UV-Vis spectrophotometer has a light detector that detects photons by current signals. But the detector itself cannot differentiate between a high energy photon (short wavelength) and a low energy photon (long wavelength). It can only measure the number. However, if we require the irradiance or power per unit area, we have to multiply the number of photons with the energy of the photons. Does the UV-Vis spectrophotometer show the power per unit area or number of photons?
My guess is that it shows the power per unit area because the Mie analytical spectra for plasmonic nanoparticles agree with UV Vis spectra.
https://www.chromedia.org/chromedia?waxtrapp=fotjtbEsHiemBpdmBlIEcCAtB&subNav=lnijabEsHiemBpdmBlIEcCAtBN
Photon flux over time gives you intensity, or irradiance. The longer the integration time the higher is the apparent intensity (or height of the response peak), even at constant photon flux. The detector is photomultiplier tube with a fixed aperture area and photon sensitivity. Then, use the definition:
photon flux = number of photons per second per unit area.
Then, multiplying by the energy of the photons using Planck’s equation gives the energy striking a surface per unit time, which is equivalent to a power density (irradiance) in W/m²
Dear @Richard,
Thank you very much for your insightful answer. So the UV-Vis spectra we obtain is after the conversion from photon flux to irradiance? I suppose the software does the conversion before it shows the spectra?
Sincerely,
Rituraj
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