A supply quote arrives with a list of luminaires, a total wattage and a delivery time. Nothing in the document states the illuminance achieved at the workstations, or whether uniformity will hold at the back of the room. The photometric study is the document that answers these two questions before the purchase.
What a photometric study really calculates
A photometric study simulates the propagation of light in a given volume. The calculation software distributes the flux emitted by each luminaire, takes into account reflections on the walls and obstacles, then gives the illuminance achieved on the defined calculation surfaces.
The result takes the form of a map of values, point by point, on which the average illuminance, the minimum, the maximum and the uniformity can be read. A simple ratio of watts per square metre says nothing about this distribution. The same calculation provides the UGR glare rating at the observer positions chosen.
The input data: plans, heights, reflectances, dust
The quality of a study depends first on its assumptions. Four families of data determine the result: the geometry of the room, the reflectances of the surfaces, the photometric files of the luminaires and the maintenance factor.
Geometry covers the dimensions, the ceiling height, the height below obstacles and the presence of racks, overhead cranes, ducts or partitions. A warehouse modelled without its racks gives a flattering result that will never be confirmed on site.
Reflectances express the ability of the surfaces to reflect light. The conventional values commonly used in offices are around 0.7 for the ceiling, 0.5 for the walls and 0.2 for the floor. In a workshop, with dark walls and a steel roof frame, these values drop and the difference in the final result becomes significant.
The maintenance factor anticipates the loss of performance over time: lumen depreciation of the sources, survival rate, dirt on luminaires and surfaces, cleaning interval. It is built from the real environment of the room and the planned maintenance schedule, not from a default value left in the software.
The documents delivered: tables, isolux, false colours, layout
Report document | What it makes it possible to check |
|---|---|
Table of results | Average, minimum and maximum illuminance, uniformity, UGR, comparison with the required value |
Isolux curves | Distribution of illuminance and location of dark areas |
False colour rendering | Immediate reading of level differences over the whole surface |
Dimensioned layout plan | Exact position of the luminaires, spacing, orientation of the optics |
Equipment list and power balance | Chosen reference, quantities, installed power and power density |
A report reduced to the false colour view alone remains unusable. The image is always read with its scale and with the table of values that goes with it.
The decisions it settles before purchase
The study settles trade-offs that are expensive once the luminaires are installed. First, the number of light points and their spacing: one fitting fewer per bay may be enough, or on the contrary take uniformity below the accepted threshold.
It then decides between optics. For the same flux, a narrow beam and a wide beam do not give the same map on the floor, and the right choice depends on the mounting height and the geometry of the room. Finally, it validates the relevance of supplementary lighting at the workstation, rather than raising the general level over the whole area.
Lastly, it acts as arbiter between two competing references. Two luminaires stated at the same flux and the same power can give very different maps, because their intensity distribution differs. The only way to compare them honestly is to run both in the same model of the room, with the same assumptions, and to read the two tables of results side by side.
Design study and verification study
Two uses coexist. The design study starts from a room and a requirement, then builds a layout that satisfies them. It comes upstream, before any call for tenders.
The verification study starts from an offer already received and checks that it reaches the stated values with the equipment proposed. It serves the project owner who is comparing two proposals, or who doubts a layout copied like for like from the existing one. The two services do not require the same amount of work, as our page on the price of a photometric study explains.
The link with the standards that apply to the room
A study is only worth something against a requirement. The calculation is compared with the applicable line: EN 12464-1 for indoor workplaces, EN 12464-2 for outdoor workplaces, EN 12193 for sports facilities, the EN 13201 series for roads.
This is the role of the note of assumptions placed at the top of the report. It states the visual task chosen, the target maintained illuminance, and the expected uniformity and UGR. Without it, an isolated illuminance value means nothing, since no one knows which requirement it was to be compared with. The benchmarks for each type of room are gathered on our page dedicated to the compliance of lighting installations.
What to provide to launch a study
- Dimensioned plans of the room, in an editable format if possible, with the heights below the ceiling and below obstacles.
- The precise use of each zone, expressed as a visual task and not as a room name.
- The nature and colour of the walls, floor and roof frame.
- The environmental constraints: dust, humidity, washing, temperature, hazardous area.
- The operating hours and whether a control or management system is in place.
- Photos of the existing installation, often more telling than a long written description.
These elements are surveyed during the site visit when they do not exist. At Luminance Consulting, this visit, the measurements, the DIALux evo simulation and the costed report are carried out free of charge as part of a project: the full process is on our photometric study page.
The photometric files of the ranges of industrial luminaires and tertiary luminaires feed directly into the calculation, which avoids simulating with one product and installing another.
To launch a study on your premises, send your plans from the contact page.
