When a beam of light encounters a substance, the energy is either absorbed by the molecules, scattered away, or passes straight through the sample. This interaction between light and matter forms the basis of many analytical techniques used in science and medicine. To quantify this interaction, scientists use two distinct measurements: transmittance and absorbance. These two values provide complementary information about how much light a sample permits to pass and how much it captures.
Understanding Transmittance
Transmittance (\(T\)) is a direct measurement of the light that successfully travels through a sample. It is formally defined as the ratio of the light intensity exiting the sample (\(I\)) compared to the light intensity that entered the sample (\(I_0\)). This ratio is a fraction between 0 and 1, representing the proportion of light transmitted. For easier interpretation, transmittance is often expressed as a percentage (\(\%T\)).
If a sample is completely transparent to the light beam, the reading is 100% T. Conversely, 0% T means no light passed through, indicating the sample absorbed or scattered all incident light.
Understanding Absorbance
Absorbance (\(A\)), often referred to as optical density, measures the amount of light the sample prevents from passing through. This measurement quantifies the light that is captured by the molecules in the solution.
Absorbance is a unitless value mathematically derived using a logarithm. A high absorbance value signifies a strong interaction, meaning the sample is absorbing a large fraction of the incoming light. For instance, an absorbance of 0 means the solution is perfectly transparent, while an absorbance of 2 means it absorbs 99% of the light.
The Inverse and Logarithmic Relationship
The fundamental difference between transmittance and absorbance lies in their mathematical relationship. They are inversely related, meaning an increase in transmitted light corresponds to a decrease in absorbed light. This relationship, however, is not a simple linear inverse.
The connection between the two is logarithmic, defined by the equation: \(A = -\log_{10}(T)\). This means that while a change in transmittance causes an opposite change in absorbance, the change is not proportional. Due to this logarithmic nature, a small change in transmittance at a low value corresponds to a large change in absorbance.
For example, a decrease in percent transmittance from 100% to 50% results in an absorbance increase from 0.0 to 0.3. A much smaller drop, from 10% to 5%, causes the same size increase in absorbance, moving from 1.0 to 1.3. This non-linear behavior makes transmittance less practical for certain quantitative work.
Why Absorbance is Used for Quantitative Analysis
Although both measurements describe the same physical phenomenon, absorbance is the preferred standard for quantitative analysis in scientific laboratories. The primary reason for this preference is that absorbance is directly proportional to the concentration of the light-absorbing substance, a relationship described by the Beer-Lambert Law.
The law states that a solution’s absorbance is equal to the product of its molar absorptivity, the path length of the light through the sample, and the concentration of the compound. Because of this linearity, doubling the concentration of a substance also doubles its absorbance value. This makes it simple to create a calibration curve by plotting absorbance against known concentrations.
This ability to create a straight-line graph makes calculating the concentration of an unknown sample reliable and efficient. Since percent transmittance is non-linear with respect to concentration, its values require an extra mathematical conversion step. Therefore, the simple, direct, and linear nature of absorbance makes it the standard metric for determining material quantity.