A recent LEUKOS review by Ali Heshmati and Barbara Szybinska Matusiak looks at an important gap in conventional lighting practice: buildings can meet visual lighting requirements while still providing weak daytime biological light exposure at the eye.
Read the open-access LEUKOS paper
Inspired by that paper, we made a few simple measurements in our own Yuji Lux office in Southern California.
All measurements were taken vertically at approximately eye level using a Sekonic C-7000.
What happens when you step inside?
Outdoors
5,680 lx vertical illuminance
5,660 lx melanopic EDI
MDER 0.996
Then we stepped inside and faced the window.
Inside, facing the window
522 lx vertical illuminance
492 lx melanopic EDI
MDER 0.943
That is an approximately 11.5× reduction in melanopic EDI simply by moving from outside to inside.
The spectrum remained relatively daylight-like, but the amount of light reaching the eye dropped dramatically.
This is why daylight should not be treated simply as “there is a window in the room.” Window size, glazing, room geometry, occupant position and viewing direction all affect the final exposure.
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Nearly the same lux. Not the same light.
At the workstation, with office lighting and some daylight:
503 lx vertical illuminance
321 lx melanopic EDI
MDER 0.638
Compare that with the measurement facing the window:
522 lx → 492 mEDI
503 lx → 321 mEDI
The ordinary illuminance differs by less than 4%.
The melanopic exposure differs by about 35%.
This is a simple demonstration of why lux alone cannot describe the biological light environment.
Two positions can look almost identical on a lux meter while delivering very different spectral and melanopic exposure at the eye.
Read more about circadian lighting
The worst case
In a workstation condition without meaningful daylight:
261 lx vertical illuminance
148 lx melanopic EDI
MDER 0.567
This falls below the widely cited 250 lx melanopic EDI daytime reference for healthy, day-active adults.
And there was another issue.
The electric lighting itself was not particularly good:
CRI Ra 86.8
TM-30 Rf 87 / Rg 97
So this condition was weak in two different ways:
relatively low daytime biological light exposure;
mediocre color quality.
This is an important distinction. Human-centric lighting should not mean trading visual quality for biological effectiveness.
We need both.
Try the Yuji Lux Spectrum Designer
What this small test shows
The takeaway is not that every office needs thousands of lux indoors.
It is simpler:
the light on the desk and the light reaching the eye are not the same thing.
A useful integrative lighting strategy needs to consider:
spectrum + intensity + timing + duration + direction + daylight
—not just CCT and horizontal illuminance.
Electric lighting can then supplement what daylight and architecture are not delivering.
At Yuji Lux, we are now restarting an internal project to improve the circadian component of our own office lighting.
The goal is practical: understand how much can be improved through relatively simple changes in spectrum, vertical light delivery, workstation orientation, daylight use and controls.
We will share the results as we go.
Explore Yuji Lux Human-Centric Lighting
Measurement Notes
Measurements were taken using a Sekonic C-7000 spectrometer, with the sensor oriented vertically at approximately eye level.
Sekonic measurements were used for illuminance, CCT, Duv, CRI and spectral data.
Melanopic EDI and Circadian Stimulus were calculated from the measured spectral power distributions using CS Calculator 2.0.
These measurements represent a real-world field demonstration from a limited number of positions and conditions and should not be interpreted as a controlled scientific study.