Diffuse Reflectance Spectroscopy Instrumentation

The instrumentation of the optical measurement system consisted of a console with five light sources for tissue illumination, two different spectrometers, and an in-house developed, handheld fiber-optic DRS probe. The light sources were identical, halogen broadband lamps (Avantes, AvaLight-HAL, 360 to 2500 nm) with integrated shutters. Of both spectrometers, one covered a visible wavelength range from 200 to 1160 nm (Avantes, AVASPEC-HS2048XL-EVO), and the other covered a near-infrared wavelength range from 900 to 1750 nm (Avantes, AVASPECNIR256-1.7-RS). The DRS probe consisted of five peripheral illumination fibers placed in a circle around one receiving fiber in the center. Each light source was coupled to one illumination fiber. The source–detector fiber distance for all fibers was 2.0 mm. All diffuse reflectance spectra were calibrated on a white reference object of spectralon in a similar manner as described in Refs. 56 (link) and 58 (link). Therefore, any disparity between the light sources would be inconsequential. A schematic representation of the optical measurement system is given in Sec. 1 in the Supplementary Material.