Background

Light

Light is a biophysical construct, meaning that the physical world and the biological world both determine the definition of light. The first definition of light was developed in 1924 based upon experiments measuring the radiant watts needed at different wavelengths to be seen by humans as equally bright. (1) This research led to the adoption of the photopic luminous efficiency function, $V(\lambda)$, which was intended to represent the spectral sensitivity of the human retina at optical radiation levels where only the foveal cones provided neural signals to the brain. Subsequently this biophysical construct of light based upon $V(\lambda)$ was “promoted” to the physical world through the SI system. Today we routinely make physical measurements and prescribe light levels in terms of illuminance, luminance, candelas, and lumens. But these physical quantities are not related to the biology of plants or other animals, nor are these measurements related to the variety of ways that humans sense optical radiation via the retina. Recognizing this limitation, the scotopic luminous efficiency function $V^{\prime}(\lambda)$ was advanced in 1951 to represent the spectral sensitivity of the human retina when only rods were active. (2) Even with the biophysical constructs of $V(\lambda)$ and $V^{\prime}(\lambda)$, many more definitions of light need to be considered to accurately relate the biology of the human retina to the physical world of optical radiation.

CL_A, CS, and the CS Calculator

Circadian-effective light (CL_A) is a biophysical construct relating the spectral sensitivity of the human retina to optical radiation for stimulation of the biological clock in the brain’s suprachiasmatic nuclei (SCN). The definition of circadian-effective light, or CL_A, was first proposed in 2005 (3) and refined in 2010. (4) Following extensive research, the definition of circadian-effective light was revised, replacing CL_A with CL_A 2.0. (5, 6) As such, CL_A 2.0 is now postulated to define the spectral sensitivity of the human circadian system to different wavelengths of optical radiation.

To quantify how different amounts, or levels, of CL_A 2.0 affect the neural signal strength reaching the SCN, the circadian stimulus metric, or CS, was developed. (3-6) CS represents the operating characteristic of the phototransduction circuits in the retina, from threshold (i.e., just barely enough to stimulate the SCN) to saturation (i.e., the maximum possible response to circadian-effective light, no matter how much optical radiation is provided to the retina).

The current version of the CS Calculator quantifies optical radiation incident on the human retina in terms of CL_A 2.0 and CS. The CS Calculator uses the relative spectral power distribution (SPD) of one or more light sources and the amount, or level, of photopic illuminance, based on $V(\lambda)$, incident on the cornea to determine circadian-effective light, CL_A 2.0. From CL_A 2.0, the signal strength, CS, is determined. A wide variety of other metrics, like chromaticity, correlated color temperature (CCT), color rendering index (CRI), and gamut area index (GAI) are also determined from the same relative spectral power distributions. A series of graphical representations accompany the numerical calculations.

The previous version of the CS Calculator, based upon the 2005 formulation of CL_A, (3) is essentially the same except circadian light is now calculated using CL_A 2.0 as defined in 2021. (5, 6)