PHOTOMETER-Transmittance,Absorbance for dmlt students

PHOTOMETER:

When light is passed through a coloured solution, certain wavelengths are selectively absorbed giving a plot of the absorption spectrum of the compound in solution. The wavelength at which maximum absorption is called the absorption  maximum (λ) of that compound. The light that is not absorbed is transmitted through the solution and gives the solution its colour.

Photometric instruments measure transmittance, which is defined as follows:

                                      Intensity of the emergent (or transmitted) light           Ie

Transmittance (T)= ------------------------------------------------------------- = ------

                                                  Intensity of the incident light                                 Io

Transmittance is usually expressed on a range of 0 to 100%.

If the concentration of the substance in solution is increased linearly, or if the path length that the light beam has to traverse is increased, transmittance falls exponentially. So a term absorbance is defined so that it is directly proportional to the concentration of the substance.

                                                           Ie

Absorbance (A)= log1/T = log  ----

                                                           Io

Absorbance has no units. Photometric instruments electronically convert the measured transmittance to absorbance values.

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