Light Pipes Explained
LED’s are a great way to display a device’s status when it’s operating. It could be a simple light to show the device is switched on or indicate if a particular event has occurred. However, what do you do if you need to protect your device from the elements, be that wet or dusty conditions, high humidity, or external electromagnetic interference (EMI)?
Enclosing the device in a hermetically sealed enclosure can provide the protection you need, but now you can’t see those LEDs. Mounting the lights on the outside of the enclosure is one option, or you could simply drill a hole in the enclosure and mount an encapsulated LED device into the hole with a bit of glue to create a seal. These are simple solutions, but they come with the disadvantage that the LEDs now need wires to connect them to the PCB. This makes manufacturing more difficult and costly, introduces new failure modes such as if a wire breaks or comes loose, and it means the replacement of a non-working LED is now near impossible.
A better solution is to keep the LEDs on the PCB where they belong and channel the light they generate outside the enclosure. Here is where a light pipe provides the perfect solution.
In its simplest form, a light pipe is a clear tube that transmits light from a source to a different place. Here the source is an LED mounted on the printed circuit board (PCB), and the endpoint is the enclosure. Fitting a piece of plastic into the enclosure is a much simpler task than trying to fit an electronic component, and there’s no wiring required. The only tricky aspect is making sure the light pipe end aligns with the LED’s position once the board is fitted inside the enclosure. This is a design challenge that, once it is achieved, doesn’t need to be revisited unless the board layout or the device housing are changed.
Light pipes are generally short in length, only a few inches, and so carry light with very high efficiency. That can also bend light, either through shallow curved pipes or using mirrors for larger angles. This allows the designer greater freedom when placing their LEDs on the PCB while still getting their light to the enclosure’s optimum point. Obviously, having a short straight light path will be the cheapest option to implement as you just need an inexpensive plastic rod for your light pipe. Another low-cost alternative is a right-angled bend of fixed size.
Using a flexible light pipe requires an additional fitting to align the pipe’s PCB end to the LED. Still, it’s good to know that you have the possibility of using a flexible light pipe to channel light around a corner.
From a design perspective, there are three key things to remember. The first is that the pipe’s PCB end needs to be aligned with the LED surface as parallel as possible. If the angle between the end of the light pipe and the light source is too acute, light will reflect off this surface rather than pass through into the pipe. If the LED can’t be located directly under the end of the rigid light pipe, consider using a flexible light pipe to achieve the required alignment.
The second point is that if you mount multiple LEDs too close together, light from one LED can leak into light pipes for adjacent LEDs, giving a poor-looking display and, at worst, a misleading indication to the user. Opaque shrouds mounted over the LEDs will prevent stray light from leaking into the other light pipes if this is an issue. Alternatively, flexible light pipes that fix over the LED are available.
The third and final point is that the closer you can locate the end of the light pipe with the LED, the more of the light will get to the other end of the light pipe. However, if the amount of light shining out of the enclosure isn’t critical, or the LED is far brighter than you need, you can get away with a larger gap if this makes the device’s manufacture cheaper or simpler. In fact, if the LED generates significant levels of heat in operation, you may well be able to use a cheaper light pipe by adjusting the distance between the LED and the light pipe. All these decisions come into play when trading off costs and light intensity.
The range of light pipes available is enormous; Digi-Key alone stocks thousands with prices ranging from 10 cents per unit for a simple plastic rod to $10 per unit for a flexible light pipe with panel mounting fittings. Things to look out for when choosing the right light pipe for your application:
- Does the viewing end of the light pipe have a formed cone shape to provide a wide viewing angle, or is it a simple circular rod with a narrow viewing angle?
- Does the viewing end of the light pipe have a frosted surface to diffuse the light and provide a uniform glow, or is it a simple smooth surface that appears brighter in the center than the circumference?
- What materials are used in the manufacture of the light pipe? Is the light pipe made from a simple transparent plastic with a low transmission efficiency? Is it made from an optical-grade polycarbonate material with high transmission efficiency? Alternatively, is it made from an acrylic material with the best optical properties and greater heat susceptibility?
- Is the viewing end of the light pipe suitable for fixing the required enclosure in terms of materials, fittings, resistance to environmental conditions, use in high-vibration environments, and resistance to cleaning materials? Here environmental factors can include direct effects such as water and dust or indirect effects such as exposure to UV and radiated heat.
One final point to make is that light pipes can come in all shapes and sizes, so they can be used to creatively form part of a company logo or maybe an alpha-numeric character. They can focus light at a small intense pointy or spread the light out across a wide area. Using colored materials can also turn a plain white LED into a kaleidoscope of colors. In fact, the options are limited only by the human imagination and the designer’s budget.
Would you like to find out more about how Altium can help you with your next PCB design? Talk to an expert at Altium.