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Treating color temperature as circadian rhythm is the biggest misconception in the industry

January 13, 2026

Color temperature is not circadian rhythm: Clarifying ‘spectrum → metrics → implementation’ all at once

In recent years, “circadian lighting” has become increasingly popular in the industry, but a common misconception has also emerged:

  • Treating color temperature (CCT) as a direct proxy for circadian strength (as if the colder the light, the stronger the circadian effect, and the warmer the light, the weaker it is).
  • Equating full-spectrum light directly with “healthier” or “more circadian.”

Here’s the conclusion upfront:

Circadian effects are not determined by color temperature alone. They are governed by the combination of:

Spectral Power Distribution (SPD) received by the eyes × Vertical illuminance at eye level (Ev) × Time (period, duration, and prior exposure).

Below, we will clarify the relationship between CCT → spectrum → CS → CAF → m-EDI → EML using a framework that moves from physical quantities, to metrics, to standards and implementation.

1) Separate Three Things: Appearance, Spectrum, and Light reaching the eyes

A. Color Temperature (CCT): An appearance parameter indicating “warmer” or “cooler” light, derived from chromaticity coordinates.

B. Spectrum (SPD): The physical distribution of energy at each wavelength.

C. Light received by the eyes: For the same lamp, vertical illuminance and spectrum at the eye level vary depending on space, angle, reflection, and distance.

Key point:

The same CCT can correspond to many completely different SPDs (common in the industry due to different LED peak wavelengths, phosphors, optical designs, or mixing algorithms).

Therefore:

  • 4000K ≠ fixed circadian stimulus
  • 6500K ≠ automatically stronger
  • Full-spectrum ≠ automatically more effective

2) What does circadian lighting focus on? Start with the “melanopsin pathway”

Human circadian rhythms and non-visual effects such as alertness are primarily linked to the ipRGC/melanopsin pathway. Therefore, modern metrics are mostly described based on melanopsin-weighted responses.

The CIE S 026 system integrates the five photoreceptor types (including melanopsin) into a single measurable framework.

3) m-EDI: Converting “spectrum × illuminance” into a comparable circadian dose metric

What is m-EDI?

m-EDI (melanopic Equivalent Daylight Illuminance) can be intuitively understood as:
The stimulation of melanopsin by the current light is equivalent to the stimulation produced by D65 daylight at a certain illuminance.

How is m-EDI calculated, and how does it relate to spectrum and illuminance?

CIE S 026 introduces a useful bridging quantity—melanopic DER (commonly called the M/P ratio):

melanopic DER = m-EDI ÷ photopic Ev

(This is the melanopic efficacy relative to photopic illuminance.)

From this, it follows directly:

m-EDI = Ev × melanopic DER

(This is an algebraic identity based on the definition, not an empirical formula.)

Why this matters: It directly dispels two common misconceptions:

  1. Discussing CCT alone without considering Ev (vertical illuminance at the eye) = meaningless discussion of circadian effect.
  2. Discussing “full spectrum” without considering DER and Ev = still meaningless for circadian impact.

4) EML: Commonly cited in the WELL framework and Its Conversion to m-EDI

    What is EML?

    EML (Equivalent Melanopic Lux) frequently appears in WELL-related contexts. Academia has clarified that EML is not a standard quantity in CIE S 026, but it can be linearly converted to melanopic EDI:

    melanopic EDI ≈ 0.9058 × EML

    Implication:

    When you see an “EML threshold” in a project, don’t worry—it can essentially be converted into the m-EDI framework, which is sufficient for practical engineering communication.

    5) How to use WELL v2? Applying “metrics” to “duration × location”

    WELL v2 (in the Light concept, L03 / Circadian Lighting Design) emphasizes not the lamp parameters, but the actual circadian light exposure received by people.

    For example, at a workstation, the standard requires that within a specified time window, a certain duration of EML or m-EDI thresholds must be met (commonly interpreted as “at least 4 hours, starting accumulation no later than noon,” etc.).

    The key insight: WELL drives real industry progress by shifting the focus of “circadian lighting” from marketing claims about fixtures to the actual exposure dose in a space, considering location, direction, time of day, and duration.

    6) CS and CAF: Why they cannot be simplified as “just another color temperature”

    What is CS (Circadian Stimulus)?

    CS comes from the LRC model (Rea / Figueiro et al.) and is used to describe the strength of light’s stimulation on the human circadian system, with modeled correspondence to melatonin suppression. It is non-linear and has thresholds and saturation regions.

    This means:

    • CS is not synonymous with m-EDI and cannot be used interchangeably in a linear manner.
    • The same m-EDI value can produce different CS responses depending on spectral composition, field of view, and adaptation state.
    • In engineering, do not forcibly convert it into a single number to compare all scenarios.

    What is CAF (Circadian Action Factor)?

    CAF is commonly used in DIN/European contexts. Essentially, it weights the spectrum by a circadian action spectrum and compares it to photopic illuminance (similar to another form of M/P ratio) to express relative circadian effectiveness.

    • CAF is also not color temperature.
    • It depends on SPD but uses a different action spectrum than m-EDI or CS, so comparisons across metrics must be done carefully.

    7) A “Relationship Diagram” to Completely Separate the Misconceptions

    You can use the following “mental model” to educate your team and clients:

    (Appearance Layer)

    • CCT, Duv: Describe “what white looks like”
    • Important: This cannot directly determine circadian strength

    (Physical Layer)

    • SPD (Spectral Power Distribution) + Ev (Vertical Illuminance at the Eye) + Time (time of day / duration / historical exposure)

    (Metric Layer)

    • m-EDI (CIE S026 system): Ev × melanopic DER
    • EML (commonly used in WELL): approximately linearly convertible to m-EDI
    • CS (LRC system): non-linear, modeled in relation to melatonin suppression
    • CAF (DIN/European system): another weighted spectral ratio expressing relative circadian effectiveness

    (Standards / Implementation Layer)

    • WELL v2: integrates threshold + duration + measurement location into rules
    • CIE S 026: provides a unified, measurable language (α-opic, especially melanopic)

    This framework separates appearance, physical measurement, metrics, and standards—making it clear that color temperature alone does not dictate circadian effect.

    8) Practical Checklist for Designers: Don’t Be Misled by CCT

    When designing a circadian lighting plan—or reviewing someone else’s—make sure to clarify these five key points:

    1. Measurement plane: Are you measuring at the eye-level vertical plane or the work surface horizontal plane? (Circadian effect depends on light entering the eyes.)
    2. Data provided: Do you have the SPD/DER, or just the CCT? (CCT alone is insufficient for serious circadian evaluation.)
    3. Daytime target dose: What is the m-EDI/EML goal, and for how long should it be maintained? (Dose = intensity × duration.)
    4. Evening/nighttime strategy: Is there an upper limit strategy to avoid phase delays or melatonin suppression? (For example, research recommends significantly lowering melanopic EDI at night.)
    5. Integrated design considerations: Have you accounted for spatial reflection, beam distribution, viewing direction, and control strategy together? (Luminaire parameters are always just the starting point.)

    This checklist ensures circadian lighting is engineered for actual human exposure, rather than relying on superficial metrics like CCT alone.

    9) Conclusion: The Scientific Bottom Line for Circadian Lighting

    If the industry continues to treat “CCT = circadian effect” and “full-spectrum = circadian effect” as common knowledge, two consequences will inevitably occur:

    1. Professionals who rigorously implement space-based exposure dosing will be driven out by less rigorous practices.
    2. The market will lose trust in circadian lighting, because delivery cannot be verified.

    The good news is: CIE S 026 provides a common measurement language, and WELL v2 embeds verifiability into the standards. What the industry truly needs is to use these languages correctly and completely, ensuring circadian lighting is both measurable and actionable.

    References & Further Reading (recommended):

    • CIE S 026 / melanopic EDI definition and measurement (Manchester Lucas group)
    • Melanopic DER (M/P ratio) and CIE S026 terminology
    • EML ↔ melanopic EDI conversion discussion
    • CS (Circadian Stimulus) and its threshold/saturation explanation (IES/LRC)
    • CAF (DIN system) definition and usage in research/engineering
    • Daytime/evening/nighttime melanopic EDI exposure recommendations (Brown et al., PLOS Biology, 2022)
    • WELL v2 L03 (Circadian Lighting Design)