Most rooms in Pakistan are lit by counting: so many tube lights per bay, so many panels per ceiling tile, whatever the last job had. The result is offices at 250 lx where people squint at drawings, and showrooms at 1 200 lx that pay for it every month. Lighting design is not complicated. A standard says how much light the task needs, a datasheet says how much light the luminaire makes, and a five-line method connects the two through the shape of the room. This guide walks through that method and the judgement that goes with it.
Lux, lumens and the working plane
A luminaire emits luminous flux, measured in lumens. What matters to the person working is the illuminance on the task, measured in lux: one lux is one lumen falling on one square metre. The same panel gives a high illuminance close to it and a low one far away, and spreads its lumens over more floor in a big room than a small one, which is why standards are written in lux and datasheets in lumens, and why you cannot pick a luminaire without knowing the room. For scale, full sun outdoors is around 100 000 lx, an overcast sky gives a few thousand, and a well-lit office is 500.
Illuminance is specified on the working plane, the surface where the task is done: desk height, 0.7 to 0.85 m, in an office; the bench in a workshop; the floor in a corridor, a warehouse aisle or a car park. The distance that matters for the calculation is from the luminaire down to that plane, not to the floor, and getting it wrong by 0.8 m in a 3 m room changes the answer by a good margin.
How much is enough: the standard's numbers
EN 12464-1, the European standard for indoor workplaces, tabulates the maintained illuminance for several hundred tasks, and Pakistani consultants and tenders generally cite it in the absence of a national code. A few of its values: 500 lx for writing, typing, reading and data processing in offices; 750 lx for technical drawing; 300 lx for classrooms, reception desks and retail sales areas; 500 lx for laboratories and kitchens; 200, 300, 500 and 750 lx for rough, medium, fine and precision industrial assembly; 100 lx for corridors and stairs; 100 lx for unmanned storage and 200 lx if the warehouse is continuously manned; 75 lx for the parking areas of a car park. The lighting calculator has a longer table.
Three things about these numbers. First, they are maintained values: the level that must still be there at the end of the maintenance cycle, not on the day of handover. Second, they apply to the task area, and the standard allows the immediate surroundings to be lower, 300 lx around a 500 lx task and 200 lx around a 300 lx one, so a whole open-plan floor need not be lit to desk level. Third, illuminance is only one of four requirements the standard sets for every space; glare (UGR), uniformity and colour rendering come with it, and a room that meets the lux figure with a few glaring high-output fittings has not met the standard.
The lumen method in five lines
The lumen method computes the average illuminance on the working plane from the luminaire flux and two factors that account for the room and for ageing:
- Lumens needed on the plane = target lux × floor area.
- Useful lumens per luminaire = luminaire lumens × utilisation factor × maintenance factor.
- Number of luminaires = (1) ÷ (2), rounded up.
- Arrange them in a grid whose spacings are close to each other and within the luminaire's spacing-to-height ratio.
- Recompute the illuminance with the grid count, and check it is not far above the target.
It is an average, and it assumes an empty rectangular room with luminaires spread evenly. That covers the great majority of offices, classrooms, shops and factory bays. What it does not do is tell you how even the light is or whether the fittings will glare, which is where the software and the photometric file come in.
Room index and utilisation factor: where the light goes
Not all the lumens leaving a luminaire reach the working plane. Some go up or sideways into the ceiling and walls, and only part of that is reflected back down. The utilisation factor, UF, is the fraction that arrives, and it depends on the luminaire's light distribution, on the reflectances of the ceiling, walls and floor, and on the shape of the room, captured by the room index:
where Hm is the height of the luminaire above the working plane. A large, low room has a high index, most of the light goes straight down, and the UF might be 0.7 or more. A small, tall room has a low index, the walls intercept more of the light, and the UF can fall to 0.3 or 0.4, lower still if the walls are dark. Luminaire makers publish a UF table for each product against room index and a set of reflectance combinations, typically 0.7/0.5/0.2 for a white ceiling, mid-tone walls and a dark floor; read the value from the row nearest your index and the column nearest your surfaces. A datasheet that offers no UF table, only a lumen figure, is not enough to design with, and a UF taken from a different luminaire is a guess.
Maintenance factor: designing for year three, not day one
An installation is at its brightest the day it is commissioned. LEDs lose output slowly with hours run; a datasheet rating of L80 at 50 000 hours means 80 % of the initial flux is still there at that point. Dust settles on diffusers and in the optics, faster in a workshop than in an air-conditioned office; walls and ceilings darken between repaints. The maintenance factor, MF, is the product of these losses over the cleaning and replacement cycle, and CIE 97 breaks it into a lamp lumen maintenance factor, a lamp survival factor, a luminaire maintenance factor and a room surface maintenance factor. For a clean office with sealed LED panels cleaned every couple of years, 0.8 is a reasonable figure; for a dusty machine shop, 0.6 to 0.7; for a foundry, lower.
The consequence runs both ways. Designing to the target with MF = 0.8 means the room starts 25 % brighter than it needs to be and settles to the target over the cycle. Some drivers offer constant light output, running the LEDs below full power when new and raising it as they age, which recovers that energy; where they are not used, a dimmable driver trimmed at commissioning does something similar until the first maintenance.
Spacing, uniformity and glare
The count from the lumen method has to be laid out, and a room is a rectangle, so 25 luminaires become 4 rows of 7 or 5 rows of 5. Aim for spacings that are close to equal in the two directions, and check each against the mounting height: the spacing-to-height ratio, SHR, should stay within the luminaire's published maximum, commonly about 1.5 for a diffuse fitting. Beyond that the light dips noticeably between luminaires. The first row goes about half a spacing from the wall so the edge of the room is not left in shadow. EN 12464-1 expresses evenness as the uniformity, the minimum illuminance divided by the average, and asks for 0.60 on most office tasks; only a point-by-point calculation can confirm it.
Glare is the other thing the average cannot see. The Unified Glare Rating, UGR, scores discomfort glare from the luminaires as seen from the working positions on a scale from about 10 to 30, and the standard caps it at 19 for offices and 16 for drawing work. It depends on how bright the luminaire surface is, how big it appears and where it sits in the field of view, so a bare high-output panel scores worse than a microprismatic one of the same lumens. The standard also limits luminaire luminance at high angles where it would reflect in screens. All of this comes from the photometric file in DIALux or Relux; the lumen method gets you to the right number of the right luminaire, and the software confirms the layout.
The example in numbers
A 12 × 8 m open-plan office with a 3.0 m ceiling and desks at 0.8 m, to be lit to 500 lx with recessed 600 × 600 LED panels of 4 000 lm and 36 W; UF 0.60 from the panel's table, MF 0.8:
- Area 96 m². Hm = 3.0 − 0.8 = 2.2 m. Room index 96 ÷ (2.2 × 20) = 2.18.
- Lumens needed: 500 × 96 = 48 000 lm. Useful lumens per panel: 4 000 × 0.6 × 0.8 = 1 920 lm. N = 25.
- Grid: 4 rows of 7, 28 panels, giving 560 lx maintained, 12 % over the target.
- Spacing 1.71 m along the length and 2.00 m across; SHR = 2.00 ÷ 2.2 = 0.91, well within 1.5.
- Load 28 × 36 = 1 008 W; power density 10.5 W/m²; efficacy 111 lm/W.
The 12 % surplus is the price of a regular grid. A 3 600 lm version of the same panel in the same 28 positions gives 504 lx, or the 4 000 lm panels can be dimmed to about 90 % at commissioning. Either is better than removing a row, which would leave 21 panels at 420 lx, below the standard.
Power density: what good lighting costs to run
The lighting power density, installed watts per square metre of floor, is the number that connects the design to the bill. The office above comes to 10.5 W/m² for 500 lx, which is typical of an LED installation; the T8 fluorescent generation that it replaces typically needed 20 to 25 W/m² for the same illuminance, and the tube-and-batten installations still common in Pakistani offices often more, because their diffusers and reflectors were never efficient and are now dirty. At 2 500 hours a year, ten hours on 250 working days, the 1 008 W office uses about 2 500 kWh a year; the fluorescent equivalent would use twice that, and occupancy and daylight sensing take a further share off whichever is installed.
On the electrical side, 1 kW of LED lighting is a small load, about 4.6 A on a single-phase circuit at a power factor of 0.95, but LED drivers draw a brief inrush at switch-on that can trip a B-curve breaker feeding many luminaires, so check the driver's inrush figure and the number the maker allows per breaker. The power and current calculator converts the load to current for each circuit, and lighting circuits, being long and lightly loaded, are one of the few places where voltage drop rarely decides the cable size. Where a building has many rooms, a lighting schedule with lux target, luminaire, count and W/m² for each is the document that lets the electrical design, the tender and the energy estimate all be checked against the same numbers; Ahmedonics prepares these as part of custom engineering work.
References
- EN 12464-1:2021, Light and lighting — Lighting of work places — Part 1: Indoor work places — maintained illuminance, UGR, uniformity and colour rendering by task; surrounding areas
- SLL Code for Lighting, Society of Light and Lighting (CIBSE), 2022 — lumen method, room index, utilisation factors, spacing-to-height ratio
- CIE 97:2005, Guide on the maintenance of indoor electric lighting systems — the maintenance factor and its four components
- IES Lighting Handbook, 10th edition, Illuminating Engineering Society, 2011