Light Guides

Light guides conduct the flow of light from a light source to a point of use. They are used to illuminate areas that are too small or too hazardous to permit the installation of a light bulb. Light guides are sometimes called light pipes (lightpipes). Typically, light guides are used in heat transfer and ultraviolet light (UV) curing applications.  

There are two basic types of light guides: liquid and fiber optic. Liquid light guides have a flexible outer sheath and a light-conducting liquid core. They are sealed with quartz windows that can be made transparent to a range of wavelengths. Fiber optic light guides consist of a non-coherent bundle of optical fibers. The fibers at each end of the bundle are tightly compressed, cut perpendicular to the axis of the fibers, and polished to permit light to pass into and out of the bundle.  

Fiber optic light guides are less flexible than liquid light guides, but are well-suited for the transmission of light in the visible and near-infrared range. Liquid light guides have little luminous loss over distance and are better suited for the transmission of UV light. Most liquid and fiber optic light guides are rigid and straight, rigid and bent, or flexible. Multi-leg devices are split along the length of the light guide so that the ends of the fibers extend separately to illuminate different points from a single light source. 

Selecting light guides requires an analysis of physical and performance specifications. Physical specifications include length, diameter, and termination method. Some light guides are terminated with a threaded or unthreaded ferrule, a tube-like mechanical fixture that confines the stripped end of a fiber bundle. Others are unterminated devices. Performance specifications for light guides include wavelength, acceptance angle, bend angle, and numerical aperture. Acceptance angle is the maximum angle measured from the axis within which light is accepted or emitted by the light guide and transmitted along its length. Bend radius is the smallest bend that fibers can withstand before fracture. Numerical aperture is a calculated, optical value that indicates a device’s ability to collect light over a range of input angles. It is equal to the sine of the acceptance angle. For fiber optic light guides, numerical aperture depends upon the core and cladding indexes of refraction. 

Light guides differ in terms of core materials and sheathing types. Common core materials include glass, silica or quartz, zirconium fluoride, chalcogenide, and plastic. Zirconium fluoride fibers offer a good combination of transmittance properties, environmental stability, and mechanical quality. Chalcogenide fibers do not have the tensile strength of silica fibers, but can be epoxied into connectors or suitably polished. Sheathing types for light guides include fluorocarbon rubber, polyvinyl chloride (PVC), stainless steel, silicone, and polytetrafluoroethylene (PTFE). Teflon^®, a registered trademark of DuPont, is a proprietary form of PTFE. 

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Part Numbers for Light Guides