Measuring melanopic illuminance and melanopic irradiance

A new way of measuring light.

The concept of melanopic illuminance as a method of quantifying light in terms of its effective brightness for melanopsin was introduced in al Enezi et al (2011). The principle was further developed by Lucas et al (2014), and has now been extended into an SI-compliant series of metrics by the CIE (CIE S 026/E:2018).

In order to appreciate the theoretical basis for this new way of measuring light and the challenges in quantifying irradiance in biologically meaningful units, we recommend reading all of those publications.

Melanopic irradiance and M-EDI

Current best practice for calculating effective brightness for melanopsin in humans is to employ the metrology as defined in international standard CIE S026.

CIE S026 provides a method for calculating effective optical power for melanopsin in terms of ‘melanopic irradiance’ (units W/m2). It further defines a derivative of that metric termed ‘melanopic equivalent daylight illuminance’ (M-EDI, unit =lx), which represents the illuminance of standard daylight (illuminant D65) required to achieve an equivalent melanopic irradiance.

M-EDI in particular is finding increasing application in both scientific research and lighting applications. An Excel toolbox is available for calculating these metrics either from a starting spectral power distribution or from predefined sources.

Melanopic illuminance for humans

Those wishing to calculate the legacy metric of ‘melanopic illuminance’ (sometimes referred to as EML) are directed to the supplementary online material for Lucas et al (2014), which includes a melanopic spectral efficiency function, as well as a toolbox and associated user guide.

The major difference between this revised melanopic function and that originally proposed by al Enezi et al (2011) is that the newer function has been scaled to ensure that melanopic illuminance and photopic illuminance are identical for a theoretical equal energy radiator (illuminant E).

Melanopic illuminance calculated according to the method defined in Lucas et al (2014) can be converted into melanopic irradiance or M-EDI by multiplying by 1.2013E-3 or 0.9058 respectively.

Melanopic illuminance for rodents

In the case of laboratory rodents, differences in pre-receptoral filtering mean that the melanopic spectral efficiency function is quite different at short wavelengths (<420nm) from that of humans.

An Excel spreadsheet comprising an appropriate spectral efficiency function and a method for calculating melanopic illuminance from measured spectral power distributions* can be downloaded below.

The spreadsheet also provides methods for calculating illuminance relevant for the other rodent photoreceptors (rods and m- and s-cone opsins) following the guidelines outlined in Lucas et al (2014) for calculating a-opic illuminances. 

At present (October 2021), versions of the CIE-S026 metrics suitable specifically for rodents are not available.

* This method is modified from that originally proposed by al Enezi et al (2011) to meet the criterion that melanopic and photopic illuminance are identical for a theoretical equal energy radiator.