used?

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I have been doing PLS analysis on NIR spectroscopy of apple and egg. In both cases absorbance are used in the Y-axis and Wavelength in X-axis.
In NIR diagram of raw data, What are absorbance, reflectance? When should each of them be used? What is the physical meaning of absorbance for apple sweetness test? What is the physical meaning of absorbance for egg freshness test? What is the unit?

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  1. External Admin
    31/05/2019 at 11:52 am

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    PLS models are typically applied to make indirect measurements of one or more properties of interest, for example, to get a measurement of an apple’s sugar content based on NIR measurements. In this case, you measure the NIR spectra for a series of samples and then for the same samples you measure the sugar content, for example using a refractometer, as the reference value. The NIR spectra will be your multivariate X in your PLS model and the corresponding sugar content would be the Y reference value. In this way, the model relates X to Y so we can use that model to find the sugar content for future samples based on only NIR, without the need to do the refractometer measurement. PLS also gives tools that allow interpretation of the model, for example different plots that help us infer which wavelengths in the NIR spectra are most important for the prediction, or most correlated with the sugar content.

    Absorbance and reflectance are different ways the spectra can be represented. Simply put, absorbance measures light that is absorbed by the sample and reflectance measures light that is reflected off the sample. You can find more details about it here: https://www.perkinelmer.com/lab-solutions/resources/docs/TCH_reflection-Measurements.pdf

    In Unscrambler, it is possible to convert absorbance to reflectance/transmittance units and vice versa among other spectroscopic transformations by accessing the Tasks menu from the toolbar and selecting Transform->Spectroscopic…

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