In a wiring workshop, an operator reports that the blue and purple conductors have been getting mixed up since the luminaires were replaced. Yet the illuminance levels are compliant, and the measurements confirm it. The parameter that moved is colour rendering, traded off a few months earlier to save a few euros per luminaire.
What the colour rendering index measures
The colour rendering index, written CRI or Ra, compares the way a source renders a series of standardised colour samples with a reference source of the same colour temperature. The result is expressed on a scale whose top value corresponds to rendering identical to that of the reference.
The general index Ra is an average calculated over eight pastel samples. This average masks the behaviour of the source on saturated colours, which are absent from the calculation. A source can therefore show a high Ra and render a bright red or a saturated green poorly.
Colour temperature and colour rendering index are two independent parameters. A neutral white luminaire is no better placed than a warm white to render colours: everything depends on the spectral composition of the source.
The limits of the general index and the value of R9
Each sample gives a particular index, written R1 to R15. R9 corresponds to saturated red, and it is the one most often lacking in low-cost white sources. A low R9 results in dull reds, grey skin tones and meat that looks lacklustre on the shelf.
Asking for R9 at the same time as Ra has become a useful reflex in sectors where colour carries information. The metrics from the TM-30 system, which distinguish fidelity and saturation of colours, provide a complementary reading when the manufacturer publishes them.
The activities where a high index becomes essential
The EN 12464-1 standard associates a minimum colour rendering with each task. An Ra of at least 80 is expected in rooms occupied by people for long periods. Colour inspection tasks fall under higher requirements, with an Ra of at least 90 combined with high illuminance.
Situation | Role of colour | Rendering requirement |
|---|---|---|
Circulation, storage without sorting | No information carried by colour | Secondary criterion |
Office, meeting room, ordinary workshop | Visual comfort and perception of people | Ra of at least 80 under EN 12464-1 |
Colour inspection, rework, fine sorting | Colour is the working data | Ra of at least 90 and R9 checked |
This reading is done room by room, starting from the task actually carried out. The details of the applicable requirements are on the lighting compliance page.
Quality control, sorting, wiring and identification
In a workshop, colour serves as a code: identification of conductors, marking of batches, sorting of parts, safety signage. Insufficient rendering turns this code into a source of error, with a cost that appears in rework or quality failures, never on the luminaire purchase line.
Visual inspection stations deserve treatment separate from the rest of the room. General lighting at Ra 80 can coexist with inspection stations fitted with dedicated, higher-performing lighting, which limits the extra cost to the area really concerned. This zoning is decided on the plan, workstation by workstation.
Retail, food processing and the perception of products
In a shop, colour carries the perceived value of the product. A textile, a fruit or a cut of meat lit by a source with a low R9 looks stale, whatever the illuminance level reached. The ranges intended for shops and boutiques build this constraint into the choice of sources.
In food processing, the requirement adds to the constraints of hygiene and washing. Visual sorting on the line, size checks and the detection of foreign bodies rely on the perception of shades. Luminaires for food processing therefore combine a high protection rating with careful colour rendering.
The trade-off between colour rendering and efficacy
With comparable technology, improving colour rendering uses up part of the available flux. The same module generally produces fewer lumens per watt in its high-rendering version than in its standard version. The trade-off therefore has three terms: the flux needed to hold the level, the power accepted and the quality of rendering required.
Handling this trade-off at the scale of the building avoids paying for high rendering everywhere. Zones where colour carries no information accept a standard choice, sensitive workstations receive the suitable product. This distribution is decided within the lighting solutions suited to each sector.
Checking the index stated on a data sheet
A few checks are enough to rule out fragile claims.
- Check that the value relates to the complete luminaire and not to the LED chip alone, before the diffuser.
- Check the wording: an Ra given as greater than or equal to a value commits more than a typical value.
- Ask for the R9 and, if it exists, the laboratory measurement report.
- Check that the stated index matches the colour temperature ordered, as versions of the same product may differ.
- Confirm that the value applies to the exact reference ordered, optic and diffuser included.
The full reading method is in our article on reading an LED luminaire data sheet. On an installation in service, a visual comparison between two areas lit differently, with the same parts placed on the same support, already gives a useful indication before any measurement.
To set the right level of colour rendering workstation by workstation on your site, request a free lighting study.
