Transmittance

The amount of light of a certain wavelength that passed through the sample is described by the quantity transmittance (lat. transmitto). It is defined

$T={\frac {I}{I_{0}}}$

where

• T is transmittance,
• I is the light intensity that passed through the sample,
• I₀ is the light intensity that entered the sample.

In practice, it would be inappropriate to measure exactly both intensities: in addition to the properties of the sample, they are also affected by absorption and light reflection on the walls of the cuvette and in the optics of the photometer, the environment, in which the measurement is taking, etc. Therefore, transmittance is usually measured relative to a blank. First, the light intensity passing through a blank sample (blank, reference sample) is measured, i.e. a solution containing all components except the colored substance to be determined. Then, under the same conditions, the intensity of light passing through the unknown sample is measured. The transmittance is then defined by the relation

$T={\frac {I_{v}}{I_{b}}}$

where

• T is transmittance,
• I_v is the intensity of the light that passed through the sample,
• I_b is the intensity of the light that passed through the blank.

If the transmittance is measured in this way, there is no need to deal with non-specific losses of light intensity. The intensity of light that passes through the blank sample is considered to be 100% (ie, the transmittance of the blank is 100%), and the transmittance of samples absorbing light of a given wavelength is always less than 100%.

The transmittance of a solution that contains a colored substance depends on

• properties of absorbing substances,
• wavelength of the transmitted light,
• amount of absorbing substance, i.e. on its concentration in the solution and on the thickness of the cuvette.

August Beer (1825–1863) first formulated the dependence of transmittance on these quantities mathematically. Provided monochromatic light is used, it does

$T=10^{-\epsilon \cdot l\cdot c}\,\!$

where

• T is transmittance,
• ε is the molar decadal absorption coefficient (a constant specific for a given substance at a certain wavelength),
• l is the optical length cuvette,
• c is the substance concentration of absorbing substances.

We can also express the transmittance by algebraic modifications

$\log T=-\epsilon \cdot l\cdot c\,\!$ or $-\log T=\epsilon \cdot l\cdot c\,\!$,

based on which absorbance and optical density are defined