Intensity Calibration of Raman Instruments: A Review †

Raman spectroscopy is a powerful analytical technique widely used in many scientific fields due to its molecular fingerprinting capabilities and non-destructive nature. However, the recorded Raman spectrum is strongly dependent on the characteristics of the instrument and on the experimental configuration: in particular, the relative intensities of Raman bands can vary substantially between instruments, limiting reproducibility and accuracy. Intensity-calibrated spectra are therefore increasingly needed, as their availability would facilitate studies based on band intensity ratios, the building of unified spectral databases, and the inter-instrument transferability of chemometric and machine learning models. This review provides an overview of the methods available for the intensity calibration of Raman spectrometers. After introducing the definition of the instrument response function and the required spectral transformations, we examine calibration approaches based on broadband lamps, luminescent materials (certified and non-certified), and Raman scatterers, discussing their experimental implementation, advantages and limitations. Insights from interlaboratory comparison studies are also reviewed, demonstrating that inter-instrument variability remains substantial, and highlighting the importance of intensity calibration. Finally, since inter-instrument transferability can be addressed both by physically calibrating the instrument response and through data-processing strategies known as calibration transfer methods, a brief overview of the latter is also included, as the two approaches are complementary and inherently intertwined.