Lighting has more effect on people than enabling us to see - it can also impact and affect our mood and health. The spectral engineering capability of LEDs presents opportunities to manipulate spectra, to enhance emission at certain wavelengths, or to improve the match to natural light. While there are differing schools of thought in our industry, delivering natural lighting is of interest to many human centric lighting advocates. This raises the question: how do we objectively quantify naturalness? Standard lighting quality metrics such as CRI and TM-30 do not fully address the naturalness question. This talk presents a new metric, Average Spectral Difference (ASD), which provides a quantitative measurement of how closely a light source matches the spectra of natural light. Spectra of various light sources will be reviewed, showing how ASD can be used to provide new insights into lighting quality, comparing and contrasting with other color quality metrics.