Why carry out a photometric study before buying your luminaires

A photometric study is a simulation that checks, before purchase, that a lighting installation will actually reach the required levels: illuminance (lux), uniformity (U0), glare control (UGR) and compliance with standards. Often left to the end of the project, when the equipment has already been ordered, it is nevertheless the only reliable way to avoid an installation that lights badly, consumes too much or fails to meet the applicable requirements. Here is what it calculates, how to read it and who to entrust it to.

What is a photometric study?

A three-dimensional model of your space

Carried out in specialised software such as DIALux or Relux, the photometric study reconstructs the space to be lit in 3D: a pitch, a workshop, a street, an office floor. It includes the actual dimensions of the volume, the height available under the ceiling or under the beams, the nature and colour of the surfaces (reflectance of the floor, walls and ceiling), the obstacles that create shadow areas (racks, overhead cranes, machines, columns) and, where applicable, the contribution of daylight.

This level of detail is not cosmetic. A light-coloured wall returns a significant share of the flux to the working plane, a high, dark ceiling absorbs much of it. Two rooms of the same dimensions fitted with the same luminaires can show markedly different results for this reason alone.

Real photometric files, not estimates

The calculation is based on the photometric files of the luminaires being considered, in IES or LDT format. These files describe the precise distribution of the flux in space: the luminous intensity curve, the beam spread, the total flux in lumens. This is what distinguishes a study from a rule of thumb: two floodlights stated at the same power and the same flux produce very different results depending on their optics.

The calculation then produces a map of illuminance, generally presented in false colours and isolux curves, together with a series of numerical indicators.

What the study calculates precisely

Maintained average illuminance (lux)

This is the quantity of light received by the working surface or the floor, expressed in lux. The notion of "maintained" is essential: it takes into account the ageing of the sources and the soiling of the optics. The study therefore calculates the level guaranteed over the operating period, and not the performance on the first day. Each use has its target. An office workstation generally targets 500 lux under the EN 12464-1 standard, whereas a circulation area makes do with much lower values.

Uniformity (U0)

Uniformity, written U0, expresses how evenly the light is distributed: it is the ratio between minimum illuminance and average illuminance over the area considered. Good uniformity avoids the alternation of light and dark areas, a source of visual discomfort, fatigue and insecurity. On a sports ground as in a workshop, it is a compliance criterion in its own right, just like the lux level. An installation that reaches its average illuminance target while showing insufficient uniformity remains non-compliant.

UGR (discomfort glare)

UGR (Unified Glare Rating) quantifies the discomfort glare felt by the user. It depends on the luminance of the sources, the angle at which they are seen from the observation positions, and the adaptation luminance of the surroundings. The EN 12464-1 standard sets, for example, a UGR of 19 or less for most office workstations. Controlling UGR is above all a matter of choosing the optics, the orientation and the mounting height, as explained in our article on UGR and glare in sports lighting.

Maintenance factor

The maintenance factor reflects the foreseeable loss of performance between commissioning and the end of the useful life of the installation. It combines the depreciation of the LED flux (expressed by the L70 or L80 values), the soiling of the luminaire and that of the room surfaces. A serious study states this factor and justifies it according to the environment and the cleaning plan chosen. An optimistic factor applied to a dusty environment produces a flattering report and a disappointing installation.

Compliance with standards

Finally, the study compares these results with the requirements applicable to the project: EN 12464-1 for indoor workplaces, NF EN 12193 for sports facilities, the NF EN 13201 series for traffic routes, to which are added the rules of the sports federations (FFF, FFR, FFT, FFBB) according to the level of competition targeted and the regulations in force. It thus provides documented proof of compliance before any commitment.

| Indicator | What it measures | What an insufficient result causes |

| --- | --- | --- |

| Maintained illuminance (lux) | Quantity of light received on the working plane | Areas too dark, non-compliance, hindrance to the task |

| Uniformity (U0) | Evenness of distribution | Marked contrasts, visual fatigue, refusal of approval |

| UGR | Discomfort glare | Discomfort for operators, players, drivers |

| Maintenance factor | Loss of performance over time | Installation compliant at the start, non-compliant two years later |

| Installed power (W/m²) | Efficiency of the solution | Overconsumption, longer payback time |

Why it is essential before purchase

Avoiding undersizing

An undersized installation reaches neither the required illuminance nor the required uniformity: dark areas, non-compliance, discomfort at the workstation. Correcting it afterwards is expensive, because light points have to be added, which means redoing cable routes, bringing back a cherry picker and sometimes interrupting operations. The study detects the problem on the plan, before purchase.

Avoiding oversizing

Conversely, overestimating needs leads to installing too much power: extra investment cost, excessive consumption throughout the life of the installation and, outdoors, avoidable light pollution. The study makes it possible to adjust the number, power and layout of the luminaires as precisely as possible, making full use of their luminous efficacy in lumens per watt.

Comparing offers on an objective basis

Without a study, comparing two proposals amounts to comparing powers and prices. With a study carried out on the same assumptions (same dimensions, same reflectances, same maintenance factor), the comparison finally focuses on what counts: the result obtained on the working plane and the installed power needed to obtain it.

Securing the decision and the budget

By quantifying the result in advance, the photometric study turns an uncertain purchase into a well-founded decision. It also feeds funding files, in particular Energy Savings Certificates, which rely on quantified and documented gains. For sports facilities subject to approval, it is even a prerequisite: see our lighting recommendations for football to FFF standards.

What the study changes in each sector

Sports facilities

Illuminance level, uniformity and glare limitation determine approval and video capture. The study also checks that there is no nuisance for residents and that the column installation angles are respected. It relies on floodlights with calculated optics, such as those in our sports and leisure range.

Industry and high ceilings

In a workshop, a warehouse or under a high ceiling, the challenge is twofold: reaching the required level on the floor while managing the shadows cast by racks and machines. The beam spread becomes decisive, and the study decides between a tight grid of wide-beam luminaires and a looser grid of concentrated floodlights. Our solutions dedicated to high ceilings cover these configurations.

Commercial buildings

On an office floor, UGR and colour rendering quality weigh as much as the lux level. The study validates the layout of office and open space luminaires and makes it possible to anticipate daylight-linked dimming scenes, in connection with the control systems of the installation.

Public lighting

On roads, the study covers road surface luminance, longitudinal uniformity and the limitation of intrusive light. It is at the heart of any public lighting renovation in a municipality and of the choice of luminaires for traffic routes.

What to gather before launching the study

• A dimensioned plan of the area, or failing that its precise dimensions and the available height.
• The exact function of the spaces and the tasks carried out there, workstation by workstation if necessary.
• The nature and colour of the surfaces (floor, walls, ceiling, roof structure), to estimate reflections.
• Fixing constraints, obstacles and areas inaccessible for maintenance.
• The target reference framework: applicable standard, federation rules, project owner's specifications.
• Operating conditions: dusty, humid or corrosive atmosphere, planned cleaning frequency.

Reading a photometric report: points to watch

A report is judged first on its assumptions. Check that the reflectances correspond to your room, that the maintenance factor is explicit and consistent with the environment, and that the working plane is defined at the right height. Finally, check that the calculation area covers the whole surface in use.

A photometric report is only reliable if the installation reproduces exactly the calculated layout: heights, spacings, orientations and tilt angles.

Who to entrust your photometric study to

Get in touch with a lighting design office such as Luminance-Consulting, a lighting solutions consultancy since 2013. A study carried out before the choice of equipment brings three things that a simple replacement calculation does not guarantee: command of the regulatory framework applicable to your project, a prior audit of the existing installation, and a report that can be relied upon and presented to your project owner, your insurer or the federation concerned.

Frequently asked questions

Which software is used for a photometric study?

The most widespread are DIALux and Relux. They model the space in 3D and calculate illuminance (lux), uniformity (U0) and UGR from the real photometric files (IES or LDT) of the luminaires selected.

Is a photometric study mandatory?

It is not systematically mandatory in the legal sense, but it becomes essential as soon as compliance with a standard or federation rules has to be proved, or a funding file justified. Some project owners and some approval procedures explicitly require it, according to the regulations in force.

What needs to be provided to launch a study?

The dimensions and plan of the area, its function, the available heights, the nature of the surfaces, the fixing constraints and the level of performance targeted. From these elements, the study proposes a layout and checks that it reaches the objectives set.

Does the study guarantee the actual result on site?

It calculates the expected result taking the maintenance factor into account, and therefore ageing and soiling. The result obtained on site then depends on installation in accordance with the study. That is why respecting the calculated layout, heights and orientations included, is decisive.

Get in touch with our technical partner, the Luminance-Consulting agency

Get in touch with our technical partner, the Luminance-Consulting agency, a lighting solutions consultancy since 2013. It carries out the audit of your installation, the photometric study in DIALux and the compliance check, based on the IES files of the luminaires being considered. For any question about the floodlights and solutions in the Rexylum catalogue, contact us · 1 Route de Bartres, 65100 Adé.