Which light source should I use to approve lab dips?

Short answer: Lab dips should be judged under agreed, controlled light, because colours can match under one light and not under another. This effect is called metamerism, and it is common when the body fabric and a waistband or trim are dyed with different dyes or on different fibres. ASTM D1729, a standard practice for visual appraisal of colours and colour differences, recommends simulated average daylight, D65, for general visual appraisal, and explains that a match is usually confirmed under another very different source, such as a low colour temperature incandescent lamp, and often under the kind of light found where the product is sold or used. It also expects observers to have normal, preferably superior, colour vision. Window daylight changes with the weather, the time of day and the room, so it is not a reliable reference. For approval, agree a primary light source, usually D65, and one or two secondary sources, such as a warm source and a typical shop light, and use the same light box settings at the brand and the factory. Instrumental colour difference, for example calculated with ISO 105-J03, can support visual approval, and the ISO 105-A02 grey scale is commonly used to rate change in colour.

Why light source matters

• Colours can match under one light and not another.
• Daylight from windows varies.
• Different fibres and dyes behave differently.

Agreeing conditions

• Primary source, usually D65.
• Secondary sources, such as warm and shop light.
• Same light box set-up at both sides.
• Trained observers with normal colour vision.

Approving the dip

• Compare against the physical standard.
• Check under each agreed source.
• Record the chosen dip and comments.

A worked example

As an illustration, a brand could state in the colour standard that dips are judged in a D65 light box first, then checked under a warm source and the light used in its retail partners' stores, and that body fabric and waistband are judged side by side.

Matching body and trims

Judge the body fabric together with the waistband, binding and thread. A dip can be correct alone but mismatch the trim it will be worn with.

When bulk and dip disagree

If bulk looks different from the approved dip, first check both under the agreed sources in the same light box, side by side, before judging. Many disputes come from comparing under different lights or comparing a small dip with a large roll.

Limits

Light boxes age, and lamps lose output over time. Visual judgement also varies between people, so instruments and physical standards help, but the agreed conditions decide the result.

Facts, recommendations and uncertainty

Established: colours can match under one light and not another, and controlled viewing conditions reduce disputes. Recommendation: agree D65 plus a second very different source and the same set-up at both sides. Uncertain: how the shade will look under every customer's lighting.

Sources and further reading

• ASTM D1729-22 — Visual appraisal of colors and color differences of diffusely-illuminated opaque materials
• ISO 105-J03:2009 — Calculation of colour differences
• ISO 105-A02:1993 — Grey scale for assessing change in colour

Standards and regulations are referenced by their published scope. Test parameters, cycles and pass levels should be agreed with your laboratory for the specific product; legal requirements should be confirmed for each destination market.