Output metrics

Circadian Stimulus (CS)

The postulated instantaneous magnitude of the spectrally weighted, circadian-effective irradiance at the cornea (CL_A) mapped to an effectiveness scale from threshold (CS ≈ 0.1) to saturation (CS ≈ 0.7). The scale directly corresponds to the amount of melatonin suppression occurring in the middle of the subjective night for one hour of exposure (1).

Circadian Light (CL_A 2.0)

A nonlinear function of spectral irradiance postulated to represent the spectral sensitivity of the retinal mechanisms that generate neural signals reaching the human suprachiasmatic nuclei (SCN). Thus, CL_A is an instantaneous, spectrally weighted quantity analogous to photopic illuminance, but because it is a nonlinear transformation of irradiance, units cannot be assigned to this metric (1).

Illuminance

The amount of radiant power per planar surface area weighted spectrally by the photopic luminous efficiency function, V(λ) (2, 3). Illuminance is measured in footcandles (fc or lm ft^-2) or lux (lx or lm m^-2). One footcandle equals 10.76 lx, although for convenience 10 lx commonly is used as the equivalent.

Irradiance

The radiant flux (power, in watts) per unit area (m^-2) received by a planar surface (4, 5). Typical units are W m^-2.

Photon Flux

The number of photons per second per unit area (4).

Melanopic EDI (m-EDI)

Per CIE S 026 definition, m-EDI is “…illuminance, produced by radiation conforming to standard daylight (D65), that provides an equal melanopic irradiance as the test source, where melanopic irradiance is the effective photobiological irradiance with the spectral irradiance, spectrally weighted with the melanopic action spectrum" (4). Thus 100 photopic lx equals 100 m-EDI for a D65 source.

Correlated Color Temperature (CCT)

A specification for white light sources used to describe the dominant color tone from reddish (warm) to neutral to bluish (cool). Color temperature, in kelvin (K), can be used to fully characterize the spectral power distribution of a blackbody radiator (an incandescent lamp and the sun are close realizations of blackbody radiators) (6). CCT extends the practice of using temperature to characterize the dominant color tone of white-light spectra. Light sources with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4500 K usually considered cool. Temperatures between that range are considered neutral in appearance.

Delta u,v (D_uv)

Defined by American National Standard Institute (ANSI), an amount that quantifies the distance between the chromaticity of a given light source and a blackbody radiator of the same CCT (7). A negative D_uv indicates that the chromaticity of the source is below the blackbody locus in chromaticity space, whereas a positive D_uv indicates that the chromaticity of the source is above the blackbody locus.

Color Rendering Index (CRI)

A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same CCT (8). Most commercially available white light sources produce CRI values of 80 or higher.

Gamut Area Index (GAI)

A measure of color rendering characterizing the degree to which object colors appear saturated, or vivid. People tend to prefer the appearance of colored objects when GAI is within the range of 80 to 100 (9). Below 80, object colors will appear desaturated and bland. Above 100, object colors begin to look too saturated and unnatural. For example, a GAI of 140 can make some types of food appear artificially colored.

Chromaticity

A numerical description of the spectral power distribution (light source) or the spectral reflectance distribution (illuminated object) based upon a system of color matching three primary lights (nominally, red, green, and blue) to the color appearance of the light source or the illuminated object (7). Thus, just three values, one for the amount of light needed by each of the three primaries to match the color appearance of the light source or the illuminated object, are needed to describe any color. Chromaticity coordinates in the CIE 1931 system reduce the three-dimensional color space (X, Y, Z) to 2 dimensions (x, y) by normalizing the absolute values (10).

CIE α-opic Irradiances

Luminous efficiency functions based upon the spectral absorption of the different in situ human retinal photopigments developed by the Commission Internationale de l'Éclairage (4).