Light Pipe Design GuideEverything You Need to Know About Light Pipes
LED Light Pipe Design Guide
Everything You Need to Know About Light Pipes (and Then Some)
But first, what the heck are light pipes?
How do light pipes work?
Light Pipe Brochure
Why use a light pipe?
Similar in name, but not the same:
A quick lesson in illuminated component terminology
Light pipe design principles:
Let’s begin with the benefits
Greater flexibility and control over
Minimized light leaks
Easily customizable solutions
Reliable, uniform performance
Light pipe characteristics that count
- Rounded corners – leveraging rounded edges inside the light pipe minimizes the amount of light that can escape before shining outside of the exit surface. A minimum radius of 0.5mm is recommended.
- Smooth, flat, or concave contoured surface – whatever matches the surface of the LED needs to be happening on the interior of the light pipe on the side where the LED is inserted (the input surface).
- Diffused exit surface – using a diffused surface where the light is visible at the exit will provide a uniform illuminated surface.
- Smooth, mirrored surface – to avoid light loss, a smooth or mirrored surface can be added to the light pipe’s exterior.
- Light-reflecting paint – for extra credit (and brightness), an optional coat of paint can be applied to the outside of the light pipe to aid in amplifying the light.
The physical features of your light pipe are only a fraction of the design decisions that will need to be made during the design phase. Throughout this guide, we’ll cover additional considerations and the science behind them when choosing the ideal light pipe for your needs.
Light pipes vary in more than just color
Rigid light pipe design guidelines
- Ideal for vertical or right-angle orientation
- Great for board-mount, press fit and surface mount LEDs
- Bend light around a fixed corner
- Available in right angle, 1-position, 2-position and 3-position
- Designed to transmit light short distances
- Length cannot exceed:
- Press fit: 1.20”
- Custom design: 1.5”
- Standard design: 2”
One additional design consideration for rigid light pipes — face shape
- Longer light pipes perform best with smooth light pipe faces or surfaces.
- Scattered or frosted textures, like a Fresnel lens help diffuse or soften the light.
Common shapes for rigid light pipe faces include:
Flexible light pipe design guidelines
Pros of flexible light pipes
- Carry light around curves
- Carry light around tight spaces
- Carry light longer distances
- Provide greater control over light optics
- Greater design flexibility
Cons of flexible light pipes
- Typically cost slightly more than rigid light pipes
- No length limits per se, but the longer the light needs to travel, the more will be lost along the way
- An extra part is required to keep the flexible light pipe-end close to the light source
What are light pipes made of?
Acrylic light pipes
Polycarbonate light pipes
- Type of light being used
- Flammability requirements
- Outdoor usage
- Vibration and moisture needs
- Operating temperature range
- Board, indicator and indicator orientation being used
For help identifying the best light pipe material for your application
One product, so many mounting types to work with
If flexible and rigid were the only options, your options might be limited. Light pipes are designed to accommodate a wide range of mounting types.
In addition to the rigid vs. flexible decision, there are several other considerations when choosing a light pipe for your design. Your power source and overall design footprint can dictate which mounting type you should use with your light pipe.
As the name implies, panel mount light pipes are those that are attached directly to the control panel or user interface.
They are attached to the panel in two ways:
- Crushable ribs that “crimp” closed
- A plastic or metal retainer ring to seal the exit surface
On the same end where the LED is attached to the board, the light pipe can also be attached to the PCB or surface. The LED fits inside the light pipe and is also attached to the PCB.
Board-mount light pipes can be rigid or right-angle. Rigid light pipes do not bend like their flexible counterparts.
When 90-degree angle alignment to the LED or PCB board is crucial, right-angle light pipes are mounted to the board directly over the LED. Right-angle light pipes are available in multiple board-mount options, including:
Board mount light pipes are designed for streamlined production and assembly. The PCB board can be printed in one line, the panel in another, and then they can be machine-assembled, saving time and costs vs. manual labor.
For high-vibration applications, the panel can also be secured to a wall for additional protection.
If your design includes a board- or panel-mount light pipe, you’ve got additional options to work with.
Flexible light pipes are also flexible in how they’re mounted — they work well with both panel and board mount designs. These light pipe types are ideal for devices where light needs to bend around corners and other obstacles.
An LED adapter attaches the LED and the flexible light pipe to the panel or board.
- Rigid or flexible?
- Panel mount or board mount?
- Right angle or not?
Light pipes are available in many LED mounting types, too
Light pipes in real-world applications
Rigid LED Light Pipes
Personal jetpack requires rugged, real-time communication
Portable device chargers turn the power up on productivity
Audio equipment combines analog looks with high-tech LEDs
Flexible Light Pipes
A parking meter that’s one smart cookie
A word about application environment
What to consider when choosing a light pipe
- How many indicators are needed?
- How far apart will the power source and LED be?
- How will the light pipe be mounted?
- How far does the light need to travel? Around curves and angles?
- How much room on board or panel?
Outdoor and high-moisture environments
Programmable Power Unit
Storage and operating temperatures
Light pipe position
Refractive Index: Medium Matters
Light doesn’t always behave the same way, which makes light pipe design that much more complex. Why?
Let’s go back to physics class and learn about refraction.
Refraction is defined as the change in direction of a wave from one medium to another or from a gradual change in the medium itself.
So, in addition to all the other variables that go into light pipe design (size, shape, brightness, distance to LED, viewing angle, and material used, to name a few), designers must also consider the Refractive Index.
The Refractive Index is used to determine how fast light travels through a medium.
Calculated as n = c/v, c is the speed of the light and v is the velocity of the light in that specific medium.
This number represents how much of the light is bent or refracted when it hits the medium. It also calculates how much light is reflected when reaching the medium.
In a vacuum, light breaks at a rate of 1.0. But no one designs light pipes to be used in this type of vacuum, so additional consideration must be given to optimize performance.
Plus, any number above 1.0 shows the decreased speed at which it travels compared to in a vacuum.
Sample Refractive Indices:
In air, the rate is 1.003
In water, the rate is 1.33
In acrylic — which is what most light pipes are made of — it’s ~1.49.
Yet, Refractive Index isn’t the only number that needs to come into play when it comes to how a surface affects light.
There’s also total internal reflection (TIR), which is the angle at which light is reflected when none of it shines through to a surface exit, instead it is reflected from one surface to another where it is visible.
TIR is a phenomenon that occurs when the angle of incidence is greater than a certain limiting angle, referred to as the critical angle.
Why does TIR matter in light pipe design?
Often, light pipes are used to guide light around corners. To help ensure that the light bounces at the critical angle and travels to the endpoint with maximum effectiveness, understanding the critical angle of various materials is essential.
For clear polycarbonate, the critical angle is 39, and for acrylic, it’s 42.
Understanding how the light reacts in different scenarios will lead to better, more optimized light pipe performance.
For example, when using a right-angle light pipe, here’s how TIR would affect the specifications:
When designing light pipes, most people assume light behaves like electric current and ignore TIR.
Because light has unique properties, designing a light pipe like the right-angle illustration shown above will result in significant light loss at the TIR. To avoid light loss, incorporate the critical angle to create a geometry to guide the light around the critical angle without having the light escape. This concept also applies to light pipes with round corners (or any geometry your design includes).
Minimal Light Loss = Design Win
Light pipes are designed to carry light from Point A to Point B as effectively as possible.
Point A: Where the input or light source is located. In most cases, it’s the surface-mount (SMD) LED.
Point B: The exit surface, or where the end-user sees the light at the interface or indicator panel of the device.
Light loss, or light bleed, occurs when some of the light traveling from Point A to Point B escapes or shines elsewhere, so less light is available to shine at the exit point.
To minimize the amount of light loss, designers can use an LED with a narrow viewing angle instead of one with a wide viewing angle to help close the distance between the light pipe and LED where light could escape. The wider the gap between the light pipe and the LED, the more room for light to escape before it enters the light pipe and travels to the exit point.
Note: there will always be some level of light that gets lost traveling from where the LED is located and the exit point where the end-user sees the light.
Good, Bad and Ugly: Light Pipes Are Everywhere
Light pipes are used in many devices and applications because of their design flexibility. However, not all devices are using optimized light pipe design.
Here are some examples of light pipes:
Notice any similarities?
Devices with good light pipe design have the following traits:
- Uniform illumination
- No hotspots
- Ideal visibility for application
Devices with less than ideal light pipe design yield bad/ugly results:
- Hot spots
- Light loss
- Difficult to see in the operating environment
Custom LED light pipe design
Before you enlist the help of an engineer for a custom light pipe, you’ll need an idea of the following:
- Number of colors needed in LED
- Number of indicators on device
- Specifics on working environment
- Desired illumination intensity
- Desired viewing angle
- How far away the light indicator needs to be viewed
- Do you need help with design and production, or just design?
- Estimated annual usage of device (EAU)
- Budget and timing
- Access to design files and lighting specs (existing designs only)
Light Ray Simulation
How does the custom light pipe design process work?
- A team of engineers collaborates with OEMs and designers to establish goals and optical requirements
- VCC uses this input to design a custom light pipe or PCA assembly.
- The team conducts ray tracing simulations using the latest light software to determine exactly how the light pipe will perform. The design is then revised and optimized until the ideal optical requirements are met.
- Once the light pipe specifications have been perfected, we use 3D printing in-house to create a functioning prototype. The prototype is tested via light performance study and any mechanical, electrical and optical changes are made as needed.
- When the design has been perfected, the light pipe moves into production where it will be ready for assembly within a few weeks.
The whole process can take as little as 2 weeks or as long as 12 — it all depends on the complexity of the design. What are some examples of custom light pipe designs?