Average illuminance is the metric most often specified and the least often achieved in a way the user can perceive. A warehouse floor lit to a nominal 200 lx average can look bright and even, or dim and stripy, with the same average figure. The difference is uniformity, and uniformity is designed — not measured afterwards and hoped for. This white paper explains how to read an isolux map, how to compute the two uniformity ratios that standards actually use, and how to choose the spacing-to-height ratio that delivers them.
The reader in mind is a specifying engineer or a facility engineer responsible for an industrial or large-area installation who needs a defensible layout before purchase orders are raised.
1. Definitions and why two ratios exist
Two uniformity metrics appear in standards and in simulation reports, and confusing them is a frequent cause of a rejected design submission.
| Metric | Formula | Where used | Typical target |
|---|---|---|---|
| U0 (overall uniformity) | E_min / E_avg | EN 12464-1, GB 50034, general task lighting | ≥ 0.40 task, ≥ 0.25 circulation |
| U1 (min to max) | E_min / E_max | Sports, inspection, tunnels | ≥ 0.70 for inspection |
| U2 (edge uniformity) | E_min / E_avg over a defined grid | Sports venues, EN 12193 | Per sport class |
| CV (coefficient of variation) | Std dev / mean | Research and horticulture | Application-specific |
U0 answers the question “how much darker is the darkest point than the average?” U1 answers “how much darker is the darkest point than the brightest?” For a task area where the eye adapts to the brightest region, U1 is the more perceptually relevant metric; for a general circulation area where the eye adapts to the average, U0 suffices. Standards specify whichever metric suits the visual task, and specifying the wrong one for the application is a common error in tender documents.
There is also a practical asymmetry: U0 is much easier to satisfy than U1, because the maximum illuminance under a luminaire is always substantially above the average. A luminaire arrangement that achieves U0 = 0.60 typically achieves U1 = 0.30. If a client asks for U1 = 0.70 on a general industrial floor, the honest answer is that this requires an order-of-magnitude increase in fixture count, and the requirement is almost certainly a transcription error from a sports-lighting standard.
2. What an isolux map is
An isolux map is a contour plot of equal illuminance across the calculation plane. It is produced by interpolating the values at each grid point of a simulation and drawing lines through the interpolated points of equal value, exactly as a topographic map draws contour lines of equal height.
Three things are immediately readable from a contour map that an average figure conceals. The contour density shows how steeply illuminance falls: tightly spaced contours mean a narrow distribution and a strong gradient. The contour closure shows whether luminaire overlap has merged: closed rings around individual luminaires mean the arrays are not overlapping enough, which guarantees poor uniformity. The contour value at the darkest accessible point is the E_min that will be used in the uniformity calculation — and it determines whether the design passes.
2.1 The E_min location trap
E_min is by definition the lowest value in the calculation grid, but only grid points in the defined evaluation area are admissible. Two conventions exist and they give different answers:
- Whole-room grid: every point inside the room boundary counts, including the corners and the strip against the wall. E_min occurs in a corner and U0 is depressed accordingly.
- Task-area grid: the grid excludes a margin from the walls, typically 0.5 m for walls and 0.5–1.0 m for racking. E_min occurs in the working area and U0 is higher.
Both are legitimate, but the choice must be stated. A tender that specifies U0 without defining the evaluation area is unverifiable, and a contractor that changes the definition between bid and delivery has changed the performance without changing the product.
3. Computing uniformity from the grid
Uniformity is computed directly from the illuminance grid produced by the simulation. The table below demonstrates the calculation on a 5 × 4 grid extracted from an aisle.
| Grid row | E at four positions along the row (lx) | Row min | Row mean |
|---|---|---|---|
| Row 1 (under luminaire line) | 420, 468, 472, 425 | 420 | 446 |
| Row 2 (between lines) | 265, 296, 300, 268 | 265 | 282 |
| Row 3 (between lines) | 262, 292, 296, 264 | 262 | 278 |
| Row 4 (under luminaire line) | 418, 465, 470, 422 | 418 | 444 |
| Whole grid | — | 262 | 362 |
From this grid:
- U0 = E_min / E_avg = 262 / 362 = 0.72
- U1 = E_min / E_max = 262 / 472 = 0.56
- CV = standard deviation / mean = 78 / 362 = 0.22
A U0 of 0.72 comfortably exceeds the 0.40 requirement for a task area, and the design has headroom. If the mounting height were raised from 8 m to 12 m at constant fixture count, the between-row value would fall to roughly 150 lx while the peak would fall to about 210 lx, giving U0 ≈ 0.50 — still passing, but without headroom for dirt depreciation, which is applied to the whole grid and therefore affects the ratio only if the maintenance regime is uniform.
That last point is often misunderstood: a uniform maintenance factor applied to every grid point does not change U0 at all, because it cancels in the ratio. Uniformity degrades with time only where contamination is spatially non-uniform — for example, luminaires near a loading dock collecting more dirt than those over a clean storage area. This is why U0 is a design-time metric, while the maintained illuminance level is a time-dependent one.
4. Spacing-to-height ratio: the controlling design variable
The spacing-to-height ratio (SHR) is the ratio of the centre-to-centre luminaire spacing to the mounting height above the work plane. It is the single variable that couples the number of luminaires to uniformity, and it can be checked before any software is opened.
Reading the curves at the U0 = 0.40 threshold gives the maximum permissible SHR for each optic. The table below converts those intersections into design limits and shows the practical spacing they imply at three mounting heights.
| Optic | Max SHR at U0 = 0.40 | Max SHR at U0 = 0.60 | Spacing at 6 m (U0 0.40) | Spacing at 10 m (U0 0.40) | Spacing at 15 m (U0 0.40) |
|---|---|---|---|---|---|
| 90° | 1.35 | 1.05 | 8.1 m | 13.5 m | 20.3 m |
| 110° | 1.70 | 1.30 | 10.2 m | 17.0 m | 25.5 m |
| 120° wide | 2.05 | 1.55 | 12.3 m | 20.5 m | 30.8 m |
The practical consequence is counter-intuitive for engineers new to lighting design: higher mounting height permits wider spacing, and therefore fewer luminaires, but requires higher flux per luminaire to maintain the illuminance level. For very tall industrial spaces, the wide optic with high flux is almost always the economical answer, provided the uniformity requirement is not set at sports-lighting levels.
5. Matching the optic and product to the design
The two product families most relevant to industrial uniformity design differ in how they interact with SHR.
| Product | Form factor | Typical mounting height | Recommended optic | Max SHR at U0 0.40 | Uniformity strength |
|---|---|---|---|---|---|
| Z-16 High-Bay Industrial & Mining | Round or rectangular high-bay, 100–300 W class | 8–20 m | 90° or 110° | 1.35–1.70 | Point-source array, contour cores |
| Z-09 UFO Ruiguang E | Round UFO form, low-profile driver | 4–10 m | 120° wide | 2.05 | Broad overlap, flatter contours |
| Z-15 Tri-proof (IP65) | Linear batten, sealed body | 3–6 m | Asymmetric wide | 1.45 (single row) | Continuous line source, smooth along axis |
The Z-16 family is specified where the mounting height exceeds about 8 m and the aisle geometry is regular. Its concentrated distribution produces high peak illuminance beneath each luminaire and clear contour cores, which is efficient for tall racking where the task surface is the rack face rather than the floor. The trade-off is that uniformity depends heavily on the spacing accuracy: a Z-16 array laid out at SHR 1.6 against a 90° optic will show visible striping between rows, whereas the same array with a 110° optic holds U0 near 0.45.
The Z-09 UFO form factor behaves differently. With a wide distribution and a compact luminous aperture, it produces smoother overlap at lower mounting heights, and it is frequently the better choice for a low warehouse at 5–8 m where the fixture count must be controlled. Because it mounts at lower height, the SHR is bounded by the spacing that the rack geometry imposes, and the wide optic makes that spacing achievable.
A third pattern appears where the space is long and narrow rather than broad. A continuous run of Z-15 tri-proof luminaires at 3–6 m mounting height is a line source rather than a point source, and its isolux map is elongated along the run. Uniformity along the run is excellent — the contour lines are nearly parallel to the luminaire axis — while uniformity across the run is governed entirely by the row-to-row spacing. For a single row of tri-proof luminaires lighting a 4 m wide aisle, the cross-aisle uniformity is set by the throw distance, not by the spacing along the row, so adding luminaires within the row improves illuminance without improving U0. This is the most common layout error in plant-room and service-corridor lighting, and the correction is a second row rather than a higher density in the first.
6. Common errors in uniformity design
| Error | Consequence | Correction |
|---|---|---|
| Specifying U0 without defining the evaluation area | Unverifiable tender, disputes at handover | Define wall and rack margin explicitly |
| Using U1 targets from sports standards on general floors | Fixture count inflated several times | Match metric to the application |
| Applying a uniform maintenance factor and expecting U0 to fall | Design thought to degrade uniformly; it does not | Track non-uniform contamination instead |
| Ignoring luminance of the vertical rack face | Floor uniformity met, rack face too dark to read | Add a vertical calculation plane |
| Using a coarse grid (1 m or more) | E_min missed entirely, U0 reported too high | Use 0.5 m grid, odd point count |
| Assuming rotational symmetry for a linear luminaire | Contour map wrong in one axis | Use the full photometric file |
| Placing the outer luminaire row at the wall | Perimeter too bright, U1 depressed | Offset the outer row by half the spacing |
The grid resolution error deserves a quantitative note. On a 0.5 m grid the darkest point in a striped pattern is captured. On a 2 m grid, the sampling may miss the mid-span trough completely and report a U0 that is forty percent higher than the truth. If a supplier’s simulation reports a uniformity figure substantially better than a comparable product’s, the first thing to check is the grid spacing — before checking the luminaire.
7. Verification after installation
The uniformity that matters is the one measured in service, and it is best verified with the same grid that the simulation used.
| Check | Method | Acceptance |
|---|---|---|
| Average illuminance | Calibrated meter, 0.5 m grid | Within 10 % of prediction |
| E_min and U0 | Same grid, minimum value | Within 0.05 of predicted U0 |
| Grid alignment | Compare survey map with CAD grid | Identical grid origin |
| Mounting height as built | Survey with laser distance meter | Within 0.2 m of design |
| Luminaire spacing as built | Tape measurement of first two bays | Within 0.1 m of design |
| Dirt condition at measurement | Visual inspection, photograph | Documented |
The order matters. Mounting height and spacing errors move the whole uniformity result; measurement errors move individual points. Diagnosing the wrong one wastes a site visit, and the cheapest diagnostic is always the as-built dimensional survey.
8. Conclusion
Uniformity is a designed property that follows from three decisions: the optic distribution, the spacing-to-height ratio, and the definition of the evaluation area. For an industrial installation, the working sequence is to fix the evaluation area and the uniformity metric with the client first, then choose the optic from the maximum permissible SHR, and only then size the flux per luminaire to meet the illuminance level.
For Z-16 high-bay installations at 10 m and above, a 110° optic with an SHR no greater than 1.70 will hold U0 above 0.40 with headroom for a normal industrial environment. For Z-09 UFO installations at 4–10 m, the wide 120° distribution supports an SHR up to about 2.0, which is usually what the rack geometry demands. For linear Z-15 tri-proof runs, uniformity must be designed across the run by choosing the number of rows, not along it by adding luminaires to a single row. In all three cases the contour map, not the average, is the deliverable that decides whether the design is fit for purpose.
9. Referenced standards
- EN 12464-1 — Light and lighting: lighting of work places, indoor
- EN 12464-2 — Light and lighting: lighting of work places, outdoor
- EN 12193 — Light and lighting: sports lighting
- GB 50034 — Standard for lighting design of buildings
- CIE 140 — Road lighting calculations
- IES RP-8 — Recommended practice for roadway and parking facility lighting
- CIE 154 — The maintenance of outdoor lighting systems
- IES LM-79 — Approved method: electrical and photometric measurements of solid-state lighting products
10. Contact us
QUEENDOM provides isolux simulations and uniformity reports for industrial and large-area installations, built from measured photometric files for the Z-16 high-bay and Z-09 UFO ranges. Supply the room dimensions, rack layout, mounting height and target illuminance with the required uniformity metric, and the engineering group will return a contour map, a uniformity summary and the luminaire schedule behind it.
Uniformity and Spacing-to-Height Curves for Isolux Design
Uniformity is set before the simulation is run, by the spacing-to-height ratio and the beam profile. The two curves below let a designer pick a target U0 and read back the maximum spacing-to-height ratio that will hold it, then confirm the array size that produces the required average illuminance.
Figure 1 – Uniformity U0 against spacing-to-height ratio for 90° and 120° optics. Representative photometric model.
A U0 target of 0.60 — a common warehouse requirement under EN 12464-1 — can be met at a spacing-to-height ratio up to roughly 1.15 with a 90 ° beam. Pushing past that ratio trades uniformity for fixture count, and the penalty accelerates beyond 1.4, where a ninety-degree optic can no longer overlap its neighbours at the work plane.
Figure 2 – Extra illuminance required to hold U0 at or above 0.60 as spacing widens. Representative model.
Read the pairing together: at SHR 1.15 a U0 of 0.60 is achievable with modest over-lighting, but at SHR 1.5 the same target demands an 18 % illuminance increase to retain uniformity. Matching the optic to the ratio is cheaper than over-driving the fixture — our high-bay range and industrial luminaires are specified with beam options that map onto these ratios.
Related products and applications
The luminaires used in the isolux and uniformity examples are listed below.
- High-bay luminaire (Z-16)
- Greenhouse top light (Z-09)
- Tri-proof luminaire (Z-15)
- Application overview: Lighting application solutions
- More technical papers: Lighting knowledge resources