I took the absorbance spectra data of fruits by using a vis-nir spectrometer, and the result showed negative absorption in most wavelengths.
Please take a look at the picture below. It's the pic of my absorbance spectra & how I measured it.
So what's wrong with my spectra data?
Absorbance A is a quantity related to the Beer-Lambert law, according to which
A = log (I0/I), where I0 is the intensity at the starting point and I the intensity after a distance d. What is tacitly assumed (which is why it is usually not known), is that I0 and I must be measured within the probed medium, which is usually impossible. Therefore, in practice, you measure e.g. the transmittance of your sample (e.g. a solution) and ratio that to the transmittance of a reference (e.g. a pure solvent).
Your instrument seems to based not on transmittance but on diffuse reflectance (which means that the spectra are not quantitatively usable IMHO). Accordingly, your reference would be an ideal scattering material like bariumsulfate in powdered form.
How exactly and in what form did you measure your fruits? What did you measure as reference?
I measured the spectra data of intact fruits.
and I wanted to predict the sugar content.
Thomas Mayerhöfer and Andrew Callender
Thanks for the reply, but i still don't understand about reference sample as you mentioned above.
I added a pic how I obtained the spectra data.
Absorbance A is a quantity related to the Beer-Lambert law, according to which
A = log (I0/I), where I0 is the intensity at the starting point and I the intensity after a distance d. What is tacitly assumed (which is why it is usually not known), is that I0 and I must be measured within the probed medium, which is usually impossible. Therefore, in practice, you measure e.g. the transmittance of your sample (e.g. a solution) and ratio that to the transmittance of a reference (e.g. a pure solvent).
Your instrument seems to based not on transmittance but on diffuse reflectance (which means that the spectra are not quantitatively usable IMHO). Accordingly, your reference would be an ideal scattering material like bariumsulfate in powdered form.
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