Lighting technology and solutions pioneer, Bridgelux, has developed a new metric to enable objective comparisons of light sources to natural light. With the growing market interest in human-centric lighting, the question of “naturalness” of light source spectra is becoming a frequently asked question. Standard lighting quality metrics such as CRI and TM-30 do not fully address the naturalness question. The Average Spectral Difference (ASD) metric has been developed by Bridgelux to address this shortcoming in available light quality metrics.
Throughout evolution, the human visual system has evolved under the natural light of sun and fire. These light sources have standardized industry spectral power definitions that describe the state of natural light. 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. ASD provides 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 CCT. The ASD value is a quantitative metric of the SPD differences shown in the figure below.
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. The Bridgelux Thrive LED light source family has been engineered to provide the closest match to natural light available in the market.
The graph and the table provided compare standard 80, 90, and 98 CRI light sources with Thrive at 3000K. The ASD values for standard light sources are much larger compared to Thrive, including that of the 98 CRI light source. While TM-30 and CRI are important color quality metrics, having high values does not necessarily correspond to high naturalness of the light source, whereas having very low ASD values always corresponds to high naturalness.
Click here to read Bridgelux's white paper further explaining the background behind the ASD metric.