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Sunday, August 18, 2013

What is Light Scattering?

Light scattering is when light rays undergo multiple reflection and refraction from fibers (and/or fillers & pigments).  Increased light scattering increases brightness in the paper making process.

As discussed in a previous post, the most important of the four things that can happen when light strikes paper, with regard to brightness, is scattering. To understand the phenomenon of scattering, first let us consider the example of a bowl of large glass beads. The colorless and transparent beads easily allow light to pass through them; therefore, the beads appear clear. If we begin to crush the beads with a hammer until they form glass chips, many additional surfaces are formed from which light rays can reflect. Therefore, the bowl of glass chips will appear to be less clear and begin to take on a whitish appearance. As we continue to crush the chips of glass with a hammer into fine glass powder, we will note that the appearance has changed from semi-clear to white. By crushing the glass into powder, we have formed an extremely large number of reflecting surfaces such that each light ray entering the mass of powder will reflect off many particle surfaces before the light ray emerges. In addition to the reflection from each particle surface, a bending of the light ray will occur whenever the light ray goes from air into a glass particle which has a greater density than the air. The light ray will bend again when it emerges from the particle of glass back into the less dense air. This bending of a light ray each time it passes through a glass particle is known as “refraction”.

The combination of multiple reflection and refraction as described above is known as light scatter. The interaction of light with cellulosic fibers in paper is very similar to the example of the glass powder as described previously. Cellulosic fibers are essentially colorless and a cross-section of paper can be thought of as a mass of intertwined cellulosic fibers. Each time light strikes a cellulosic fiber, some rays will specularly reflect from the surface of the fiber and other rays will pass through the nearly transparent fiber, being refracted (bent) as they pass through. After the direction of the light ray has been changed by one fiber it will then encounter another fiber and its direction will be changed again. The numerous direction changes cause by multiple reflection and refraction inside the mass of cellulosic fibers results in light rays being scattered in all directions. In sufficient amounts of all wavelengths of light are scattered back in the direction of the viewer, the sheet will appear to be white. This is why refining (breaking the fibers into more small pieces) and additions of pigment/fillers (more surfaces) are used; they increase scattering and, therefore, brightness.

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