Light Pipe Overview

Hi, I am Ra'ef, an engineer here at Edmund Optics. Today I’m going to discuss the different types of light pipes we offer and provide you with information that will help you choose the light pipe best suited for your application. Light pipes are sometimes also referred to as light guides, homogenizing rods, homogenizers, or light funnels. They are typically used to increase the uniformity of a source or to direct light. Light pipes are used in many different types of applications. Automobiles and electrical instruments use light pipes to illuminate their indicators. Backlight displays use light pipes to evenly distribute light. Solid-state lighting uses light pipes as mixers, mixing colors from multi-color LEDs, like we will see in a little bit, and as homogenizers, to create uniform spatial and angular light distribution. Lithography uses light pipes to create uniform illumination to produce and inspect microchips. In addition, projection systems use them for creating uniform projected light distribution. Light pipes accept a cone of light and can be made into two different configurations: as a mirrored tube with a hollowed center or as a transparent material. Hollowed designs have reflective side walls to channel input light, whereas solid material configurations can be made out of glass, plastic, or a crystalline material, and use total internal reflection to channel the light to the output end. Solid light pipes can be clad, like an optical fiber, but are generally bare. For solid light pipes, light loss is primarily due to reflection loss at the input and output faces, which can be anti-reflection or AR coated to reduce reflection, whereas hollow light pipes are more susceptible to light losses, due to the metallic absorption on reflection. Edmund Optics offers several types of light pipes to address your application needs. Our N-BK7 and Fused Silica Light Pipe Homogenizing Rods have equally-sized input and output hexagonal faces. Both use total internal reflection to give a more spatially uniform illumination output. The hexagonal cross section provides 35% less light loss as compared to a square cross section configuration. Because the uniformity of the output is proportional to the number of reflections inside the light pipe, Edmund Optics offers a variety of lengths and apertures. Selecting the right length depends on the divergence of the incident light source in other words, whether the source has low or high numerical aperture. The Numerical Aperture, or NA, defines the size of the cone or angle of light that can enter the light pipe. For low NA sources, longer light pipes are recommended to obtain better light uniformity. The fused silica rods are primarily used for UV sources for maximum UV transmission and uniformity, whereas the N-BK7 performs well in the visible range. Now, our Tapered Light Pipe Homogenizing Rods have square input and output faces that differ in size by a magnification factor of either 2x or 3x. This means that the light exiting the larger aperture face has its divergence reduced by the same magnification factor, so you can increase or decrease the size of the light output. Using the TechSpec light pipe mounts, standard or tapered light pipes can easily be positioned and integrated into optical setups, and the end-clamps provide minimal contact between the mount and the rods, which allow for greater throughput of light. So here we have a simple optical setup to show us how these light pipes work. Here on this monitor we have our four basic colors: red, green, blue and yellow. And we are going to use our largest light pipe to mix those colors together and produce white light. So, as you can see here, the light coming out of the rod is blue and yellow, red, now we have green. And when we move towards the center, when the four colors are mixed, we produce white light. I hope this helps you select the best light pipe for your application. You can browse our website for more of our technical application notes and videos to find answers to common questions. Or please call our Technical Support to discuss your application in more detail.