Absorbance optics Optical absorbance is the most widely used detector for AUC. The XLI absorbance system requires at least a minute to scan a single sample, and has notoriously poor wavelength reproducibility, limiting its utility for many applications. The prototype absorbance system built for the XLI uses a continuous light source and software synchronization (Laue et al. 2006), acquires data from all samples simultaneously, has a 100-fold better wavelength reproducibility, has a tenfold higher sensitivity and precision, and acquires data at twice the radial resolution than the XLI.