So I wanted to do an article for our readers on the most optically clear plastics available. But then I got so caught up in researching what “transparency” really means that I decided this topic really deserves two articles.
Here’s a rundown of two of the major ways of measuring transparency in plastics (and other materials)-the refractive index and optical clarity. Keep your eye out for a second post listing highly transparent plastics in the next few weeks.
1) The Refractive Index
The refractive index is a measure of how much light is bent (or refracted) as it passes through a substance. It is defined by: n = sin i /sin r, where i and r are the angles of incidence and refraction respectively. The refractive index is also the ratio of the speed of light in a vacuum to the speed in the transparent material. The refractive index will vary slightly with the wavelength of the light used to measure the refractive index. If ‘white’ light (a mixture of various wavelengths) is used as the incident beam, then the variation in the refractive index for the various wavelengths will lead to splitting of light into the colors of the spectrum, a process known as dispersion. To allow comparison of refractive for materials, the light used is generally the sodium D line (a specific wavelength). When light enters a dense material from a less dense material then the refracted ray is bent towards the normal. When light enters a less dense material from a dense material the refracted ray is bent away from the normal. When light passes through a transparent material with parallel sides, the refractions ‘cancel out’ and the path of the light is displaced due to the presence of the transparent material.
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 possibly not acceptable for another observer. It is possible to measure the degree of light transmission using ASTM D-1003 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics). This test method is used to evaluate light transmission and scattering of transparent plastics for a defined specimen thickness. As a general rule, light transmission percentages over 85 are considered to be ‘transparent’. The perceived transparency or optical clarity is dependent on the thickness of the sample used for assessment, and the optical clarity will decrease with increasing thickness. Standard glass can be relatively optically clear in thin sections but will show a green tint (due to iron in the glass) as the thickness increases. Optical clarity can 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, PMMA’s transmission loss is around 93%, and PS’s is around 88%. These surface reflections can come from two basic causes: specular reflection, which is the normal reflection from a smooth surface, and 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.
Questions? Comments? Let me know in the comments section below.
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