Lightfastness / print permanence rating.

A figure for how long a print keeps its color on display, produced by accelerated fading tests and meaningful only alongside the conditions it was measured under. ISO 18937 is a multipart series: general requirements in Part 1, xenon-arc exposure in Part 2, filtered to simulate indoor indirect daylight through window glass, and LED lamp exposure in Part 3, published in 2026, while most fading data on color prints from the past thirty years came instead from glass-filtered or UV-filtered cool white fluorescent lamps. Filtering, light level, sample surface temperature and humidity all move the number, and none of these regimes is a print hanging in a sunlit shop window. Quote a bare figure in years and the complaint arrives later, when the customer's print has visibly gone well inside it.