Bridgelux has developed a metric for comparing light sources to natural light: average spectral difference (ASD).

“With the growing market interest in human-centric lighting, the question of ‘naturalness’ of light source spectra

is becoming a frequently asked question,” according to the company. “Standard lighting quality metrics such as CRI and TM-

30 do not fully address the naturalness question.  The metric has been developed by Bridgelux to address this shortcoming in available light quality metrics.”

It is promoting ASD alongside its ‘Thrive’ LED light range. In 2015, Bridgelux was advocating gamut area index’ (GAI) instead of CRI, alongside its Décor Class A products.

On the subject of ASD, it went on to say: “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 standardised natural light source of the same correlated colour temperature. ASD value is a quantitative metric of the SPD differences.”

Unlike CRI and TM-30 where higher values indicate higher quality light, lower ASD values are closer to natural light. “The ASD values for standard light sources are much larger compared to Thrive,” said Bridgelux.

Bridgelux has written a white paper to describe ASD (flick to page 8 to skip a lot of marketing).

It takes 266 1nm wavelength bands from 425nm to 690nm for its calculation (see paper – below – for why this range).

The difference in radiometric power between the artificial light source and a reference light at each band is measured and expressed as a percentage deviation.

Bridgelux-ASD-formulaThe absolute value of all 266 values is averaged to produce a single value.

Reference sources are the black body curve for lights of  4,000K and below, and the daylight spectrum (D50, D57 and D65 standard illuminants) for 5,000K and above.