Adaption and Rhythmicity in the Contribution of Cones to Irradiance Coding in Circadian and Non Image Forming Pathways

Measuring melanopic illuminance

Update (Sept 2021):

The concept of melanopic illuminance introduced by al Enezi et al (2011) and Lucas et al (2014) has been extended into an SI-compliant series of metrics by the CIE (CIE S 026/E:2018). CIE international standard S026 defines methods 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, units=lx), which represents the illuminance of standard daylight (illuminant D65) required to achieve an equivalent melanopic irradiance. For most purposes, these fully standardised metrics of CIE S026 (excel toolbox available https://bit.ly/2T9QLTL) should be used in favour of melanopic illuminance. In particular M-EDI is finding increasing application in both scientific research and lighting applications.

The melanopic illuminance (often referred to as EML) can be converted into melanopic irradiance or M-EDI by a multiplication with 1.2013E-3 and 0.9058 respectively. At present (Sept 2021) versions of the CIE-S026 metrics suitable specifically for rodents are not available.

Link to CIE S 026 website: https://cie.co.at/publications/cie-system-metrology-optical-radiation-iprgc-influenced-responses-light-0

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The concept of melanopic illuminance as introduced in al Enezi et al (2011)(https://doi.org/10.1177/0748730411409719) has been developed in a more recent publication by Lucas et al (2014) (https://doi.org/10.1016/j.tins.2013.10.004). 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 both of those publications.

Those wishing to calculate melanopic illuminance for humans are directed to the supplementary online material for Lucas et al (2014) which includes an updated melanopic spectral efficiency function as well as an excel-based toolbox (https://documents.manchester.ac.uk/display.aspx?DocID=57019) and associated user guide (https://documents.manchester.ac.uk/display.aspx?DocID=57020) to help calculate melanopic illuminance. The major difference between the revised melanopic function and that originally proposed by al Enezi et al (2011) is that the function provided by Lucas et al (2014) has been scaled to ensure that melanopic illuminance is equivalent to photopic illuminance for a theoretical equal energy radiator. Melanopic illuminance calculated according to the al Enezi et al (2011) method can be converted into the updated Lucas et al (2014) version by multiplying by 1/5.4.

In the case of rodents, differences in pre-receptoral filtering mean that the melanopic spectral efficiency function is quite different at short wavelengths (<420nm) from that of humans. In order to ensure consistency with the function for humans proposed in Lucas et al (2014), the method for calculating rodent melanopic sensitivity function proposed by al Enezi et al (2011) has been updated to meet the criterion that melanopic and photopic illuminance are identical for a theoretical equal energy radiator. An excel spreadsheet comprising the revised spectral efficiency function and a method for calculating melanopic illuminance from measured spectral power distributions can be downloaded by clicking here: rodents. 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.

Although the method outlined in Lucas et al (2014) should now be used to calculate melanopic illuminance, for those who are interested, the legacy melanopic spectral efficiency functions proposed by al Enezi et al (2011) are available upon request (robert.lucas@manchester.ac.uk).