Measuring Transparency, Lensing, And Optical Clarity

Published: February 15, 2021
by Charles Busada
Displaying lensing and clarity of two types of transparent tubing laid over a pattern of repeated dots

2) Transparency

So how do scientists measure clear plastic? The boundaries between ‘transparent’ or ‘clear’ and ‘translucent’ or ‘opaque’ are often highly subjective. What is acceptable for one observer is not necessarily acceptable for another. One test for measuring light transmission uses the ASTM D-1003 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics). This test method evaluates light transmission and scattering of transparent plastics for a defined specimen thickness.

As a general rule, light transmission percentages over 85% are considered as ‘transparent’. But, thickness of the specimen effects the perceived transparency or optical clarity: it will decrease with increasing wall thickness. Standard glass, for example, is relatively optically clear in thin sections but shows a green tint (due to iron in the glass) as the thickness increases. Optical clarity is only be achieved when the refractive index is constant through the material in the viewing direction. Any areas of opaque material (such as colorants) or areas of different refractive index, will result in a loss of optical clarity due to refraction and scattering.

Optical clarity is also dependent on surface reflections from the sample. The surface reflections at the air/plastic interface create significant transmission losses. For example, acrlyic’s transmission loss is around 93%, and polystyrene is around 88%.

These surface reflections come from two basic causes: 1) specular reflection, which is the normal reflection from a smooth surface, and 2) diffuse reflection, which is dependent on the surface flatness of the sample. The transmission loss as a result of surface roughness or embedded particles is more often termed ‘haze’, and this is generally a production concern and not a property of the material. In producing blown film, haze can be caused either by melt fracture at the surface or by interfacial instability between the layers of the film.

 

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