Two samples can produce the same (X,Y,Z) tristimulus under one light source, they look identical, and completely different under another. This is illuminant metamerism. It is why the upholstery the client approved in your office looks wrong in their hallway, why dental crowns fail at examination tables, and why automotive paint matching is an entire industry.
Four ideas to carry into the next specification conversation you have.
Two colours look identical under one light, different under another. The match depends on the spectrum of the illuminant.
The eye integrates SPD into 3 cone signals. Two different curves can produce the same trio, and the same perceived colour, but only under that exact light.
Even Ra 95+ lamps have spectral dips. If two pigments have reflectance peaks in those dips, they mismatch.
Approve materials under the actual installation lighting, or under D65 for daylight projects. Never approve under store fluorescents.
Four flavours of conditional match, and which one bites you in a project.
| Type | What varies | Where you see it |
|---|---|---|
| Illuminant | Light source SPD | Sample approved in office, wrong at home |
| Observer | Person's cone response | Two viewers disagree on a colour |
| Field-size | Visual angle | Small swatch matches, large wall doesn't |
| Geometric | Viewing angle | Car paint shifts in different light angles |
Five real-world cases, and how good lighting design prevents the callback.
Client approves a marble slab under 6500K showroom LEDs. Installed in their 3000K lobby, the veining looks olive. Always approve under installation CCT.
Dentists examine under 5500K/Ra 97 lights for this exact reason. A crown matched in the lab under 4000K Ra 80 fluorescents will fail under the chair light. ISO 7551 mandates the standard.
Repaired fender matches body in the shop, then looks off under sodium street lights. Why automotive specifies multi-illuminant metamerism tests (D65, A, F11).
Garment colour approved under D65 light boxes in QA. If retail uses 4000K LEDs with low R9, reds shift visibly. Brands now spec lighting in stores.
Skin colour determines burn-degree and oxygenation calls. NHS guidance and IES RP-29 specify ≥ Ra 90 with high R9 in exam rooms because metameric skin mismatches are dangerous.
For retail, healthcare, and hospitality projects, NLC ships fixture batches with documented SPD match across an entire order. Same spectrum across hundreds of units, no metameric surprises.
Practical lighting know-how from NLC Academy. No spam, unsubscribe anytime.
By subscribing you agree to receive NLC emails.