Spread spectrum lighting for excitation-emission matrix measurements in ambient light environments

An excitation-emission matrix (EEM) provides valuable information for industrial applications such as detecting invisible hand dirt and food residues, and inspection techniques to determine food quality. However, ambient light interferes with EEM measurements in field environments. Conventionally, EEM signals are subtracted from the reflected ambient light intensities to eliminate these effects, removing only time-invariant components and not time-variant ones. To address this problem, we propose a spread spectrum lighting technique, wherein the light at each excitation wavelength is modulated according to different orthogonal codes and simultaneously irradiated. The fluorescence intensities are detected, and the EEM is reconstructed. The experimental results indicated that the time-invariant components were eliminated almost to the same extent as those of the conventional method, with the time-variant components reduced to 55%–57% of those of the conventional method. This demonstrates the potential of the proposed method for more accurate EEM measurements in ambient light environments.