Optical Brighteners: OBA Index and FI

Most printing paper is not white. It is slightly yellow paper with a dye in it that turns invisible ultraviolet into visible blue, so it looks white. That dye is why the same sample can measure two different colours, and why this library is published on M2.

What a brightener actually does

An optical brightening agent absorbs ultraviolet — which the eye cannot see — and re-emits it as blue light at around 420 nm, which the eye very much can. The paper therefore returns more visible light than fell on it in the blue part of the spectrum. Blue against a yellowish base reads as neutral, and neutral reads as white.

It works beautifully, and it has one condition: there has to be ultraviolet in the light. Under a window on a bright day, plenty. Under an LED panel, almost none. So a sheet that looks brilliantly white in the studio can look distinctly cream in a shop — without anything about the sheet having changed.

Two numbers, two questions

They are constantly confused, and they answer different things.

An index of 0 means a stock with no brightener at all. ChromaSpot’s own measuring stock reads 6.46, which is a fairly bright office-grade paper — and knowing that number is what lets the library be honest about which of its data is measured and which is modelled.

What it does to this library

Every sample was measured under all three conditions, so the effect is not an estimate. Across all 4253 samples:

2354 samples — 55% of the library — move by more than 1 ΔE₀₀ when the ultraviolet is switched on, and 1017 by more than 2. The library’s own neighbouring colours sit 2.5 apart on average. So for a quarter of these colours, turning the UV on is a bigger change than stepping to the next swatch in the book.

That is the whole argument for M2 in one paragraph, and it is measured rather than asserted. Measuring and Viewing covers the standards on both sides of it, and Why M2? the rest of the reasoning.

Three samples that fluoresce more than the paper

0P5.79, 0B5.92 and 0P4.90 return more fluorescence than the unprinted stock does. Either the ink carries a fluorescent pigment of its own, or that sheet was brighter that day. They are published as measured and flagged, not quietly corrected — a library that tidies away its own anomalies is a library you cannot check.

Your paper is not our paper

ChromaSpot describes a colour, not a sheet. But if you know the OBA index of the stock you are actually printing on, the whole library can be recomputed for it: an index of 0 for a UV-free board, 6.5 for something like ours, up to about 9.7 for a markedly brighter sheet.

M0, M1 and M2 are measured and ship with the library. Anything between or beyond them is computed, and computing it is a feature of the desktop application rather than of this browser. Being explicit about which numbers are measured and which are modelled matters more than having both in the same place.