This document specifies procedures for the measurements and the definition of a one-dimensional parameter approximating the translucency magnitude of a flat materials that absorbs and scatters light. This one-dimensional translucency material property is termed “Alpha” and can be used in appearance-based workflows for colour and translucency reproduction. While the measurements conditions and reference materials used to define Alpha are designed for current 3D printing workflows, the use of Alpha is not limited to reproducing objects with additive manufacturing technologies. This document does not address: To be discussed today based on the nature of the comments. Also an informative Annex provides guidance for a visual meaningful colour measurement.