Circadian light

Abstract

The present paper reflects a work in progress toward a definition of circadian light, one that should be informed by the thoughtful, century-old evolution of our present definition of light as a stimulus for the human visual system. This work in progress is based upon the functional relationship between optical radiation and its effects on nocturnal melatonin suppression, in large part because the basic data are available in the literature. Discussed here are the fundamental differences between responses by the visual and circadian systems to optical radiation. Brief reviews of photometry, colorimetry, and brightness perception are presented as a foundation for the discussion of circadian light. Finally, circadian light (CLA) and circadian stimulus (CS) calculation procedures based on a published mathematical model of human circadian phototransduction are presented with an example.

Figure 1

Photopic and scotopic luminous efficiency functions [10] and other spectral sensitivity functions measured with humans (mesopic: Rea et al. [13], glare: Bullough [14], brightness, central: CIE [10], brightness, peripheral: Weale [15]). Peak wavelengths for each function are noted in the legend.

Figure 2

Chromaticity diagram based on the Judd correction [22], showing contours of equal B/L value [23]. Also shown are the chromaticity coordinates for a red light (RED) and for a green light (GRN) that, when added, produce a yellow light (YEL) with the illustrated chromaticity coordinates as described in the text.

Figure 3

Nocturnal human melatonin suppression data from Brainard et al. [26] and Thapan et al. [27] for narrowband spectra (symbols), and a spectral sensitivity function resulting from exposure to narrowband illumination (solid curve). Also shown is the spectral sensitivity for broadband illumination when spectral opponency is exhibited [36].

Figure 4

Nocturnal human melatonin suppression data [24,26,27,30,38,39] (left ordinate), plotted as a function of CL quantities (abscissa) predicted by the model of Rea et al. [36]. Also shown is the best-fitting four-parameter logistic function [40] to all of the data from threshold to saturation. The circadian light quantity CL was formerly named circadian stimulus (CS) [36]; CS (right ordinate) now refers to the effective stimulus based on nocturnal melatonin suppression.

Figure 5

Close-up photograph of the Daysimeter. Two light sensors are located near the plane of the cornea, calibrated in terms of their absolute, spatial and spectral response to provide both photopic and circadian light readings. The rear housing attached to the earpiece contains accelerometers for measuring activity as well as memory and control circuitry, all powered by a coin-cell battery.