How Is a Lighting Design Prepared? Step-by-Step Calculation and Layout

Good lighting does not start by lining up a model you like from a catalogue at equal intervals on the ceiling; it starts with a calculation. Luminaires of the wrong wattage or the wrong beam angle in a space create either dark corners and eye strain or unnecessary energy consumption and glare. A lighting design is the way to find the right point between these two extremes.

1. Needs analysis: what will the space be used for?

Every project starts with the same question: what will be done in this area? Reading and screen work at an office desk, showing a product's colour accurately in a shop and reading shelf labels in a warehouse all need different light. At this stage the following information is collected:

• The use of each area: work, sales, storage, circulation, worship, sport.
• Geometry: architectural plan, ceiling height, ceiling type (suspended, concrete, timber), columns and beams.
• Surfaces: wall, ceiling and floor colours; light-coloured surfaces reflect light and reduce the number of luminaires needed.
• Expectations: colour temperature, dimming, sensors, daylight-linked control, budget and energy targets.

2. Setting the target values

For indoor work areas the targets are taken from the EN 12464-1 standard. Four values are set for each area: average illuminance (Ēm, lux), uniformity (Uo, the ratio of the minimum illuminance to the average), the glare limit (UGR) and colour rendering (Ra / CRI).

The values in the table are typical targets from EN 12464-1; in a project, the table in the current edition of the standard and the specific conditions of the space should be used.

3. Preliminary sizing: the lumen method

The lumen method is used to estimate quickly how many luminaires are needed:

Number of luminaires = (E × A) ÷ (Φ × UF × MF)

• E: target average illuminance (lux)
• A: area (m²)
• Φ: luminous flux of one luminaire (lumens, the system value in the datasheet)
• UF (utilisation factor): the proportion of the light leaving the luminaire that reaches the working plane; depends on the room proportions, surface reflectances and the luminaire's light distribution
• MF (maintenance factor): an allowance for dirt and light loss; usually about 0.8 in clean interiors

Example: 500 lux is targeted in an office of 10 m × 8 m = 80 m². With 4,000-lumen linear luminaires, UF = 0.6 and MF = 0.8: (500 × 80) ÷ (4,000 × 0.6 × 0.8) = 40,000 ÷ 1,920 ≈ 21 luminaires. This is a starting value; layout and uniformity are checked with software.

4. Selecting luminaires

The result of the calculation is not just a quantity. The luminaire's light distribution (wide, narrow, asymmetric), glare rating, colour temperature, CRI, IP rating and control compatibility must match the space. Low-glare linear luminaires with diffusers stand out in offices with screens, high-CRI track spotlights in shops and IP65+ industrial luminaires in dusty warehouses.

5. Verification with software and the layout plan

The photometric file of the chosen luminaire is loaded into lighting calculation software and placed on the architectural plan. The software calculates the illuminance at every point; if uniformity is low the spacing is reduced, and if there is glare the luminaire type is changed. The result is a calculation report for each area and a layout plan showing the positions of the luminaires.

6. Control and energy

The same illuminance can also be achieved with less energy: sensor dimming in areas with daylight, presence sensors in unused areas and scene settings in meeting rooms. These decisions are made in the design because the driver type (DALI, 1-10V, Triac) and the wiring must be chosen accordingly from the start.

Common mistakes

• Choosing luminaires by wattage without looking at lumens and light distribution.
• Mistaking the chip lumens in the datasheet for system lumens.
• Ignoring the maintenance factor; after a few years illuminance falls below the target.
• Not checking glare (UGR) in areas with screen work.
• Trying to add the control system later; incompatible drivers then have to be replaced.

Conclusion

A lighting design is the plan for bringing the right light to the right place with the least energy. The target values come from the standard, the number of luminaires from the calculation and the layout from the software; the part left to intuition is kept to a minimum. If you would like us to prepare a calculation and layout plan for your project, take a look at our lighting design and consultancy page.