Substrate Adaptation

In the hand a sheet of paper is white. On a screen the same sheet, shown as measured, is grey — and a brightened stock is violet. Substrate Adaptation closes that gap. It is on by default.

Why a measured paper looks wrong on screen

Our brightest stock measures L 93, and few papers are brighter. Drawn exactly, beside a white page or window, it reads as light grey: the eye takes the brightest white in view as white, and on a screen that is the display itself.

In the hand there is no brighter white beside the sheet. The eye settles on the paper and calls it white. The blue of a brightener shows only next to another white, and only to someone looking for it.

How far — the slider

The strength sits beside the switch on the Ink Drawdown page. 85 % is where colour appearance models put the eye on a display and it is what the page opens at. Move it and the picture follows the thumb.

0 % is the measurement, the same as switching adaptation off.
100 % makes the sheet and the white of the next window the same colour, which they are not — worth seeing once, and the reason the default is not 100.

The setting is remembered for next time. A link may carry its own with ?adapt=0.75, and that wins over what was remembered.

What it does

The paper you chose is shown most of the way to your display’s white — about 85 % — and every colour on it moves with it. Not all the way: the white of the next window is on the same screen, and a sheet of paper should still differ from it. Colour appearance models put the eye’s adaptation to a display near that figure, and never at 100 %.

One sheet reads as paper on any display and for any pair of eyes, because it borrows the white you already accept.
Where two sheets are in view, the whiter one is white and the other shows its real difference, as in the booth.

The paper list also offers 100/0/0, the ideal white. It is not a sheet anyone can hold, so the colours stay on the library’s own stock and the ideal white shows as what it is: whiter than any paper, with N.93 a hair below it.

The library browser and every colour page use it too, anchored to the library’s own stock under the light you chose. The switch is in the top bar of the library, and beside the observer on the Ink Drawdown page.

The numbers on the page do not change. They are the measurement; the picture is the appearance.

Off: as measured

Switch it off and the paper is drawn at its measured values against the display’s white. That is correct, and nobody accepts it as paper. Use it to compare stocks against a fixed reference.

A simplified form

How much ultraviolet the light carries is the M selector on the same page: M1 with it, M2 without. The finer settings — a display white matched by eye, the light of a particular booth — belong to desktop applications, where proofs are signed off.