Figure 1. Feasibility boundaries for achieving 250 m-EDI.
By Bill Chan, President, LiteController Inc.
Shortly after my recent IES FIRES article, “Circadian Lighting in Offices: Feasibility Boundaries and Design Trade-Offs” was published, an architectural lighting designer left a comment on LinkedIn:
“Nowadays, it’s getting [to be] a real Mission Impossible!”
That brief comment perfectly summarizes the challenge facing many lighting designers today. We are expected to provide lighting that supports occupants’ circadian health while simultaneously satisfying visual comfort, recommended office illuminance, and high energy efficiency.
The question is no longer simply, “How do we achieve 250 melanopic Equivalent Daylight Illuminance (m-EDI)?” Instead, it has become: “How do we achieve it without creating glare, exceeding recommended office illuminance, or compromising energy efficiency?”
Four Constraints, One Design Challenge
A typical office design must satisfy four practical constraints simultaneously:
- Achieving the WELL Building Standard target of 250 m-EDI.
- Maintaining horizontal illuminance (Eh) within the 300–500 lux range recommended by IES RP-1.
- Providing sufficient vertical illuminance without exceeding the commonly accepted office comfort criterion of UGR < 19.
- Maintaining high luminous efficacy while satisfying industry expectations such as DLC 6.0 efficacy requirements.
These constraints are closely interconnected. Increasing vertical illuminance usually requires a wider light distribution, which can increase glare. Improving melanopic effectiveness through spectral optimization may also affect luminous efficacy. The challenge is therefore to balance competing design requirements rather than maximize any single lighting parameter.
A Simple Framework: The Demand Ratio, k
The IES FIRES article introduced a simple analytical relationship:
m-EDI = Eh × (Ev/Eh) × m-DER
Dividing both sides by Eh gives the demand ratio:
k = m-EDI / Eh = (Ev/Eh) × m-DER
where:
- Eh = horizontal illuminance (lux)
- Ev/Eh = vertical-to-horizontal illuminance ratio
- m-DER = Melanopic Daylight Efficacy Ratio
For the WELL target of 250 m-EDI, the required value of k depends only on the selected horizontal illuminance. For example:
- Eh = 500 lux → k = 0.50
- Eh = 300 lux → k = 0.83
The feasibility map (Figure 1 above), reproduced from the IES FIRES article, illustrates how combinations of Ev/Eh and m-DER determine whether a design is practically achievable.
The map also provides an intuitive way for designers to visualize these trade-offs. For example, if Ev/Eh = 0.60, then a minimum m-DER ≈ 0.83 is required to achieve the WELL target at Eh = 500 lux, corresponding to the minimum demand ratio of k = 0.50. Designers can therefore use the map to visualize the combinations of optical distribution and spectral performance that satisfy the circadian-lighting objective.
From Theory to Practice: A Viable Solution
The obvious question is: “Can this actually be achieved in practice?” One promising design approach is to combine an appropriate optical distribution with a high-melanopic-efficacy spectrum. Figure 2, below, illustrates one such example using a direct/indirect pendant luminaire together with an experimentally characterized 5000 K LED package incorporating a 465 nm blue-pump LED.
The LED package achieved an m-DER of approximately 0.91 with a luminous efficacy of approximately 181 lm/W at a forward current of 40 mA, suggesting that high melanopic efficacy can be achieved while maintaining the efficacy expected of DLC 6.0-qualified indoor luminaires. This level of melanopic efficacy is higher than that of many conventional white LED packages using a 450 nm blue-pump LED, making it easier to satisfy the proposed feasibility boundary while maintaining high luminous efficacy.
To demonstrate how this concept may be applied in practice, a DIALux simulation was carried out using the direct/indirect pendant luminaire in a typical open-plan office with 16 workstations. Horizontal illuminance was evaluated on the workplane at 0.75 m above the floor, while a 0.3 m × 0.4 m vertical calculation surface, centered at 1.2 m above the floor, was used to evaluate vertical illuminance and UGR for each workstation. The simulated design achieved an average horizontal illuminance of 495 lx, with a minimum-to-average uniformity ratio (Emin/Eavg) of 0.91, indicating excellent workplane uniformity. The corresponding average vertical illuminance was 279 lx, giving an average Ev/Eh ratio of 0.56 and an average UGR of 18.5. Combined with an experimentally measured m-DER of 0.91, the resulting m-EDI was approximately 254 lx. These results demonstrate that the proposed design satisfies the WELL target of 250 m-EDI while maintaining horizontal illuminance below 500 lx, UGR below 19, excellent horizontal illuminance uniformity, and high luminous efficacy.
Figure 2. Example of a practical circadian-lighting solution combining a direct/indirect pendant with an experimentally characterized 5000 K, 6 V, 465 nm blue-pump LED package (m-DER ≈ 0.91; 181 lm/W at 40 mA).
This example also illustrates an important design principle. Rather than simply increasing light output, successful circadian-lighting design requires simultaneous optimization of both the optical distribution and the spectral characteristics. In other words, the goal is to find the “sweet spot” where horizontal illuminance, vertical illuminance, visual comfort, and melanopic efficacy are balanced simultaneously.
What This Means for Designers
The demand ratio, k, provides a simple way to visualize the trade-offs among horizontal illuminance, vertical illuminance, spectral effectiveness, and energy efficiency. Once designers understand these relationships, they can make more informed design decisions and avoid many of the “Mission Impossible” situations encountered in practice. Circadian lighting is not limited by a lack of interest. It is constrained by the need to satisfy multiple competing requirements simultaneously.
Perhaps circadian lighting is not a “Mission Impossible” after all. Instead, it is a design optimization problem that requires balancing horizontal illuminance, vertical illuminance, glare, melanopic effectiveness, and luminous efficacy.
Rather than asking, ‘What CCT should I use?’, designers may achieve better results by asking, ‘How do I optimize Eh, Ev/Eh, and m-DER while maintaining visual comfort and energy efficiency?’. Hopefully, this framework will encourage further discussion on practical approaches to circadian-lighting design by balancing these design parameters, rather than optimizing any single parameter in isolation.
Readers interested in the complete analytical derivation and supporting discussion are invited to read the full article published by IES FIRES:
Circadian Lighting in Offices: Feasibility Boundaries and Design Trade-Offs
All images courtesy of Bill Chan, President of LiteController Inc.
About The Author
Bill Chan is the President of LiteController Inc., a manufacturer of Circadian Lighting LED fixtures and modules. Bill has also been an Honorary Professor of Electrical Engineering at Tongji University for 32 years. Bill lives and works in Markham, Ontario, Canada. The author can be reached with questions at bill@litecontroller.com.
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