Black versus white backing on the same sheet
Thin paper and film show through. Black backing and white backing on the same patch can differ by several colour-difference units. Opaque board is less affected, but you still follow the contract. ISO 13655 treats white backing and black backing as two different reports. One job uses one of them, written next to every reading.
Thin paper and film show through. Black backing and white backing on the same patch can differ by several colour-difference units. Opaque board is less affected, but you still follow the contract. ISO 13655 treats white backing and black backing as two different reports. One job uses one of them, written next to every reading.
What shows through is the table
White backing reflects light from behind the sheet, so thin paper looks brighter. Black backing swallows that light; the reading is closer to “this sheet only.” If film or thin paper sits on whatever white paper is on the table, a small table colour change moves colour difference. Black backing cuts that. Thick coated board shows through little, so the two backings sit close — close is not a reason to skip writing it.
If the OK sheet was measured on white backing, production uses white backing. Switching to black mid-run is switching the target set.
Which one to use
| Material | Common practice if the contract is silent | Must do |
|---|---|---|
| Opaque board | Often white backing | Still write white backing |
| Thin paper, film | Black backing, so the table does not show through | Match the OK sheet |
| OK sheet already names backing | Follow the OK sheet | Do not switch to the “more reasonable” one |
When you record, write
- Every reading: wb or bb, or white backing / black backing.
- Change backing, remeasure the whole column.
- Before comparing numbers with the customer, match backing. If they do not match, stop.
