Application of JETI Spectroradiometer specbos 1211

Circadian Measurement

Since 2001 the action spectrum of the melatonin regulation is known thanks to the work of Brainard and Thapan. Additionally the responsible sensors were found distributed on the retina of the human eye. These sensors are not locally resolving, but their integral signal causes the suppression of melatonine secretion during night. The experimentally found circadian action spectrum c(λ) is shown in the following diagram:
circadian curve
Circadian action spectrum c(λ)


The sensors are specifically effective for blue light. The spectrum consists of the transmission of the human eye in the shortwave range and a mean distribution of the Brainard/ Thapan experimental data in the longwave range. The maximum of c(λ) at 450 nm is set to 1, as usual in action spectra.

This spectrum is the basis for the circadian metrics.  Generally an effective radiometric quantity is defined according to the following formula:

Xes - effective radimetric quantity, k – conversion factor, X- absolut spectral radiometric quantity, s(λ) – action spectrum, λ1 to λ2 - wavelength range of calculation 


Spectral Irradiance E is mainly used as radiometric quantity in circadian metrics. The conversion factor k is set to 1. Therefore one gets the following formula for the calculation of the total effective circadian Irradiance Eec:

The relation between the photometric and circadian effect of a light source is given by the circadian action factor :  
Circadian measurements can be done with a detector which sensitivity is specially adapted to the circadian action spectrum or with a spectroradiometer and weighting of the obtained spectrum.  The software JETI LiVal, which can be used with specbos 1211 and spectraval, allows to calculate the circadian radiometric quantity and the related action factor.
limes circadian calculation
Screen shot of Circadian metrics window of JETI LiVal


Additionally the measured Correlated Color Temperature CCT is given in the Circadian metrics window. The reason is that there was found a linearity between a and CCT [3]

cct av
Relation between CCT and acv


[1]  Brainard, G.C. Action Spectrum for Melatonin Regulation in Humans: Evidence for a Novel Circadian Photoreceptor. J. of Neuroscience 21 (2001) 16, pp. 6405 - 6412

[2]  Thapan, K.: An action spectrum for melatonin supression: evidence for a novel non-rod, non-cone photoreceptor system in humans. J. of Physiology 535 (2001) 1, pp. 77 -82
[3]  Gall, D. Die Ermittlung von Licht- und Farbfeldgrössen zur Bestimmung der spektralen Wirkung des Lichtes. Tagung Licht 2004 Dortmund