A lab dip is approved in the booth under daylight. Bulk is dyed, matched and shipped. Then the buyer's store reports that the trousers and the matching jacket are visibly different colours on the rail.
Both items matched — under one light. That is metamerism, and the only defence is to look at the shade under more than one light before anyone signs.
What metamerism is
Two samples are a metameric pair when they match under one set of viewing conditions and do not match under another. It happens because the two samples reflect light differently across the spectrum but happen to produce the same colour sensation under a particular source.
The more different the colorants, the more likely it is. A cotton dyed with reactive dyes matched to a polyester trim dyed with disperse dyes, or a fabric matched to a printed paper card or a plastic chip, are classic metameric pairs: the reflectance curves cannot be the same, so the match can only hold under some lights.
Three kinds, three causes
| Type | What changes | How it shows |
|---|---|---|
| Illuminant metamerism | The light source | Match under D65, mismatch under store or incandescent light |
| Observer metamerism | The person looking | Two assessors with normal colour vision disagree under the same light |
| Geometric metamerism | The angle of light or view | Match when viewed straight on, mismatch at an angle — common with sheen, pile and lustrous yarns |
Most shade disputes are illuminant metamerism, and that is the one a light cabinet is designed to expose.
The sources and what each represents
A typical multi-source cabinet carries:
| Source | Represents | Why it is in the booth |
|---|---|---|
| D65 | Average daylight, about 6500 K | The primary reference for textile colour in most specifications |
| TL84 | Store and showroom fluorescent, about 4000 K | Where many garments are first seen and compared |
| A | Incandescent, domestic | Home lighting; a warm, red-rich source that exposes many metameric pairs |
| UV | Ultraviolet (blacklight) | Shows optical brighteners and fluorescent dyes |
| D50 (optional) | Horizon daylight, about 5000 K | Print and packaging work; ISO 3664 |
CIE illuminant A is defined at a correlated colour temperature of about 2856 K. Lamps in cabinets approximate it; what matters is that every lab in the chain uses a comparable source, so ask your buyer which lamps their reference booth uses.
A sequence that works
Agree the light sources with the buyer in writing before the first lab dip. A common pattern is one primary source on which the match must be good, and one or two secondary sources on which the match must be acceptable.
At the booth:
- Start with the primary source, usually D65. Judge the match: depth, hue, brightness.
- Switch to each secondary source in turn without moving the samples. Watch for the moment of switching — a metameric pair often visibly "jumps" apart.
- Record direction, not just pass/fail. "Under TL84 the batch is greener than standard" tells the dyehouse what to do; "fail under TL84" does not.
- Check UV on whites and pale shades if brighteners may be involved. A white that matches in daylight but glows differently under UV will look different in any light rich in UV.
- Return to the primary to confirm the eye has re-adapted before the final call.
Give the eye a moment after each switch. Colour adaptation is real, and a judgement made in the first instant after changing the light is not the same as one made after the eye has settled.
When the booth says "metameric"
A booth finding is a diagnosis, not a cure. Metamerism is fixed in the recipe, not at the cabinet: by choosing dyes whose spectral behaviour is closer to the standard's. If the standard is a paper card or a different fibre, it may not be fully fixable at all; the honest answer is to agree a primary light and a tolerance on the secondary ones.
A spectrophotometer helps here. Measuring both samples and computing colour differences under several illuminants shows how strongly the pair is metameric and under which source, and a metamerism index puts a number on it. Use it alongside the booth, not instead of it: the buyer will still look.
Controlling the other two kinds
Observer metamerism cannot be removed, only managed. Screen assessors for colour vision, use more than one assessor on critical approvals, and keep a record of who approved what.
Geometric metamerism is managed by holding samples in the same orientation and at the same angle, especially for pile, brushed or lustrous fabrics. A viewing stand inside the booth makes that repeatable. Lay pile in the same direction on both samples.
Booth maintenance is part of the method
A metamerism check is only as good as the sources. Daylight simulators and fluorescent tubes drift with age; a "D65" lamp at the end of its life may no longer reveal the pair it was bought to reveal. Track lamp running hours, replace on schedule, and check the cabinet periodically with an instrument. Keep room lights low during assessment so that the booth light is the only light on the samples.
Checklist
- Primary and secondary light sources agreed in writing with the buyer?
- Samples assessed under each source without moving them?
- Findings recorded with direction (redder, greener, darker)?
- UV checked on whites and pale shades?
- More than one assessor, colour vision screened, on critical approvals?
- Lamp hours within the replacement schedule?
TLI-TexLabInstruments manufactures multi-source light assessment cabinets and colour viewing stations in its own workshop in Istanbul.