A calculation report arrives attached to an offer: a few pages, a very even false colour rendering, an average announced above the requirement of the standard. Six months after commissioning, a lux meter survey in the same room gives noticeably lower values, and nobody can say where the gap comes from.
In almost every case, the gap does not come from the calculation engine. It comes from three or four assumptions entered at the modelling stage.
What the software calculates and what it ignores
DIALux evo solves a problem of light propagation between surfaces considered as diffusing, based on the luminous intensity curves of the luminaires and the geometry of the room. It gives illuminances, luminances, uniformities and glare ratings calculated under the conditions described by the model.
On the other hand, it has no way of knowing whether the real room resembles the model. It ignores the dust settled on the diffusers, the pallet stored in an unexpected place, the paint redone in dark grey, the steel structure added afterwards and the luminaire finally delivered with a different optic from the one tested.
This distinction is the key to reading a report. The calculation is exact with regard to its assumptions, and it is the quality of the assumptions that decides the value of the document.
Modelling the room: geometry, obstacles, furniture
Entering the geometry goes beyond the plan dimensions. The usable ceiling height and the presence of beams, ducts, sprinklers and overhead cranes change the mounting height really available and create shading.
In a warehouse, the racks are the main obstacle and cannot be ignored. A simulation carried out on an empty volume gives a flattering uniformity that disappears as soon as the aisles are lined with full racking.
The same reasoning applies in offices with high partitions, cabinets and screens. Modelling these elements takes time, and it is this time that makes the difference between a sales image and a usable study.
Reflectances of the room surfaces
Surfaces reflect part of the light they receive, and this share determines the result, especially in small rooms or with indirect lighting. The EN 12464-1 standard gives recommended reflectance ranges for the main surfaces of a workplace.
Surface | Recommended reflectance range |
|---|---|
Ceiling | 0.60 to 0.90 |
Walls | 0.30 to 0.80 |
Floor | 0.10 to 0.50 |
Work surfaces | 0.20 to 0.60 |
The default values offered by software correspond to a light, clean room. An industrial hall with a dark steel roof frame, a heavily marked concrete floor and dusty cladding sits at the bottom of the ranges, and keeping the default values is enough to overestimate the final result.
The right reflex is to record the real colours during the visit and document them in the report. A reflectance assumption written down in black and white can be discussed; an implicit assumption cannot.
Importing the photometric files of the luminaires
The luminaire enters the calculation in the form of a photometric file, in IES or LDT format, which describes the distribution of its luminous intensity in space. Consistency between this file, the data sheet and the product actually ordered determines everything else.
Three checks are enough to rule out most errors: the luminaire output flux stated in the file matches that of the data sheet, the absorbed power is that of the chosen source and driver combination, and the optic modelled is the one that will be delivered. A product available in several beam angles has as many files as optics, and swapping two of them completely changes the uniformity.
Defining the task areas and calculation surfaces
The EN 12464-1 standard reasons by task area, surrounded by an immediate surrounding area about 0.5 metre wide, itself bordered by a background. Each has its own illuminance requirement and its own uniformity requirement.
Positioning these zones in the model is a design decision, not a cosmetic setting. A calculation surface placed in the middle of the room, far from the walls, mechanically produces a better uniformity than the reality perceived at the workstation at the edge of the bay.
The height of the calculation plane is chosen according to the task: work surface height for a seated or standing workstation, floor level for a circulation area. A calculation plane placed too high or too low makes the report impossible to compare with the measurements taken at acceptance.
The maintenance factor, the assumption most often botched
The standard requires a maintained illuminance, that is, the value reached at the end of the maintenance period, not the value obtained on the day of commissioning. The maintenance factor links the two.
It combines several effects: the drop in source flux over time, luminaire failures, dirt on the luminaires and dirt on the room surfaces. A cutting workshop, a joinery and an air-conditioned office clearly do not have the same dust levels, and therefore not the same factor.
Using a default value without linking it to a real cleaning interval amounts to borrowing from the future. The report must state the factor used, the associated maintenance interval and the environment considered, otherwise the announced compliance cannot be checked.
Checking a report before approving an order
A report is reread starting with the assumptions, even before looking at the images. Six points are enough to form an opinion.
- The dimensions and mounting height match the room surveyed on site.
- The reflectances are consistent with the real colours.
- The maintenance factor is stated and justified by an environment and an interval.
- The photometric files match the references and optics of the costed proposal.
- The task areas and calculation planes are placed where the work is done.
- Significant obstacles, racks, partitions, machines, appear in the model.
These checks are part of the ordinary work of a photometric study carried out before the works, and they decide whether the compliance requirements that apply to the room are met. The amount of data entry also explains what weighs in the price of a photometric study: the modelling, not the click on the calculate button.
The photometric files of the industrial luminaires and high bays ranges are available to be imported directly into the model.
To have a calculation report received from a supplier checked, or to have your room simulated, send your plans to our design office.
