What is Melanopic Lux?

Lighting quality parameters are now a standard in architecture and design, with evolving standards. Originally, human-centric lighting (HCL) meant understanding the impact of CCT on humans and choosing “cool” or “warm” light fixtures appropriate to the purpose and activities of different indoor spaces. Understanding the melanopic ratio and incorporating melanopic lux as a factor in lighting layout designs is the next level of HCL lighting.

Lux is a measurement used by lighting designers to quantify the effect of an interior lighting scheme. Luminous flux is a measurement of the total amount of light a light source emits, integrated over the entire angular span of the light. We quantify luminous flux in units of lumens (lm), a photometric unit of measurement. The amount of light that shines onto a surface is called illuminance, measured in lumens per square meter (lm/m²), also called lux.

Lux has always been an essential consideration in lighting design for spaces where humans live and work, but now science has increased our understanding of the human circadian response to light. Light affects the release of an essential hormone, melatonin, which helps regulate sleep and wakefulness cycles. Thus, “melanopic lux” is a new metric that more accurately quantifies how lighting in an interior space will affect its human inhabitants.

Equivalent Melanopic Lux (EML)

The WELL Building Standard describes healthful light in Equivalent Melanopic Lux (EML). The EML formula stands beside other calculations for visible light as a method to quantify the biological (melanopic) effect of lighting on humans. The formula is weighted to emphasize the response of the retinal ganglion cells (ipRPGs), rather than the cones, which is the case with traditional lux.

An additional circadian cycle standard includes the Circadian Standard (CS) from Rensselaer Polytechnic Institute’s Lighting Enabled Systems & Applications (LESA) Center. Recently, Underwriters Laboratories (UL) decided to adopt the Rensselaer model.

Vertical Lux vs. Horizontal Lux

Previously, industry standards accounted only for horizontal lux. Horizontal lux measures task lighting that shines on a flat surface such as a desktop or the floor. The new approach considers vertical lux, light that shines vertically and enters the pupil at eye level in the image below. which directly impacts our melanopic response. (Vertical lux and horizontal lux are common shorthand terms. Technically, vertical lux refers to the vertical photopic illuminance, E_V, and horizontal lux refers to the horizontal photopic illuminance, E_H.)

WBS defines “eye level” as 1.2 meters/4 feet above the floor. Lighting designers following the WBS must specify EML in both the vertical and horizontal plane. WBS suggests a daytime minimum target of 200 EML. A building’s overhead lighting also contributes to the horizontal lux experienced by a human occupant. When you design lighting, you must convert horizontal lux to vertical lux.  If the vertical lux is known (or measurable) then

EML = E_V * MR

where MR is a calculated value based on the SPD of the LED package and can be supplied by Luminus.

For a more detailed discussion of how to perform this conversion and calculate the EML of a lighting scheme, refer to the white paper Lighting For Health: Human-Centric Lighting. 

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