Determining Light Intensity, Timing and Type of Visible and Circadian Light From an Ambulatory Circadian Monitoring Device

Abstract

During last decades, the way of life in modern societies has deeply modified the temporal adjustment of the circadian system, mainly due to the inappropriate use of artificial lighting and the high prevalence of social jet-lag. Therefore, it becomes necessary to design non-invasive and practical tools to monitor circadian marker rhythms but also its main synchronizer, the light-dark cycle under free-living conditions. The aim of this work was to improve the ambulatory circadian monitoring device (ACM, Kronowise^®) capabilities by developing an algorithm that allows to determine light intensity, timing and circadian light stimulation by differentiating between full visible, infrared and circadian light, as well as to discriminate between different light sources (natural and artificial with low and high infrared composition) in subjects under free living conditions. The ACM device is provided with three light sensors: (i) a wide-spectrum sensor (380-1100 nm); (ii) an infrared sensor (700-1100 nm) and (iii) a sensor equipped with a blue filter that mimics the sensitivity curve of the melanopsin photopigment and the melatonin light suppression curve. To calibrate the ACM device, different commercial light sources and sunlight were measured at four different standardized distances with both a spectroradiometer (SPR) and the ACM device. CIE S 026/E:2018 (2018), toolbox software was used to calculate the melanopic stimulation from data recorded by SPR. Although correlation between raw data of luminance measured by ACM and SPR was strong for both full spectrum (r = 0.946, p < 0.0001) and circadian channel (r = 0.902, p < 0.0001), even stronger correlations were obtained when light sources were clustered in three groups: natural, infrared-rich artificial light and infrared-poor artificial light, and their corresponding linear correlations with SPR were considered (r = 0.997, p < 0.0001 and r = 0.998, p < 0.0001, respectively). Our results show that the ACM device provided with three light sensors and the algorithm developed here allow an accurate detection of light type, intensity and timing for full visible and circadian light, with simultaneous monitoring of several circadian marker rhythms that will open the possibility to explore light synchronization in population groups while they maintain their normal lifestyle.

Keywords: Kronowise®; ambulatory circadian monitoring; chronobiology; circadian light; ipRGCs; melanopsin.

FIGURE 1

Kronowise^® ACM device.

FIGURE 2

Variables monitored by Kronowise^® as exported by its programming software (Kronoware 10.0) from a representative recording. From top to bottom: skin wrist temperature (in °C), visible light, blue filtered light, infrared light, maximum visible light (all in lux), integrated acceleration (expressed in g-values), integrated time in movement (expressed as motion events in 30 s epochs) and tilt of X axis in grades. Time in movement graph is clipped because, under natural conditions, it could never exceed 300 s × 10^-1/30 s. Light variables are clipped because the device is linear up to 43000 lux and above that it gets saturated.

FIGURE 3

Relative spectral response of full (black line) and infrared channels (red line) of the visible light sensor of ACM Kronowise provided by the manufacturer, compared with the normalized curve of human photopic (blue dashed line, Vos, 1978) and scotopic vision (black dotted line, CIE, 1951).

FIGURE 4

Sun light spectrum measured in irradiance with a spectroradiometer (orange line). Blue line represents the sun light spectrum filtered by the blue filter (also measured with the spectroradiometer), while green line corresponds to ipRGCs sensitivity curve according to the model from Enezi et al. (2011), and red line indicates melatonin suppression curve by light according to the model from Brainard et al. (2001).

FIGURE 5

Light intensity according to the measurement angle for the three light Kronowise channels from a light source placed at 50 cm from the device. More than an 80% of maximal light intensity was detected by the three channels in a range of 50°. Blue line represents the circadian channel recordings, the red line denotes the infrared channel recordings, and the orange line corresponds to the visible channel recordings.

FIGURE 6

Normalized visible light spectra from the nine different light sources used in this study measured with spectroradiometer. Normalization has been performed to the maximum peak of irradiance. Infrared band is highlighted as a red area and ultraviolet band in violet.

FIGURE 7

Bland–Altman plot comparing visible light intensity recorded by spectroradiometer and ACM. (A) Before correction for infrared content. Orange dots correspond to incandescent light and blue dots to low infrared content lights and sun light. (B) After correction for infrared content. Orange dots correspond to incandescent light and blue dots to low infrared content lights and sun light.

FIGURE 8

Bland–Altman plot comparing circadian light intensity according to CIE S 026 standard, recorded by spectroradiometer and ACM. (A) Before correction for infrared content. Orange dots correspond to incandescent light and blue dots to low infrared content lights and sun light. (B) After correction for infrared content. Orange dots correspond to incandescent light and blue dots to low infrared content lights and sun light.