Basic Units of a Spectrometer

Spectrometers measure light-matter interactions using units such as absorbance (Au), transmittance (%T), and wavelength (nanometers, nm).Wavelength

Spectrometers measure the wavelength of light that interacts with a sample, typically expressed in nanometers (nm) for visible and ultraviolet light, or micrometers for infrared light. The wavelength identifies which part of the electromagnetic spectrum is being absorbed or transmitted by the sample, allowing the identification of specific substances based on their spectral signature .

Absorbance

Absorbance (A) quantifies how much light a sample absorbs and is often reported in absorbance units (Au). Absorbance is calculated as the negative logarithm of the transmittance: A=log10(I0/I), where I0 is the incident light intensity and I is the transmitted intensity. Absorbance is unitless because it is a ratio of two identical light intensities, but it is commonly expressed in Au for practical purposes .

Transmittance

Transmittance (T) measures the fraction of light that passes through a sample and is expressed as a percentage (%T). A transmittance of 100% means all light passes through, while 0% means no light is transmitted. Transmittance is directly related to absorbance but follows an exponential relationship, making absorbance more suitable for quantitative analysis .

Intensity

Spectrometers also measure the intensity of light, which is the amount of energy detected after passing through the sample. While intensity itself has physical units like watts per square meter (W/m²), in spectrophotometry it is often used in relative terms to calculate absorbance and transmittance .

Summary

In practice, a spectrometer provides data in three main forms:

• Wavelength (nm) – identifies the spectral region
• Absorbance (Au) – quantifies light absorbed, unitless but expressed in absorbance units
• Transmittance (%T) – fraction of light transmitted through the sample These units together allow scientists to analyze chemical composition, concentration, and optical properties of materials accurately .