Designing and fabricating LED light pipes

Designing and fabricating LED light pipes

When you see an LED indicator on a computer or piece of machinery, chances are you are seeing light that has passed through an LED light pipe. Light pipes transmit light from a light source such as an LED to an output area where a user can see it.

Light pipes are not widely discussed, but they are used in a range of industries, from consumer electronics to aerospace. They have many benefits, including clearer lighting and flexibility when it comes to the design of a part’s housing or interface. This means that many product designers incorporate light pipes into their products.

This article looks at the basics of LED light pipes: different types of light pipes, how to make light pipes, and the best materials for light pipes.

What is a light pipe?

An LED light pipe — sometimes called a “light tube” or “light guide” — is an important component in many electronic devices. It is a clear plastic tube through which light travels between a light source (such as an LED) and a user interface (such as an ON/OFF indicator).

Note that “light pipe” can also refer to much larger components that function in a similar way. A large light pipe may be used to transmit sunlight to an underground parking lot, for example.

In electronic devices, which are the focus of this article, light pipes exist so that the light source does not have to be located next to the user interface. Why is this helpful? Well, the simplest and most cost-effective placement for an LED is at one end of a PCB, but the PCB may not be located next to where the light needs to end up — by a switch on the device’s housing, for example.

By using a light pipe, engineers do not have to install a LED mount at the output location (which is fiddly and time-consuming), nor do they have to compromise on the placement of either the light source or the output; they can simply transmit light from one place to another using a practical and low-cost component.

Light pipes can be made from different clear materials and manufactured in different ways. They can be straight, angled, or curved, and they can also be rigid or flexible.

Light pipe uses and applications

Where are light pipes used? Examples of LED light pipes can be found in a surprising number of places. Light pipe applications span many industries, including consumer products, aerospace, industrial, and medicine.

Light pipes can be seen in computers, communication devices (WiFi routers, telephones, radios), remote controls, industrial machinery, medical devices (life support equipment, bed adjustment controllers), smoke and security alarms, cash machines, aircraft and automobile components, toys, and many other devices.

Light pipe advantages

There are several benefits to using light pipes (as opposed to directly mounting an LED to function as an indicator). These include:

  • Simple installation: Many light pipes are designed to simply snap into place
  • Control over light visibility: Light pipes can be modified to provide optimal brightness
  • Focuses light: Light pipes prevent light from escaping to other areas of a component (providing unwanted glare on a screen, for example)
  • Reduces number of LEDs required: Light pipes can transmit light from one light source to multiple outputs, helping to cut costs and improve part reliability
  • Added safety: Light pipes allow product designers to keep electrical components like LEDs within the housing of a part, reducing the chances of electric shock for the user

Light pipe design

Different light pipe applications require different types of light pipe. Light pipes can be rigid or flexible, and they can have different shapes: straight, angled, or curved.

Nonetheless, all variants share some important light pipe design principles. These include:

  • Matching the internal surface of the light pipe with the surface of the LED: concave, flat, or smooth
  • Using round edges inside the light pipe to minimize light leakage
  • Using a diffused surface at the output to evenly distribute light
  • Adding light-reflecting paint or a mirrored surface to the outside of the light pipe

Placement of the light pipe is also important. In general, shorter distances between the LED and the output results in less loss of light and superior brightness. But if the light pipe is too close to the LED, it may be affected by the heat generated.

More specific design choices depend on factors such as:

  • Shape of part housing
  • Distance from light source to output
  • Obstacles between light source and output
  • Required brightness
  • Number of outputs/indicators
  • Mounting methods
  • Indoor/outdoor use, high/low temperature
  • Budget

Rigid vs flexible light pipes

Light pipes can be either rigid or flexible. Rigid light pipes are ideal for maximizing brightness over a short distance, while flexible light pipes are suitable when the PCB and output are further away from one another or placed at awkward angles. Rigid pipes are typically used with press-fit, surface-mounted, or PCB-mounted LEDs.

Flexible light pipes are usually made from a flexible polycarbonate fiber, while rigid light pipes can be made from a few different materials.

Rigid light pipes

Flexible light pipes

Advantages

Disadvantages

Advantages

Disadvantages

Low cost

Limited distance

Transmit light around bends and through tight spaces

More expensive

Single component

Cannot bend at nonstandard angles

Can cover longer distance

Extra mounting component required

Ideal for straight or right-angle connections

Allows designer to adjust placement of output

Light pipe materials

Because light pipes are partially transparent, there is a limited number of materials that can be used to make them.

The main materials are:

  • Polycarbonate (PC): PC can be rigid or flexible, is heat-resistant, and provides good light transmission
  • Acrylic (PMMA): Acrylic has excellent optical properties
  • Photopolymerizable resins: Clear resins for stereolithography (SLA) 3D printing can be brittle but offer properties comparable to molded plastic

Light pipe manufacturing

There are multiple light pipe production techniques. Although typically made via injection molding, light pipes can also be 3D printed — this is ideal for custom light pipes and prototypes.

Injection molding is an excellent manufacturing technique for fabricating larger quantities of molded light pipes. It produces the best surface finish and therefore delivers good optical qualities; it also works especially well with acrylic.

3D printed light pipes have their own advantages. 3D printing (usually a vat photopolymerization process like SLA) enables on-demand printing of one-off custom parts at very low cost. It shortens lead times and accelerates time-to-market. Although it cannot match molding for optical quality, it is excellent for light pipe prototypes.

3ERP is a prototyping and low-volume manufacturing specialist with expertise in molding and 3D printing. Request a quote for your next light pipe order.