Metameric Orange: A Rejected Batch of Machine Guards
A paint shop must supply 40 identical welded steel machine guards in 'safety orange' to match a customer's approved master panel. The shop has no spectrophotometer of its own but can borrow a portable one from a neighbouring laboratory once. Tinting is done in-house with a machine fed by a white base and 12 tinters.
For batch 1 the mixer added, per 4 L of base, 18 g of an organic red tinter, 2.5 g of a yellow and 0.4 g of black, and matched the master panel under the booth's D65 daylight lamp. Batch 2 was mixed from the same formula but from a freshly opened container of the red tinter. At the customer's site, under sodium street lighting, it looked distinctly redder than the guards from batch 1, and the whole lot was rejected as 'a different colour'. The guards were primed with a mid-grey primer; on the flat faces the colour matched, but the folded edges look visibly darker and redder.
The shop manager argues that the laboratory measured ΔE 1.2 against the master for batch 2, so the rejection is unfair.
1. Explain how two panels can look different under one light although their measured colour difference is small.
Read the worked answer
Metamerism. A pair of colours is metameric when their spectral reflectance curves differ but they look the same under one illuminant. The eye integrates reflectance across the visible spectrum weighted by the light source; change the source, and a difference that was hidden at one wavelength band becomes obvious. Under a sodium lamp, emission is concentrated in a narrow band around 589 nm with a very low colour rendering index. Any difference in reflectance at that wavelength is exaggerated, and very little of the rest of the spectrum is present to balance it out. So a pair matched under D65 can visibly separate under sodium. This is a genuine colour mismatch, not a perception fault - but sodium is a bad judging light either way.
The ΔE of 1.2. ΔE is a single number computed for a defined illuminant, observer and geometry - usually D65, 10° observer. It describes the size of the difference, not the shape of the spectral curves. A metameric pair can show a low ΔE under D65 and a large apparent difference elsewhere. Also, ΔE 1.2 is above a typical industrial match tolerance of about 1.0 for solid colours, and a single number cannot catch a hue shift that appears only under other lighting.
Batch variation. A fresh container of red tinter may have been more concentrated, or the machine's tinter had settled and was not recirculated, so the same nominal weights gave a different pigment loading. Unrecorded tinter strength is a common source of drift.
2. Give a practical procedure to settle the dispute with the customer and to stop it recurring.
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Settle the dispute first, on agreed ground. Get the master panel, a batch 1 guard and a batch 2 guard side by side in a colour viewing cabinet and judge them under at least three sources - D65/daylight, a warm tungsten source and a triphosphor fluorescent (TL84 type) - with the samples flat and the same distance and angle. Invite the customer and record the result. Explain in writing that sodium lighting has too low a colour rendering index to judge any colour, and agree in future which illuminant and which geometry constitute acceptance. Also note that curvature and gloss change appearance: comparing a flat master panel with a folded guard is not like for like.
Then measure once, usefully. Use the borrowed spectrophotometer to measure Lab* for all three panels under D65 and under a second illuminant and calculate the metamerism index, not just D65 ΔE. That converts an argument into data.
Prevent recurrence: write the illuminant, observer and ΔE tolerance into the order; keep a retained sample of the approved master and of every batch, sealed and labelled with date and formula; record tinter batch numbers and weights on the batch card; stir or recirculate the tinting machine before drawing tinters, and check the machine with a control panel daily; and measure spectrophotometrically at batch release rather than relying on the booth lamp. Where the master is a high-chroma organic orange, try to match it with the same pigment type rather than a visually equivalent mix, because that is what causes metamerism.
3. Why do the folded edges look darker and redder than the flat faces, and how would you correct it?
Read the worked answer
Film build. On a folded edge the spray deposits less paint and the electrostatic or air path thins it further, so the dry film thickness is lower than on the flat. A high-chroma organic orange contains relatively little hiding pigment; the mid-grey primer shows through the thinner edge film, darkening and dulling a colour that is meant to be bright and opaque. The effect is a change in both lightness and chroma, which the eye reads as 'darker and redder'.
Geometry and lighting. A folded edge also reflects light away from the eye and presents two faces at different angles, so the shaded side always looks deeper. Some of the effect is unavoidable in any solid colour, but the primer show-through is not.
Correction:
- Measure it. Use a comb gauge and an eddy-current or magnetic thickness gauge on flats and on edges; compare against the datasheet dry film thickness. If the edges are below the minimum, the colour cannot be right.
- Improve hiding. Either apply an extra full coat overall, or use a light grey or white undercoat/primer so that a thin edge film shifts less in colour. A mid-grey primer under a high-chroma orange is a poor choice.
- Adjust technique. Spray edges and returns first with a narrower fan and gun angled to bisect the fold, then coat the flat and overlap onto the edge; keep 50% overlap on the last pass and do not increase distance to 'save' paint at the edges.
- Verify with a film thickness survey and a visual check of the finished part against the master under the agreed illuminant, not under the booth lamp alone.