Lighting calculation and visualisation: avoiding design errors before construction

Key facts in brief: A lighting calculation proves before construction that illuminance, uniformity and glare meet the requirements of the standard – over the entire service life, because the maintenance factor is included. The visualisation shows clients how the room will look. Both belong in the early HOAI work stages, when changes still cost little.

What a lighting calculation delivers

Programs such as DIALux evo or Relux calculate a 3D model of the room using real luminaire data (manufacturers' luminous intensity distributions) and surface reflectances. The result is reliable values instead of estimates:

Maintained average illuminance Ēm per assessment area
Uniformity U₀
Glare rating (UGR) and illumination of walls and ceiling, as required by EN 12464-1 ^[1]
Number of luminaires, connected load and energy demand

The maintenance factor – the most common source of error

Standards specify maintained values: the average illuminance must never fall below the required level, regardless of the age and soiling of the installation. A higher initial value is therefore installed. The rule is: maintained value = maintenance factor × initial value ^[2]. If the maintenance factor is set too optimistically, the installation falls below the standard after a few years; if it is set too pessimistically, energy and investment are wasted.

The designer must state the maintenance factor, list all the assumptions behind it and draw up a maintenance schedule with intervals for cleaning and replacement ^[5]. Typical assumptions are 0.67 for clean rooms with a three-year maintenance interval and up to 0.8 for LED luminaires with annual cleaning ^[5].

Typical errors a calculation reveals

Luminaire grid does not match the furniture layout, workstations lie in shadow
Glare at screen workstations caused by unsuitable luminaires
Walls and ceilings too dark – rooms feel gloomy despite sufficient desk illuminance
Underestimated mounting heights in halls and atria
Missing control and daylight zones

Relation to the HOAI

The German HOAI fee scale generally divides services into nine work stages, from establishing the basis (LP1) to project support (LP9) ^[3]. Basic services are those generally required for the proper fulfilment of a commission ^[4].

In the German fee scale HOAI, lighting installations belong to the service profile Technical Equipment, system group 4 high-voltage installations ^[6]. The object list assigns low-voltage installations including lighting installations, as well as outdoor lighting installations, to fee zone II, and lighting installations with special design requirements – for example lighting simulations using elaborate methods for museums or special rooms – to fee zone III ^[7].

Work stage	Contribution of lighting design
LP1–2 Basis, preliminary design	Requirements, lighting concept, first rough calculation
LP3 Design	Complete lighting calculation, visualisation, cost calculation
LP5 Detailed design	Luminaire layouts, details, controls
LP6–7 Tendering	Manufacturer-neutral specification, checking alternative offers by recalculation
LP8 Site supervision	Acceptance, verification measurement if required

Visualisation as a decision aid

False-colour images show where light is missing; photorealistic renderings show clients and users the effect. Design decisions are made before anything is ordered and installed.

What a verifiable lighting calculation includes

Room geometry, furnishings and surface reflectances
Evaluation areas with position, height and grid size
Luminaire data with luminous intensity distribution (EULUMDAT/LDT or IES) and power ratings
Maintenance factor with all assumptions and the associated maintenance schedule
Results: Ēm, U₀, UGR, cylindrical illuminance, wall and ceiling values, energy demand

Frequently asked questions

Can I do the calculation myself with free software?

The programs are freely available, but the results depend on correct luminaire data, reflectances, maintenance factors and assessment areas. Proof of compliance requires expertise.

Do you also check alternative offers?

Yes. We recalculate proposed substitute luminaires so that "equivalent" is also equivalent in photometric terms.

What documents do you need for a calculation?

Floor plans and sections (DWG, DXF or PDF), details of the use and – if already chosen – the photometric data of the luminaires in EULUMDAT (LDT) or IES format.

Sources (in German)

Author: Paul Rostand Ndjeundoun, M.Sc. (Hons), Certified Lighting Expert (LiTG), Registered European Lighting Expert · As of November 2026. Standard values have been carefully researched but do not replace consulting the currently valid edition of the standard.