A percentage spectrometer measures the fraction of light transmitted or absorbed by a sample, expressed as percent transmittance (%T) or percent absorption (%A).

Concept of Percent Transmittance and Absorption

In spectroscopy, when light passes through a sample, some of it is absorbed while the rest is transmitted. Percent transmittance (%T) is the ratio of transmitted light (P_T) to incident light (P_0), multiplied by 100: %T = (P_T / P_0) × 100 . Conversely, percent absorption (%A) quantifies how much light is absorbed by the sample: %A = (1 – T) × 100% . These two measurements are inversely related: if a sample absorbs 100% of light, %T = 0%, and if it absorbs 0%, %T = 100% .

Relationship with Absorbance

Absorbance (A) is a logarithmic measure of light absorption: A = -log(T), where T is the transmittance expressed as a fraction (not percent). Percent transmittance can also be calculated from absorbance using: %T = 10^(-A) × 100 . This relationship allows spectroscopists to convert between absorbance and percent transmittance for quantitative analysis.

Instrumentation

A spectrometer or spectrophotometer is used to measure these percentages. Light of a specific wavelength passes through the sample, and the instrument detects the transmitted light. Modern spectrometers can measure across UV, visible, and sometimes infrared regions, providing data on %T or %A at each wavelength .

Applications

• Chemical analysis: Determining concentrations of solutions using the Beer-Lambert Law.
• Drug testing: Measuring absorption of compounds at characteristic wavelengths.
• Environmental monitoring: Detecting pollutants or contaminants in water or air.
• Material identification: Studying molecular structures and chromophores in organic and inorganic compounds .

Practical Considerations

Accurate measurements require clean cuvettes, correct wavelength selection, and properly calibrated instruments. Percent absorption is particularly useful for comparing unknown sample concentrations to standard curves, enabling precise quantitative analysis . In summary, a percentage spectrometer provides a direct, quantitative measure of how much light a sample transmits or absorbs, expressed as a percentage, which is fundamental for analytical and research applications in chemistry and related fields.