ThetaMetrisis - Absorbance & Transmittance Measurements

_Absorbance/Transmittance measurements

The Absorbance/Transmittance spectra show the Absorbed/Transmitted light through the sample under investigation vs. wavelength.

The sample under investigation can be either solid or liquid. Figure 1 shows a generic Absorbance/Transmittance holder for liquid samples mounted on the working area of a FR-tool. For more detailed description on how to perform the absorbance measurement please refer to section on FR-Monitor manual, in Download section.

Figure 1. Absorbance/Transmittance set up for liquid sample with cuvette of 1cm optical length.

The definition of absorbance (A(λ)) is:

where:

I_o is the reference light intensity, typically the emission light, e.g. with empty cuvette and

I_s is the light intensity through sample of interest.

For precise measurements the dark spectrum should be subtracted from both sample and reference spectra, so eq. 1a becomes:

where:

I_ref is the reference light intensity, i.e. the light intensity of the light source used, as it is recorded by the spectrometer without passing through the sample of interest, e.g. with empty cuvette for liquid sample.

I_sam is the light intensity after passing through the sample i.e. the light intensity recorded by the spectrometer when the light from the light source passes through the sample of interest.

I_dark is the dark intensity i.e. the light intensity recorded by the spectrometer with the light source switched-off or blocked and without sample.

Usually the absorbance value is in the [0, 2] range reflecting the [0-99]% absorbance range.

On the other hand, the definition of transmittance (T(λ)) is:

and by taking into account the dark spectrum as in the absorbance case:

The transmittance value is in the [0, 100]% or [0-1] range.

With FR-tools, absorbance and transmittance measurements can be made in UV, VIS or NIR spectrum (please refer to the product section) on semi-transparent and transparent samples (solid or liquid) as well as and of un-supported (suspended) films can be performed. For details on available solutions please refer to the product/accessories section.