Lot-to-lot or piece-to-piece colour consistency is an important indicator of quality for many plastic items. The process of pressing a plaque for the purpose of colour measurement is commonplace in plastics processing, with well-established ASTM guidelines for best practices. Plastic plaques can range from opaque to translucent or transparent, so require different measurement techniques depending on the sample type.
Opaque Plaques are impenetrable by light and are best measured using a Directional 45°/0° reflectance instrument or a Diffuse d/8° reflectance instrument:
Translucent Plaques allow light to pass through, but only diffusely, so that objects on the other side cannot be clearly distinguished. Both reflective and transmittance measurement modes may work well depending on the translucency of the sample.
Transparent Plaques allow light to pass through with little or no interruption or distortion so that objects on the other side can be clearly seen. These plaques can only be measured using transmission instrumentation.
Additionally, certain metrics and indices such as CIE L*a*b* Colour, Whiteness, Yellowness and Haze require the use of specific geometries and specific measurement modes.