【Part Two】 A “Threshold Cheat Sheet” + Nine “Healthy Light Guidelines” to Build the Framework of Healthy Light Design
In many project discussions, I often hear designers express this confusion:
“I roughly know that the lighting should be ‘healthy’ and ‘human-centered,’ but when it comes to concrete numbers, I feel completely lost.”
On one side, there’s a bookshelf full of standards and research papers. On the other, there’s the overwhelming project schedule and client pressure. What designers truly need is a framework that’s easy to reference and apply:
- Telling you what to look at first, and what to look at next
- Showing you what is ‘enough’ and what is clearly insufficient
In this edition, we attempt to provide that framework for designers using 9 healthy light guidelines + a “threshold cheat sheet.”
1. Build the Framework Before Talking Style
Whether it’s interior design, architecture, or lighting design, we’re used to starting from concepts and styles:
- “I want this area to feel quiet and warm.”
- “That area should feel high-tech and futuristic.”
- “Here, the vibe should be Instagram-worthy.”
But when it comes to health, sleep, attention, or mood, relying on “feelings” alone is clearly not enough.
Take the lighting sections in international mainstream healthy building standards as an example. The lighting-related clauses can generally be summarized into four levels:
L01–L03: Is there enough and proper light?
- L01 Light Exposure (basic light dose)
- L02 Illuminance for Visual Tasks (Eh)
- L03 Circadian Light Exposure (Ev / EML related to circadian rhythm)
L04–L07: See clearly, and comfortably
- Glare control, brightness contrast and balance, and key visual areas such as stairways and pathways
L08–L09: Light source quality and control
- Spectrum and color rendering
- Flicker and short-term visible flicker risk
- Color consistency and long-term stability
- Scene and adaptive control strategies
Cross-cutting considerations: “Timeline” and “User Differences”
- Day vs. night, weekdays vs. weekends
- Adults vs. children, office vs. healthcare, hotel vs. residential
In other words:
Healthy light design is not about “adding a few more lamps” or “buying a few smart switches.”
It’s about finding a logical path between Quantity (illuminance), Quality (spectrum), Time (timeline), and People (users).
2. Designer’s “Threshold Cheat Sheet”: No Need to Memorize Every Clause, Just Remember the Key Anchors
The numbers below are not absolute values, but serve as anchor points for judging direction, persuading clients, and guiding brand teams during a project. You can put them on a small card for your desk or notebook.
1. L01 Light Exposure: At Least “Enough”
For areas with long-term occupancy (offices, classrooms, etc.), daytime vertical illuminance at eye level (Ev):
- Near windows: 250–300 lx or higher
- Interior areas: 150–200 lx minimum
- On cloudy days, in winter, or in north-facing buildings:
“Only turning on half the lights” is usually far from sufficient. Use measurements to see how much artificial lighting is needed to reach a truly alert level.
One line summary:
“Seeing enough during the day does not mean circadian stimulation is sufficient.”
2. L02 Illuminance for Visual Tasks: The Task Surface Must Do Justice to the Task
- Office / design workstations: Eh ≥ 500 lx
- Home / children’s desks: Eh 300–500 lx
- Corridors / circulation areas: Eh 50–200 lx
More importantly:
- Don’t just look at the maximum value — also check: average, minimum, and uniformity (min/avg)
Practical note:
Lighting only a few “spots” while other areas rely on intuition often results in “good-looking renderings, but uncomfortable real-world experience.”
One line summary:
“Task surfaces must be bright enough and uniform enough.”
3. L03 Circadian Light Exposure: The Day Needs a “Main Line”
Circadian-related metrics (e.g., EML / m-EDI) often seem complicated. From a design perspective, focus on two things:
- Daytime: Define a clear “main period” for alertness
- Example: 09:00–12:00 Ev and circadian stimulation should be noticeably higher than early morning or before closing.
- Evening: Intentionally “wind down”
- 1–2 hours before sleep, lower illuminance and color temperature to signal that it’s time to rest.
One line summary:
“Keep people truly alert during the day, and truly able to sleep at night.”
4. L07 Brightness Balance & Contrast: Avoid Extreme Bright-to-Dark Fluctuations
For visual comfort, avoid two extremes:
- Entire space uniformly bright with no layers (too flat)
- Large brightness difference between work surface and background (too extreme)
Practical guideline:
- Work surface vs. surrounding environment brightness ratio: roughly 1:0.5 – 1:3
- Temporary contrast increase can guide vision or emphasize focus
- But for long-term work, study, or reading areas, extreme contrast will cause fatigue and complaints
One line summary:
“Moderate contrast creates layers; excessive contrast creates torture.”
5. L08 Light Source Quality: Don’t Let Poor Light Ruin Good Design
Light source quality may seem like an “engineering” or “product” issue, but designers can gain significant influence by understanding it. Key anchor points:
- Color Rendering:
- CRI ≥ 90, or TM-30: Rf ≥ 90, Rg close to 100
- Color Consistency:
- Indoor spaces: SDCM < 5, to avoid noticeable color drift
- Flicker:
- Flicker < 20%, SVM < 0.6, Pst LM < 1.0
These numbers are based on research and standards, but for designers, knowing them gives you the confidence to say “no” to clients or suppliers when specifications fall short.
3. The Purpose of Tools Is Not to Show Off, but to Make This Logic Actionable
Many designers, when facing measurement equipment, often think:
“Will this be complicated? Do I have to become an engineer?”
In fact, tools that are truly suitable for designers should allow you to:
- Press a button and see Eh / Ev / CCT / EML
- Automatically calculate min / avg, contrast, and flicker metrics
- Even provide preliminary assessments based on WELL clauses
When LRS collaborated with various manufacturers to develop In.Licht Ultra / Pro, the goal was to turn these “seemingly complex parameters” into information designers can quickly read and act on onsite, rather than an additional burden.
The real point is not how expensive your equipment is, but whether you can clearly explain to your client:
- “Why should the lights be arranged this way?”
- “Why is this space worth a bit more budget?”
4. What Can LRS Do for Design Teams?
In many companies and organizations, the understanding of light across design, product, and marketing teams is often fragmented:
- Design focuses only on effect and atmosphere
- Product focuses only on parameters and cost
- Marketing focuses only on story and selling points
When collaborating with lighting brands, real estate groups, and design agencies, LRS typically conducts three types of workshops or internal trainings:
1. Build a “Healthy Light Framework” for Design Teams
Using L01–L09 and the “Threshold Cheat Sheet,” we help integrate scattered knowledge into a practical, actionable design logic.
2. Redefine “Light Formula” Language for Product & R&D Teams
Move beyond just “3000K / 4000K / 5000K” to scene-based vocabulary like “suitable for children’s learning, healthcare spaces, or night-shift offices.”
This helps brands clearly explain to the market who the product serves and what problem it solves.
3. Build a Unified External Narrative for Marketing & Leadership Teams
Elevate “healthy light” from a technical term to a long-term brand and business pillar.
Communicate with clients, partners, and associations using rigorous yet understandable language.
If your company or organization is considering:
“How can we systematically upgrade our team’s healthy light capability?”
LRS is happy to work with you to customize a training and co-creation session that truly translates into everyday design practice, based on your project type and team structure.
[Next Edition Preview]
In the next article, we will explore project practice and business models: how designers can move from a single delivery to long-term “light environment stewardship” when light can be measured and verified:
- How to apply FES (Field Evaluation System) thinking in real projects
- How to extend one-time lighting design into a Light as a Service model
- What new opportunities this opens for designers, lighting companies, and clients
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