Filament calibration card

Photographic relief printing predicts colour from one number per filament — TD, how far light travels into the plastic before it is blocked. It cannot be looked up: it changes with pigment, so it differs between brands and between colours of the same brand. This prints a card that measures it.

One spool per card. The U1 changes filament by height rather than by area, so every pad above the base is the same colour — calibrating four spools means four prints.

Backing filament (loaded first)Dark and fully opaque works best — it is the colour that has to stop showing through.Filament being measured (loaded second)The spool you want a real TD for.Steps: 14Layer heightUse the same layer height you print reliefs at, or the number will not transfer.

After it prints

Print the card flat, with no supports. It is a thin plate, so it takes minutes rather than hours.
Photograph it straight on under even, indirect light. Avoid glare and coloured lamps — the measurement is the colour, so the light matters.
Note the colour at the centre of each pad, plus the bare backing and the thickest pad.
Those readings give TD. Feeding them back in automatically is not built yet — send the photo and I will fit it by hand until it is.

Experimental. The card and the maths are tested; what has not been tested is a real print, which is exactly what this is for.