The VPR reference pages are produced under optimal printing process conditions. For each batch, conformity is determined metrologically according to ISO 12647-2. Even if the print run is statistically assessed as conforming, individual pages may be outside the tolerance under certain circumstances. Another factor is the age and storage conditions of the printed sheets. Paper is subject to a natural ageing process, even under light-protected conditions. If the sheets are not stored protected from light in the black envelope supplied for this purpose, the ageing process accelerates and the colour appearance changes.