Getting Light Right
By: Craig DiLouie

The Lighting Research Center (LRC) at Rensselaer Polytechnic Institute recently released a circadian stimulus (CS) calculator. Based on the CS metric, this tool can aid lighting professionals to select light sources and light levels that will increase the potential for effective circadian light exposure in buildings.

Lighting and health is emerging as a significant lighting trend. While a lot of conversation is happening around tunable-white lighting, color spectrum is only part of the story. When specifying lighting for the circadian system, light level, spectrum, timing, and duration of exposure must all be factored in, in addition to previous light exposure (i.e., photic history).

The CS metric is based on an LRC model of how the retina in the eye converts light stimulation into neural signals for the body’s circadian system.

“Lighting for the circadian system employs lighting design objectives that differ from those typically used in traditional architectural lighting design and therefore requires metrics that differ from those currently used by lighting designers,” said Professor Mariana Figueiro, Light and Health Program director at the LRC.

The metric centers on determining weighted spectral irradiance distribution of the light incidence at the eye’s cornea, or circadian light (CLA). From this distribution it is then possible to calculate CS, which expresses CLA’s effectiveness from threshold (CS = .1) to saturation (CS = .7).

Exposure to a CS of .3 or greater at the eye, for at least one hour in the early part of the day, is an effective method for stimulating the circadian system and is associated with better sleep and improved behavior and mood.

In an October 2016 article in LD+A, Figueiro and other LRC researchers point to several ways in which designers can deliver prescribed amounts of CS:

  •  Request the spectral power distribution (SPD) of light sources as this information is more revealing than correlated color temperature (CCT). Light sources with a higher CCT (5000K to 6500K) generally provide higher CS, but this is not always true.

  • Design for vertical (at the eye) not just horizontal (at the workplane) light levels and use luminaires that provide the best horizontal-tovertical light-level ratio. The LRC evaluated a variety of luminaires and found that a direct-indirect optic provides the best ratio. Other solutions include luminous workstation panels and task lighting that offer vertical brightness. 

  •  Light level and spectrum work together. Lower light levels generally produce lower CS values unless compensated by an SPD that delivers more power at shorter wavelengths (cooler light source). Figueiro pointed out hat when designing for an average workplane light level of 30 footcandles (fc), the researchers found that a 6000K source was needed to achieve the target CS threshold of .3. A 4500K source was needed for a workplane light level of 40fc. 

Figueiro advised that the design should also consider light exposure all day, who will be using the space, and layering the light to deliver lighting that is both functional and capable of circadian stimulation.

To use the CS calculator, designers should formulate a base condition by evaluating the space using the calculator and software such as AGi32. The design can later be fine-tuned again by using the CS calculator, while also accommodating IES recommendations, energy codes, and owner requirements.

The development of the CLA and CS metrics and calculator is potentially exciting for the lighting industry. With metrics and tools based on scientific research, the industry can begin developing and vetting practical design concepts aimed at stimulating a circadian response.

Download the free CS calculator at 

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