Introduction
LEDs are essential to products across a range of markets. Although much focus is placed on consumer electronics and devices, LEDs are pervasive in a number of more challenging industries, including agriculture, communications, healthcare, manufacturing, renewable energy, and transportation. OEMs use LEDs to communicate status at a glance, improve usability, increase functionality, and enhance appearance. When properly specified, LEDs can bring significant value to a product; that said, they can also present special issues for design teams, especially for first timers. Light pipes – structures for transmitting light from the LED to the point of use – represent a powerful, easy-to-use technology for addressing these issues. By taking advantage of light pipes and following best practices, OEMs can mitigate those challenges to bring their best possible product to market for even the most challenging environments.
LED illumination categories
From an OEM perspective, we can divide LED illumination into three classes:
Indication: LEDs communicate the status of a device or system. Implementation options include changing colors, changing display modes (solid or flashing at various rates), or both. Examples include the front panel indicators of servers in data centers or of PLCs on the factory floor.
Functional illumination: LED illumination supports the usability or function of a device. Examples range from illuminating power and function buttons to sophisticated assemblies that provide technicians with the detailed information they need on fault conditions to make a repair.
Aesthetic illumination: LEDs enhance the appearance of products, vehicles, and structures by backlighting logos, adding design elements, etc.
The challenges of illumination applications
LED illumination is undergoing rapid growth in a number of market sectors. At the same time, these applications present greater complications than just lighting up a living room.
Agriculture
Crops aren’t all that’s growing in agri-business – so is the use of technology. From irrigation controls to moisture sensors to tractor panels and door keypads, high tech has become increasingly common on the farm. Whether indoors or outdoors, devices deployed in these environments need to survive repeated exposure to dust, dirt, and liquids of various types, as well as shock and vibration.
Communications infrastructure
Walk into a data center and you’ll see indication LEDs winking on every server, router, and switch. In these climate-controlled environments, the issue is not contamination but light bleed among side-by-side indicators. A secondary issue may be board-level heat buildup from processors and other components, especially if board congestion is an issue.
Communications infrastructure also encompasses wireless networks. With the continuing rollout of 5G, private deployments are likely to become increasingly popular in areas like factories and processing plants. In these situations, the equipment may not be as protected from environmental contamination and temperature swings, even when indoors.
Electric Vehicle Charging Stations & Diagnostics
Supporting the electric vehicle revolution requires an infrastructure of charging stations and diagnostics. Charging stations incorporate functional illumination, with meters that light up as the vehicle charges. They also use indication and aesthetic illumination for branding purposes. Hotspots and fadeouts are potential issues for all of these types of designs.
Many of these stations are located outdoors, where they are exposed to the elements. Housings need to incorporate some type of ingress protection while ensuring that lighting elements are clearly visible.
Healthcare & Medical Devices
The medical device market presents its own challenges. LEDs are widely used for indication and functional illumination in products ranging from infusion pumps to surgical robots to patient beds. The clinical environment can subject devices to shock and vibration, as well as unexpected contamination. An emphasis on compact products also makes heat buildup a risk.
Particularly in the wake of COVID, home healthcare has become increasingly popular. Devices need to be designed to tolerate these conditions, which tend to be far less controlled than the clinical environment.
Industrial Automation, Robotics, & Process Controls – IP, shock and vibration
Today’s smart factories and processing plants make heavy use of motion control, robotics, smart sensors, edge computing, and automated guided vehicles (AGVs). Indication and functional illumination play key roles in equipment like drives, PLCs, HMIs, sensors, and computers. And with the surge in digitalization, machines and facilities are connected with both wired and wireless networks.
The industrial environment exposes equipment to punishing conditions, including temperature extremes, shock and vibration, and contamination with dirt, grease, and harsh chemicals. Although devices can be protected in climate-controlled cabinets, a trend toward distributed control architectures means that more and more drives and controllers are installed out on the machine.
Renewable Energy
Renewable energy makes heavy use of electronics, and LED illumination plays its part, from supporting sophisticated applications like monitoring and control to more simple tasks like illuminating keypads to control access. Windmills generate continuous vibration, while solar arrays may be in areas where the temperatures soar. One constant is that equipment is frequently exposed to harsh environmental conditions while facing stringent reliability specifications.
Transportation (Rail, Marine, Avionics) – IP, shock and vibration
Transportation modalities, including rail, marine, aviation, and vehicles make heavy use of LED illumination. Devices include operator panels, external door controls, monitoring equipment, and more. Because of the lives at stake, transportation applications are the very definition of high reliability. At the same time, equipment needs to survive punishing conditions, including wide temperature swings, dusting contamination, extreme weather, and high levels of shock and vibration.
The challenges of LED illumination
As electronic components, LEDs are vulnerable to the typical issues that trouble electronics. Contamination by dust and liquids can cause premature failure. The devices can be damaged by shock and vibration, as well as voltage or current spikes. LEDs are also subject to issues unique to the technology:
Heat
LEDs are known for having high conversion efficiencies, but they still generate a large amount of waste heat on the board side. If it’s not properly managed, excess heat can lower output and shorten lifetime, which is a particular drawback for support technologies like indication and functional illumination. Customers will be frustrated if the device still operates but the status indicators do not.
Heat generated by the LEDs is not the only issue. To serve these applications, LED output needs to be visible at the panel. Unfortunately, this is also popular real estate for other components. If an LED is placed in the middle of a congested board layout, waste heat generated by adjacent electronics just worsens the situation.
Hotspots
In terms of footprint, the small size of LEDs (a few millimeters in diameter or less) is a benefit. At the subsystem level, however, it can be a problem. Many functional illumination applications require the combined output of many LEDs to fully illuminate the region. Unfortunately, that can result in hotspots – regions of excess brightness that create an uneven appearance.
Fadeout
Elsewhere, the combined LED output may not fully illuminate the shape, causing fadeout at the edges. Because many of these subassemblies perform roles beyond pure aesthetics, such as illuminating the meter on a battery charger to show progress, this effect can impact communication with the user.
Light bleed
In rows of tightly spaced indication points, light bleed – spillover from one indicator to the next – can cause confusion. Light bleed also can result in light escaping into the housing, reducing the amount of light that reaches point of use. This unwanted housing glow represents wasted energy, in addition to affecting appearance.
Light pipes can be used to address all of these problems. At the same time, they are economical, robust, and easy to install. Successful use just requires an understanding of the options and the best ways to use the technology to address specific issues.
What are light pipes?
Light pipes are plastic or optical fiber structures designed to capture LED output at the PCB and transmit it to the point of use. Light propagates through a light pipe by means of total internal reflection. Depending on the design, a light pipe can transmit input through multiple angles and planes of orientation.
Types of Light Pipes
Light pipes can be categorized as rigid or flexible, each with its own advantages, limitations, and sweet spots (see Table 1).
Rigid light pipes
Rigid light pipes are monolithic structures molded of optical plastics. They guide light in straight segments, redirecting it via prisms or lower-loss curves (see Figure 1). Rigid light pipes are ideal for paths 3 inches long or less, with limited turns. Straight and right-angle designs are commonly offered as commercial-off-the-shelf (COTS) devices. For projects involving specialized requirements, custom light pipes may be more effective. The process is straightforward, although it will involve somewhat more effort and lead time. The key is to focus on managing optical loss, which is greatest at turning points.
![Figure 1: Rigid light pipes can redirect light using prisms (left) or lower-loss curves (right). [Source: Bivar]](https://www.bivar.com/wp-content/uploads/2023/05/rigid-light-pipes-pp1.png "rigid-light-pipes-pp1")
Rigid light pipes are robust, easy to install, and available with a variety of mounting options, including some that provide very high levels of ingress protection against dust and moisture.
![Figure 2: Flexible light pipe works well for longer distances, complex paths, or skirting obstacles. [Source: Bivar]](https://www.bivar.com/wp-content/uploads/2023/05/flexible-light-pipes-pp1.png "flexible-light-pipes-pp1")
Flexible light pipes
Flexible light pipes are based on optical fiber, enabling them to transmit light over longer distances and around obstacles (see Figure 2). For very complex requirements, a flexible light pipe solution is generally less expensive, faster, and easier solution than a custom rigid light pipe. Flexible light pipes do need to be board mounted using an adapter, but this is generally not a problem for most applications.
Flexible light pipes can support complex routing on the board. They do have one limitation, which is a minimum bending radius of 10X the fiber diameter, or about 1 cm.
Table 1: Comparison of rigid and flexible light pipes
| Parameters | Rigid | Flexible |
| Distance | Up to 3 inches | Up to 100 meters |
| Bending or Shaping Requirement | Vertical, Right Angle | Bend around obstacles |
| Mounting Options | Panel Press-Fit, Board Mount | Board Mount Adapter |
| Built-In LED Option | SMD | SMD, Through-Hole |
| Ingress Protection | Yes | Yes |
| Side by Side SMD LED Placement | Array, ZeroLightBleedTM | ZeroLightBleedTM |
Light pipe materials
The most common materials for rigid light pipes are polymethylmethacrylate (PMMA) and polycarbonate. Each has its trade-offs. PMMA has an optical transmittance of about 93% across most of the visible spectral region. It’s straightforward to mold and economical. On the downside, it’s more hydrophilic than other optical plastics, so it’s not as dimensionally stable in humid conditions.
Polycarbonate is less hydrophilic than PMMA, making it better suited for humid conditions. With a softening temperature of 130° C, it can tolerate higher temperatures. The drawback is that its optical transmittance is around 80 to 90% across the visible waveband, with a particular drop at shorter wavelengths.
Benefits of light pipes
Light pipes can be used to address many of the challenges presented by LEDs and their applications. Light pipes enable LEDs to be placed at a distance from point of use, making it possible to avoid board congestion and waste heat from other devices. The design team has much more flexibility with overall board layout.
Light pipes reshape LED output to optimize it for each use case. They can homogenize the output of multiple LEDs to eliminate hotspots and prevent fadeout, particularly if diffusing materials are added. Specialty light pipes have been designed to prevent light bleed, even from closely spaced indication lights.
Light pipes offer other benefits. Certain mounting styles offer ingress protection against dust and liquids, while others increase tolerance of shock and vibration.
Mounting options to serve challenging applications
The wide variety of light-pipe mounting options make it possible to find an ideal fit for almost any set of operating conditions. The key is to choose an approach that will meet the needs of the application in terms of protection without over specifying, which can increase cost and installation time.
Before we can delve into mounting options, we should review the ingress protection (IP) rating system defined by the International Electrotechnical Commission’s IEC 60529. It is expressed as a code in the form of IPxy, where x defines the ability of an enclosure to prevent the ingress of particulates and objects (see Table 2), and y defines its ability to protect against liquids (see Table 3).
Table 2: Protection against particulates and objects
| x | Protection Provided |
| 0 | No protection |
| 1 | Protected against solid objects > 50-mm (2-in.) diameter |
| 2 | Protected against solid objects > 12.5-mm (0.5-in.) diameter |
| 3 | Protected against solid objects > 2.5-mm (0.1-in.) diameter |
| 4 | Protected against solid objects > 1.0-mm (0.04-in.) diameter |
| 5 | Limited protection against dust (no harmful deposit) |
| 6 | Fully protected against dust (dust tight) |
Table 3. Protection against liquids
| y | Protection Provided |
| 0 | No protection |
| 1 | Protected against vertically dripping water for 10 min. |
| 2 | Protected against vertically dripping water for 10 min. when tilted 15° from vertical |
| 3 | Protected against spraying water for 5 min. when tilted up to 60° from vertical |
| 4 | Protected against water splashed from any direction for 5 min. |
| 5 | Protected against low pressure water sprayed from all directions for 3 min. |
| 6 | Protected against high volume jets of water from all directions for 3 min. |
| 7 | Protected against 30 min. of immersion in water to a depth of 1 m (3.3 ft.) |
| 8 | Protected against immersion in water to manufacturer-specified pressure |
| 9K* | Protected against high-pressure and high-temperature water jets |
*Defined by German standard DIN 40050-9
Common IP ratings for light-pipe applications include:
IP54: Dust and water resistant – protects devices against accidental splashing and all but the finest dust. This is a good rating for a device that may be occasionally splashed with liquids under unusual circumstances. Having IP54 protection should be considered a failsafe rather than a response to continued operating conditions.
IP67: Dustproof and water resistant – protects devices against short periods of immersion at low pressure. This is a good rating for a device that will have limited exposure to the elements, such as an outdoor device located under an overhang or an automated forklift moving inventory from outdoor storage to indoor shelving.
IP68: Dustproof and waterproof – details are determined by user and vendor. This is a good rating for devices that will be subject to harsh weather. It can also survive punishing indoor conditions, including large volumes of water but at low pressure. It cannot tolerate high-pressure washdown, however; that is the province of IP69K, which is not discussed here, since it’s typically only used for food and beverage production.
Light pipe mounting styles can be classed as panel mount (attached by the panel) and board mount (attached to the board).
Panel mount light pipes
Panel mount light pipes maintain position as a result of their contact with the front panel. They do not touch the LED or the board but maintain position just above it. They can be divided into front mount and rear mount types.
Front mount
Front mount light pipes are inserted from the front face of the panel. A flange provides a stop to prevent the light pipe from going all the way through. Because these light pipes are installed from the front, they can be put in place after the PCB, maximizing the flexibility of the assembly process. Front mount light pipes are available with various types of output faces, including flat, Fresnel lens, hemispherical dome, and parabolic dome, depending on the viewing angle desired.
Front mount light pipes are available offering various degrees of ingress protection. Here are two examples:
IP54: The simplest versions of front mount light pipes are built with crush ribs that deform upon insertion to provide a friction fit. Adding a sealing gasket between the flange and the panel can give it an IP54 rating, although it will not be able to tolerate shock and vibration (see Figure 3).
![Figure 3: Adding a sealing gasket between a friction fit light pipe and the panel can provide IP54 protection. [Source: Bivar]](https://www.bivar.com/wp-content/uploads/2023/05/figure-3-pp1.png "figure-3-pp1")
IP67: A threaded light pipe with a sealing gasket on the front side and a washer and hex nut on the backside can raise ingress protection to IP67 (see Figure 4). This type of light pipe system would be suitable for an EV charging station installed under a shelter or an irrigation controller inside a greenhouse.
![Figure 4: Adding a washer and threaded light pipe/nut can increase protection to IP67. [Source: Bivar]](https://www.bivar.com/wp-content/uploads/2023/05/figure-4-pp1.png "figure-4-pp1")
The hex nut also provides some degree of vibration resistance; more robust versions incorporate locking washers.
Rear mount light pipes
Rear mount panel mount light pipes are inserted into the panel hole from the rear. They are friction fit with a retaining ring to limit travel. The crush ribs provide a degree of stability but can’t withstand significant axial force. As a result, rear mount light pipes are best used with graphic overlays, for example to illuminate a logo or an access keypad. They are not designed to provide any ingress protection or resistance to shock and vibration.
Hybrid mount light pipes
Hybrid mount light pipes combine a rear mount light pipe with a front mount press-fit lens cap to deliver an IP68 rating (see Figure 5). An adhesive sealing gasket provides a tight fit around the panel hole. The lens cap features flexible tabs that compress when the cap passes through the aperture and then spring back once in place to put the gasket under compression. As long as the panel thickness meets tolerance, the lens cap will deliver the rated performance. The rear mounted light pipe is stabilized by the standard friction fit and the lens cap protects it from axial force.
Sample applications include exterior door access keypads, solar-array controls, and unprotected outdoor EV charging stations.
![Figure 5: Hybrid mount light pipes combine IP rated front mount lens caps with press-fit rear mount light pipes to achieve ratings as high as IP68. [Source: Bivar]](https://www.bivar.com/wp-content/uploads/2023/05/figure-5-pp1.png "figure-5-pp1")
Board mount light pipes
Board mount light pipes connect to the board using an adapter that encloses the LED, adding stability, protecting the device from damage, and preventing light loss. The adapter connects directly to the board. Once it is in place, the end of the light pipe is pushed into the adapter well for a simple press fit. As a result, the technology can be used with both rigid (front mount) and flexible (front mount, rear mount, and hybrid) light pipes. An internal flange limits the travel of the light pipe to maintain a safe distance (see Figure 6).
![Figure 6: Board mount light pipes attach to the board using adapters; note the internal ledge that limits travel to maintain a safe separation between light pipe and the LED. [Source: Bivar]](https://www.bivar.com/wp-content/uploads/2023/05/figure-06-pp1.png "figure-06-pp1")
Board mount light pipes offer greater resistance than panel mount devices to shock and vibration. Press-fit adapters are attached to the board by inserting posts into pre-drilled holes on the board; posts can be solid with crush ribs, for regular retention, or split prong, for additional stability. Solder-on adapters consist of three elements: A mount, a surface mount device (SMD) LED attached to the board via reflow soldering, and the adapter that interfaces with the light pipe (see Figure 7). The three are supplied as a single component that is attached to the board during assembly. Solder-on adapters provide a very firm connection for applications with higher levels of shock and vibration.
![Figure 7: Exploded view of a solder on adapter (left) shows the three individual elements that are combined into the single device supplied to the customer (right). [Source: Bivar]](https://www.bivar.com/wp-content/uploads/2023/05/figure-07-pp1.jpg "figure-07-pp1")
Because board mount light pipes can be used with various types of mounts, they complement the increased robustness with various levels of ingress protection, making them good solutions for applications that combine harsh environments with shock and vibration.
Sample applications include indication lights and control panels on factory AGVs and door access panels on trains and windmills.
Board mount light pipes offer another big benefit, which is the ability to control light bleed. Because the adapters enclose the LED, board mount light pipes virtually eliminate bleed over, murky colors, and enclosure glow. They are available as single devices (flexible light pipes and rigid light pipes) and as arrays (rigid light pipes only; see Figure 8).
![Figure 8: Board mount light pipes are available as arrays to prevent light bleed among closely spaced indication LEDs. [Source: Bivar]](https://www.bivar.com/wp-content/uploads/2023/05/figure-8-pp1.png "figure-8-pp1")
What to know about your application to specify a light pipe system
Knowing the various design options is just the start – that information needs to be put to use during specification. Start by considering the design constraints and the conditions of the application, then use that information to guide the specification process.
The wavelength of the LED: Polycarbonate has greater losses at shorter wavelengths, so if the application uses blue or UV LEDs, consider whether you can use PMMA, instead.
Output distribution of the LED: If the LED has already been selected, then the light pipe diameter needs to be sized appropriately to maximize input coupling. This is particularly important for rigid light pipes, which need to be both white enough to capture most of the LED output and long enough to cover just above the LED. Not all LEDs are ideal for use with light pipes. To optimize performance, footprint, and cost, select the LED and the light pipe at the same time to ensure that the LED has the best output distribution for coupling into the light pipe.
Distance from LED to point of use: Rigid light pipes are excellent for short-distance light transmission. The generally accepted maximum is 3 inches. Beyond that length, choose a flexible light pipe. Below that length, either rigid or flexible light pipes can work, so choose the type that provides the best fit for overall requirements. For example, both rigid and flexible light pipes are capable of redirecting light, flexible light pipes are better options for paths with multiple turns or tight turns.
Environmental conditions: As we’ve already discussed, a wide variety of options exist to tailor light pipe systems for environmental conditions.
Contamination – Be very specific with your vendor when discussing factors affecting IP ratings. Remember the difference between unexpected events and ongoing operational conditions – you can make the difference between a product that fails frequently and one that delights your customers for years.
Temperature – Polycarbonate has better heat resistance than PMMA.
Shock and vibration – Board mount light pipes provide the best resistance to shock and vibration. For applications requiring ingress protection, look for threaded designs incorporating locking washers.
Tips for success
Start light pipe design early in the process. Although it seems like just a support technology, it’s still crucial to your operation. Just as important, it can improve other aspects of the design. The right light pipe, for example, can add freedom to board layout. Selecting the LED and light pipe at the same time is more likely to result in the desired output.
Be sure to meet the parameters and tolerances on the data sheets. Only by following the numbers can you expect to achieve the promised performance.
Consider integrated products. Complete LED illumination subsystems arrived preassembled, reducing assembly time, scrap, and bill of materials.
Although it may require a bit more lead time, custom subassemblies are simpler to execute than you might think and may ultimately be a more efficient and cost-effective solution.