Verifying a High-Bay LED Lighting Layout for a Packaging Hall
A pharmaceutical secondary-packaging hall in Borg El Arab is 36 m × 20 m with a clear ceiling height of 8 m and a working plane at 0.85 m. The client's brief requires 500 lx maintained on the working plane, UGR ≤ 22 and Ra ≥ 90 in the print-inspection bay, where operators compare coloured cartons against a reference sample. The lighting contractor proposes 24 LED high-bay luminaires, each 150 W and 18 000 lm with a 120° beam, arranged on a 6 m × 4.5 m grid at 7 m above finished floor. He quotes a utilisation factor of 0.62 and a maintenance factor of 0.80 and argues that 'the fittings are 120 lm/W, so the scheme must be efficient'. The electrical consultant must approve the layout before the order is released, and the client also wants a lux measurement protocol written into the acceptance criteria.
1. Check the proposed layout against the 500 lx requirement. Show the lumen method calculation and the number of luminaires actually needed.
Read the worked answer
Lumen method: E_avg = (N × Φ × UF × MF)/A, with A = 36 × 20 = 720 m².
For the proposed 24 fittings: E_avg = (24 × 18 000 × 0.62 × 0.80)/720 = 214 272/720 = 298 lx.
That is only about 60 % of the specified maintained illuminance. The scheme is under-designed by a wide margin, and no amount of efficacy (120 lm/W) can compensate: efficacy governs the energy bill, not the illuminance, and the lumens delivered to the working plane are what the brief specifies.
Required number: N = (E × A)/(Φ × UF × MF) = (500 × 720)/(18 000 × 0.62 × 0.80) = 360 000/8928 = 40.3 → 41 minimum, adopt 42 for a workable grid (7 × 6).
Practical points to put to the contractor:
- The utilisation factor of 0.62 should be challenged. At a mounting height of 7 m, a 36 × 20 room has a room index of roughly 2.3, and a 120° high-bay with a decent reflector should reach 0.65–0.70; re-running the calculation from a photometric file may change the count by two or three fittings, which is worth doing before ordering.
- The maintenance factor of 0.80 assumes a clean, filtered space and a planned cleaning cycle; in a packaging hall with paperboard dust, 0.70 is more defensible, which would push the count to about 47.
- Confirm the 18 000 lm is the luminaire output at the declared ambient, not the unverified LED source lumens.
2. Assess uniformity, glare and colour rendering for the proposed arrangement, and recommend what should change.
Read the worked answer
Uniformity. Mounting height above the working plane Hm = 7 − 0.85 = 6.15 m. Spacing/height is 6.0/6.15 = 0.98 longitudinally and 4.5/6.15 = 0.73 transversely - inside the spacing-to-height ratio of a typical 120° high-bay, so the grid is not outrageous. But uniformity U0 = E_min/E_avg is what matters, and with only 24 fittings producing 298 lx average, the minimum will fall well below the 0.40 floor that EN 12464-1 sets for general tasks (0.60–0.70 where the task demands it). With 42 fittings the grid becomes 5.14 m × 3.33 m, which raises E_min and U0 together. Ask for the calculated U0, not an assertion.
Glare. UGR ≤ 22 at a 7 m mounting height with a 120° unshielded beam is difficult; a broad beam at 500 lx will put significant luminous intensity into the 65–85° zone. Specify the luminaire's actual UGR at the design layout, verified in a DIALux or Relux file using the manufacturer's photometric data, and prefer a fitting with a controlled optic or a diffuser rather than a bare reflector.
Colour. Ra ≥ 80 is standard for industrial tasks, but the print-inspection bay demands Ra ≥ 90, ideally with R9 (red) above 50 - colour comparison against a reference carton fails on an 80-CRI source even at 500 lx. Give the inspection bench its own dimmable task luminaire at Ra ≥ 90 over a neutral grey background, plus vertical illuminance on the sample, and do not rely on the general high-bay for that task.
Recommendation: reject the 24-fitting layout; require a full photometric calculation at 42–47 fittings showing E_avg, E_min, U0 and UGR at every grid point, plus the separate inspection-bay solution.
3. Write the illuminance measurement protocol the client should put into the acceptance criteria, and identify the pitfalls that would make a measurement misleading.
Read the worked answer
Protocol:
- Condition of the installation: measure after at least 100 h of burn-in so the luminous flux has stabilised, with all luminaires operating, the specified supply voltage, and the room clean and in its normal state. Record the lamp and driver serials actually installed.
- Grid: divide the working plane into a grid whose spacing does not exceed about 0.2 × the mounting height above the working plane (here Hm = 6.15 m → spacing ≈ 1.2 m or finer), with measurement points at the centre of each cell. Add specific points at the inspection bench and at the worst-lit corners.
- Instrument: a calibrated illuminance meter (calibration certificate current, cosine- and colour-corrected), held in the plane of the working surface, shielded from the operator's body, with a note of the meter's orientation and any daylight contribution.
- Conditions: measure at night or with the roof lights fully blacked out, otherwise daylight inflates the result and the acceptance figure becomes meaningless.
- Reporting: report E_avg, E_min, U0 = E_min/E_avg and the UGR verification, and compare against the design values. Agree in the specification whether acceptance is against initial or maintained values; if the contract is written on maintained values, the as-measured figures should exceed 500 lx by at least the design maintenance factor, and the certificate should say which basis was used.
Pitfalls that would make the reading misleading: measuring with new, un-burned-in lamps; leaving daylight in the room; measuring on the floor rather than the 0.85 m working plane; using an uncalibrated or phone-based meter; taking a single central reading rather than a grid; and accepting an average without a minimum, which hides dark pockets under the racks.