Light Obscuration
Measurement of the ability of a sheet of paper to hide marks or print on the underlying surface defines the optical density of the material. The opacity iso 2471 standard quantifies this property as a percentage of the light reflected by a single sheet compared to a thick stack. It is a fundamental metric for preventing the show-through of text in books and brochures.
Reflectance Measurement
Testing involves placing the specimen over a black cavity and then over a white backing with a known reflectance. The ratio between the two readings indicates how much light is absorbed or scattered rather than transmitted through the fibres. A value of one hundred percent represents a completely opaque sheet where no light passes through to the other side.
Visual Interference
Controlling the optical properties is achieved at the mill through the selection of pulp types and the addition of mineral fillers. High levels of opacity iso 2471 are required for lightweight papers used in bibles or pharmaceutical leaflets to ensure that the small text remains legible. If the value is too low, the ghost of the image from the reverse side becomes visible which reduces the perceived quality of the printing.
Proper monitoring of this property allows for the use of thinner substrates without sacrificing the clarity of the finished document.
