How do you measure the reflectance of acrylic sheet?

When light is directed at a medium such as acrylic, there are many things that can happen to it. A portion of the light can be reflected off the incident surface, typically measuring about 4% for acrylic sheet. The remaining light that then enters the material is refracted or bent, as it passes into the material. The amount of bending is determined by the index of refraction. Depending on the material, some light may then be absorbed. When the light strikes the second surface of the material, 4% is again reflected off the inner surface. The remaining light is then transmitted through the sheet. For colorless acrylic, about 92% of the light passes completely through the sheet. Total internal reflection is a special situation in which light is completely reflected from an interface, inside a material.

Reflection

Incident light obeys the law of reflection in that the angle of reflection equals the angle of incidence.

For objects such as acrylic, with surfaces so smooth that any microscopic hills and valleys on the surface are smaller than the wavelength of light, the law of reflection applies. All the light traveling in one direction is reflected from the acrylic sheet in one direction, as opposed to being scattered. This type of reflection from such objects is known as specular reflection.

Most other objects exhibit diffuse, or scattered reflection, with light being reflected in many directions. All objects obey the law of reflection on a microscopic level, but if the irregularities on the surface of an object are larger than the wavelength of light, which is usually the case, the light reflects off in all directions. This is typical of an acrylic material such as ACRYLITE® non-glare (P99) or ACRYLITE® Satinice products.

Refraction

When asked about the speed of light, the key to this statement is that it is the speed measured when traveling in a vacuum. This value is 3.00 x 108 m/s. When light travels through something else, such as glass, plastic, or any other material, it travels at a different speed. The ratio of the speed of light in a vacuum to the speed of light in another material is known as the index of refraction (n). It can be measured with an Abbé Refractometer by ASTM D 542.

The change in speed that occurs when light passes from one medium to another results in the bending of light, or refraction, which takes place at the interface. If light is traveling from one medium into another medium, and angles are measured perpendicular to the interface, the angle of transmission of light into the second medium is related to the angle of incidence by Snell's law:

At some angle, known as the critical angle, light traveling from a medium with a higher index of refraction to a medium with a lower index of refraction will be refracted at 90°, or along the interface. If the light hits the interface at any angle larger than its critical angle, it will not pass through the material. Instead, it will be trapped in the material by repeated reflections of its internal surfaces, a process known as total internal reflection. When light is constantly reflected internally from one surface back to the other, it is often referred to as light piping.

The critical angle can be found from Snell's law, putting in an angle of 90° for the angle of the refracted light. This gives:

critical angle: sin θc = n2/n1 (n1>n2)

For any angle of incidence larger than the critical angle, Snell's law can not be solved for the angle of refraction, because it will show that the refracted angle has a sin larger than 1, which is not possible. In this case, all the light is totally reflected off the interface, obeying the law of reflection.