Light Pipe Technologies (original) (raw)

Light Pipe Technologies

In illumination engineering, it is often desirable to transport light from a source and to distribute it in a particular area. Optical fibres can be used effectively to guide light for small-scale applications. But, optical fibres are unsuitable for large-scale lighting applications since both the cost and the mass of a large solid-core fibre are prohibitive. For these reasons, hollow light guides are preferred in these cases.

There are a number of possible methods for guiding light in a hollow guide. Perhaps the most obvious approach using metallic reflection. In such a guide, the inner surface of a cylindrical tube is with a metallic coating so that light rays in the guide undergo a reflection each time they reach the inner surface and propagate down the guide. However, even with a highly polished surface with 95% reflectance, for instance, the 5% attenuation of the light associated with each metallic reflection is sufficient to cause severe attenuation as the light propagates along the guide.

The prism light guide provides a solution to this attenuation problem. There are several different geometrical configurations, but typically the prism light guide is a cylindrical transparent structure with circular cross-section. The outer surface of the guide is lined with a sheet of prismatic microstructures, as shown in the figure below. These microstructures are typically 200 micron high right-angle prisms running lengthwise along the pipe.

Prism light guide cross section and isometric view

Light travels mainly in the hollow air space inside the guide and undergoes TIR when it strikes the prismatic surface, provided the angle, a , which the light ray makes with the axial direction, is less than the prism light guide acceptance angle, a plg , given by:

where n is the refractive index of the prismatic material. Typical prism light guides are made of polycarbonate resin ( n =1.59), so this acceptance angle is about 30 � . Since this is a practical degree of collimation for powerful light sources, the prism light guide is well-suited to large-scale illumination applications.

One key advantage of the prism light guide is its low level of absorption. In any hollow guide, light rays spend most of their time in the enclosed air space where losses are negligible. As a result, the loss of light at each wall reflection dominates. Although there are fundamental losses associated with diffraction of light when it encounters the prismatic microstructures, these losses are extremely small. In a practical prism light guide, the main loss is due to bulk absorption and scatter in the material from which the microstructures are made and surface roughness and optical imperfections of the prismatic structures. Typically these amount to less that 2% per wall reflection, most of which takes the form of light escaping rather than the light being “lost”. In other words, most of this light escapes from the guide, where it can often be used to provide illumination.

Prism light guides are currently widely used around the world, primarily in applications where it is desirable to separate the source from the region being illuminated. Common installations include tunnel lighting, industrial spaces such as warehouses, high ceiling areas, and architectural highlights on buildings.

Prism light guides are hollow structures which pipe light by means of total internal reflection (TIR), and can achieve high efficiency and uniform illumination. These guides use materials known as �extractors�, usually a piece of white diffusive film, to scatter the light required for uniform illumination in the room below. Currently, in conventional prism light guides the shapes of extractors have to be designed specifically for each guide. This is undesirable since it means that every installation requires customized extractors and, once designed, the level of extraction cannot be adjusted, so the resulting level of illumination cannot be changed.

In one of our research projects, we developed a practical method for actively controlling the light intensity in the light guide and also generalizing the shape of the extractor.

In order to extract light out of the light guide, TIR is prevented or �frustrated� by moving scattering micro-particles into optical contact with the optical lighting film (OLF) forming the guide. These 0.5 m m diameter particles are electrostatically charged and suspended in a low refractive index liquid. The suspension is contained within a gap between the OLF and a substrate material. The position of the particles is electrophoretically controlled by applying an electric field across the suspension, between transparent conductive layers deposited on the substrate and the OLF surface. This approach is schematically demonstrated in the figure below, with the reflective state shown on the left hand side of the figure and the scattering state shown on the right. As a result, the light intensity can be electronically varied in different portions of the light guide with a general shaped electrophoretic extractor.

Principle of an FTIR-based extraction technique

To see how this technology has impacted the commercial world, click here.