Light the Eye, Not the Desk: Circadian Lighting Is Measured on the Vertical Plane

The number the spec sheet forgets

Walk into most new offices and the lighting has been signed off against a single figure: horizontal illuminance at the working plane, usually 300 to 500 lux measured flat on the desk, per long-standing standards such as IES RP-1 and EN 12464. That number tells you whether people can read a document without strain. It says almost nothing about whether the same light is keeping their body clock properly set. Those are two different jobs, run by two different parts of the eye, and for a century lighting has been designed for only the first.

The gap matters because the second job, circadian entrainment, is now something buildings are actively assessed on. The metric is melanopic equivalent daylight illuminance, or melanopic EDI, and it is not measured where desk lux is measured, nor with the same sensitivity curve. Get the distinction wrong and a room that looks bright and correct on paper can still leave its occupants under-lit for health.

Two systems, two peaks

Human vision as most people picture it is cone-driven and peaks in the green, at 555 nanometres, which is the basis of the photopic luminous efficiency function that defines the lux. But a separate class of retinal cell tells a different story: intrinsically photosensitive retinal ganglion cells, or ipRGCs, carrying the pigment melanopsin. These cells are not really for seeing. They are irradiance detectors wired to the brain master clock, the pupil reflex, alertness and mood. Their peak sensitivity sits far to the blue of vision, around 480 to 490 nanometres.

That offset is the whole point. A lamp can score high in lux while delivering relatively little of the short-wavelength energy the melanopsin system reads, or the reverse. So melanopic EDI reweights the same light by the melanopic action spectrum instead of the photopic one, then expresses the result against a common daylight reference, the CIE D65 illuminant at roughly 6500 kelvin. A melanopic EDI of 250 lux means light as biologically stimulating as 250 lux of that reference daylight, regardless of how many ordinary lux the meter reads.

Vertical, at the eye, and why the desk reading lies

The second break with convention is geometry. Desk lux is horizontal: sensor face-up on the working plane. But the melanopsin cells face the world through the pupil, so what reaches them is the light arriving on a vertical plane at eye level, roughly 1.2 metres above the floor for a seated person, aimed forward. That is where melanopic EDI is specified and measured.

The two planes rarely agree. Most office light comes from overhead luminaires pouring downward; it lands generously on the desk and grazes the eye. The ratio of vertical to horizontal illuminance in a typical downlit fit-out can sit near 0.6, meaning the eye receives well under two-thirds of what the desk does. A confident 500 desk lux can translate to a vertical figure far below the daytime target once that geometry and the spectral reweighting are both applied. The desk reading, in other words, systematically flatters the room.

The 250 that health research settled on

The daytime figure now widely cited comes from a 2022 expert consensus published in PLOS Biology, led by Brown and colleagues: for the hours people are awake and active, a minimum of 250 melanopic EDI lux at the eye, in the vertical plane at about 1.2 metres. The WELL Building Standard turned a version of this into a compliance test. Its circadian lighting credit asks for at least 200 equivalent melanopic lux at three-quarters of workstations, on the vertical plane, sustained through the middle of the day, roughly 9am to 1pm.

The consensus does not stop at daytime. Its companion limits are the more demanding ones: in the evening, from about three hours before bed, no more than 10 melanopic EDI lux; overnight, under 1. The daytime floor and the evening ceiling are one rule, and a home or workplace that hits the first while ignoring the second is only half-designed.

Why a bright office still fails

Because the target lives on the vertical plane and weights the blue, three levers move it, and only one is brightness. The first is spectrum: warmer, low-CCT sources carry a lower melanopic ratio, so pushing correlated colour temperature up toward 4000 to 5000 kelvin during the day does real circadian work that simply adding lumens at 3000 kelvin cannot. The second is direction: light has to reach the eye, not just the desk, which favours brighter walls and ceilings, indirect and wall-washing components, and high surface reflectance that bounces energy back up into the vertical field. The third is daylight, still by far the most efficient melanopic source there is; a seat near a window can clear the target on daylight alone, while an interior desk two rooms deep cannot.

The reason this is a design problem and not just a bigger-lamp problem is that the levers fight each other. Widening a fixture distribution to lift vertical illuminance also widens its glare footprint. Chasing melanopic content by cooling the spectrum can cost luminous efficacy and read as clinical. And simply flooding the room to brute-force the eye figure blows past the 300-to-500 lux the desk actually needs, and the energy budget with it. Analyses from the Illuminating Engineering Society frame the real work as balancing three quantities at once: horizontal lux, the vertical-to-horizontal ratio, and the spectral melanopic ratio, landing inside all of them rather than maximising any one.

The rule has an evening

The most common mistake is to treat circadian lighting as a daytime brightness upgrade and stop there. The biology is symmetrical: the same melanopsin cells that need a strong blue-rich signal at noon must be left almost undisturbed at night, or the clock the morning light set gets pushed back out. A genuinely circadian building therefore reads its own clock, cool and vertical and generous by day, warm and low and horizontal after dark, and measures success not at the desk, but at the eye.