Coming to Terms with LED Lighting

It’s More Than Meets the Eye

by | Aug 21, 2019 | Blog

The rush to switch to LED lighting continues to accelerate.  A Washington Post headline recently caught our eye.  Headlined “Why you should switch to LED lightbulbs right now, before the law requires it,” the article, like most in the media about LED lighting, talks mostly about energy efficiency and cost savings.  Less has been reported about the scientific evidence that provides a new understanding of LED lights’ profound effect on our biological health.  Not all LED lights are created equal, and before rushing out to change all the bulbs in your office building or home, it helps to understand these terms in assessing how light that impacts on health and well-being is measured.


Light is a stimulant that increases brain activation and alertness. It is the role of the eye to process this light information. It does so through its photoreceptors, which are specialized retinal cells.  Two types of photoreceptors – rods and cones – were first observed microscopically by Leeuwenhoek in 1684,[1] although it wasn’t until the 19th century that their role was more fully elucidated.  It is cones that are responsible for color perception, while rods mediate vision in low light conditions.

ipRGCs (intrinsically photosensitive retinal ganglion cells)

It wasn’t until 2002 that scientists discovered that there is a third class of photoreceptors in the human eye, photosensitive ganglion cells[2] that mediate human non-image forming (NIF) response to light, including changes in alertness, gene expression, and hormone secretion, but they do not contribute to vision.  These ipRGC photoreceptors, which express the photopigment melanopsin, can respond to light in the absence of all rod and cone photoreceptor input.[3]  The understanding of the role ipRGCs has become mainstream over the past 20 years.

Photopic vision

Photopic vision is related to color and detail vision when we are in a place well-lit by sunshine or by artificial means.  It’s the perception of the visual world around us, e.g. what makes a red apple look the color and shape of that fruit as distinguished from another round object such as an orange.  Photopic vision is regulated by cones.

Photopic sensitivity

Photopic sensitivity, or photopic spectral sensitivity, refers to visual sensitivity under well-lit (or photopic) conditions, where radiant energy stimulates the cones, the retinal photoreceptors responsible for color perception.[4]  The photopic sensitivity curve that peaks at 555nm is used to define and quantify lumen output from a light source.

Photopic lux

Photopic lux describes the average response of the color vision receptors, cones.  It is a measure of the intensity of light that hits or passes through a surface as perceived by cones. One lux is equal to one lumen per square meter.

Correlated Color Temperature (CCT)

CCT is a measurement of the color appearance of a white light—that is, its apparent “warmth” or “coolness”, measured in degrees Kelvin.  The CCTs of most commercial LEDs range between 2700K and 6500K. The high end of this range produces a bluish, cool color, while lights with a CCT of 3000K or lower appear to be cozier, calming and more intimate.  Although CCT can have an impact on mood and emotional well-being, it is not an adequate measure of the biological impact of the lighting because of the unreliable linkage between CCT and ipRGC stimulation.

Scotopic Vision

Scotopic vision refers to monochromatic vision at low light levels, such as at night.  Scotopic vision is mediated by rods, which have a much higher sensitivity than the cones. However, the sense of color or detail is essentially lost in scotopic vision.  At low light levels such as in a moonless night, objects lose their colors and only appear to have different gray levels.

Scotopic sensitivity

Scotopic sensitivity is visual sensitivity to low light conditions. In low light, it is the rods, which are achromatic and contain only one type of pigment, that are stimulated.

Melanopic response

The melanopic response essentially recognizes that in addition to the perception of the visual world around us, our body has a non-image forming response to light that impacts our biological functions, including the production of  the hormone melatonin (hence “melanopic”), which is key to our circadian rhythms, including the alternating patterns of wakefulness and sleep.  During the night the production of melatonin is suppressed by the presence of rich blue light, since melanopsin, the photopigment expressed by ipRGCs, has a peak sensitivity at 480nm (490nm in vivo).  As blue light wanes at night, we fall asleep.

Since the discovery of the melanopic response, manufacturers of LEDs have attempted to address biological needs by adjusting the amount of blue light in their lighting solutions so that artificial light can be maximized to allow us to be attentive to our work during the day, and minimized at night to help us fall asleep.  These lighting solutions have had mixed results.  Many are simply adjusting correlated color temperature to make health claims.  However, adjusting correlated color temperature alone does not have direct biological impact.  Other solutions compromise energy efficiency when using filters or a combination of multiple color LED chips.

Melanopic lux

While the mechanisms involving non-image forming responses and circadian systems are only beginning to be understood, a simple but effective way of quantifying the biological impact of light is to weight the light stimulus according to the input in the melanopic sensitivity function.  This is the range of the color spectrum where our body has a non-visual response to light.  This occurs from 400-590 nm and peaks at 480nm (490 in vivo).









Melanopic Ratio

M/P ratio is simply the ratio of melanopic lux to photopic lux at any given plane and can help to measure of the biological effectiveness of the light. During the day an M/P ratio of greater than 1 is believed to provide sufficient amount of ipRGC activation to stimulate alertness, while at night, an M/P ratio of < 0.3 is desired to promote sleep.   Depending on lighting conditions, the M/P ratio of natural sunlight is approximately 1.0.

Equivalent Melanopic Lux (EMl)

Equivalent Melanopic Lux (EML) is one of the models that attempts to measures our body’s biological response to light.  Mathematically, Equivalent Melanopic Lux = Photopic Lux x Melanopic Ratio.

Human-Centric/Circadian Friendly Lighting

Human-Centric and Circadian Friendly lighting are industry marketing terms that are increasingly being used to describe lighting solutions that aim to align with circadian rhythms.  However, when assessing healthy LED lighting solutions, it is important to consider how “healthiness” or “circadian friendliness” is measured.  Truly healthy LED lighting affects more than correlated color temperature.

A Word About LED Lights, Energy Efficiency and Impact on Health

In addition to defining LED terms, another important consideration is energy efficiency, often measured in lumens per watt or lumens per dollar.  Current industry efforts use multi-colored LEDs.  These multi-colored LEDs are expensive and compromises energy efficiency.  Only Aurea Lighting uses a patented dye film to convert traditional low cost, high-efficiency blue LED light into a light that syncs with the body’s biological needs – without sacrificing energy efficiency or light quality.

For more on the impact of blue light on health and well-being, please see

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