Lighting specifiers, designers, and luminaire manufacturers regularly consider color quality metrics such as CRI and TM-30, as well as individual spectral power distributions (SPDs), when making decisions regarding light source selection. However, until now, there has been no metric that quantifies the naturalness of a light source. Standard lighting quality metrics such as CRI and TM-30 do not fully address the naturalness question. In order to address this shortcoming, Bridgelux has developed the Average Spectral Difference (ASD) metric, providing an objective measurement of how closely a light source matches natural light over the visible spectrum, averaging the differences of the spectral peaks and valleys between a light source and a standardized natural light source of the same colour correlated temperature (CCT). The ASD value is a quantitative metric of the SPD differences. Unlike CRI and TM-30, where higher values indicate higher quality light, ASD represents the difference in spectra compared to natural light, therefore the lower the ASD value, the closer the match to natural light.