Transmittance and Absorbance

Figure 2.4 Comparison between scales of Absorbance and Transmittance.

Two samples of the same phosphor crystal have quite different thicknesses, so that one of them has a peak optical density of 3 at a frequency of vo. while the other one has a peak optical density of 0.2 at vq. Assume a half width at half maximum of Av = IGHz and a peak wavelength of 600 nm, and draw the absorption spectra (optical density versus frequency) for both samples. Then show the absorbance and transmittance spectra that you expect to obtain for both samples and compare them with the corresponding absorption spectra. (To be more precise, you can suppose that both bands have a Lorentzian profile, and use expression (1.8), or a Gaussian line shape, and then use expression (1.9).)... 

A vernier adjustment (scale expansion) between fixed ranges is also available for calibration of absorbance and transmittance. The dual flow cells have a capacity of 8 /il and a 10-mm optical pathlength. 

Absorbance and transmittance are two important terms used in absorption measurement. If a beam of radiation of intensity P(> passes through a layer of solution, a part of the light is absorbed by the particles of the solution thus, the power of the beam weakens. The transmittance T of the solution is measured as the fraction of incident light transmitted by the solution. If the intensity of the transmitted light is I then transmittance is measured as,... 

The following is a conversion table for absorbance and transmittance, assuming no reflection. Included for each pair is the percent error propagated into a measured concentration (using the Beer-Lambert law), assuming an uncertainty in transmittance of+0.005.1 The value of transmittance that will give the lowest percent error in concentration is 3.368. Where possible, analyses should be designed for the low uncertainty area. 

From Beer s law, the absorbance and transmittance of a material can be expressed as... 

The intensity of an incident beam is redneed by 60%. Calcnlate the absorbance and transmittance of this beam. 

The following spreadsheet calculation and plot of absorbance and transmittance illustrates the exponential change of absorbance as a function of a linear change in transmittance. 

Explain clearly what is meant by absorbance and transmittance, and derive a relationship between them. 

The ordinate scale may be presented in % transmittance with 100% at the top of the chart. It is commonplace to have a choice of either absorbance or transmittance as a measure of the band intensity. The relationship between these two quantities will be discussed in Chapter 5. Figures 2.2a and 2.2b show the infrared spectra of lactic acid and illustrate the difference in appearance between absorbance and transmittance spectra. It almost comes down to personal preference... 

Table I. Absorbance and Transmittance Calculations For Each Wavelength of a Fusion H Bulb Medium Pressure Mercury Arc Lamp...

In practice, the method requires the use of a reference beam attenuation solution as in the ASTM method to allow the scale reading to be observed. The monochromatic absorbances are determined using 1mm cuvettes and 10 mm cuvettes for the observed absorbances. Note that the term differential absorbance and transmittance ratio are equivalent. 

IR analysis use absorbance and transmittance properties of the sample and defined as follows ... 

From this point we use the terms absorbatwe spectrum to refer to a spectrum that is plotted linear in absorbance and transmittance spectrum to refer to a spectrum that is plotted linear in transmittance or percent transmission. Note that we use the term transmission spectrum to refer to a spectrum that is measured in the transmission mode. 

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