Light pipes are structures designed for transmitting light from the LED source to the point of use. Although many characteristics are involved in specifying a light pipe, the choice of material is most important and generally least well understood. Here, we review the most common questions that our applications engineers encounter when working with customers to build a successful system.
What materials are used for light pipes?
Although light pipes can theoretically be made of glass, they are generally formed of optical plastics and resins, for reasons of weight. The most common materials used for light pipes are:
- • Poly (methyl methacrylate) (PMMA): PMMA is the default material for light pipes. It’s easy to mold and economical.
- • Polycarbonate: Polycarbonate is the backup material for certain applications in which PMMA won’t work.
- • Cyclic-olefin polymer (COP): COP is a good solution for certain applications with specialty requirements.
The table below presents key properties of select light-pipe materials. Please note, these are representative values only. Exact numbers vary from manufacturer to manufacturer.
Table 1: Key properties of select optical materials (representative values only)
|Properties||Poly (methylmethacrylate) (PMMA)||Polycarbonate||Cyclic-olefin polymer (COP)|
|Average Optical transmittance, (UV-near IR)||92||87-89||92|
|Working temperature (°C)||85-105||120||130|
|Water absorption (% per 24 hrs)||0.3||0.2||<0.01|
Is material performance wavelength dependent?
Yes. The optical absorption of optical plastics and resins varies as a function of wavelength. PMMA has an average of 92% optical transmittance from the UV to the near IR spectral regions, falling off at about 300 nm. Its transmittance drops slightly around 800 nm but still remains above 90%.
The transmittance of COP across the UV to near-IR spectral range is somewhat lower but still remains above 90%. In contrast, polycarbonate only achieves about 87% to 89% transmittance across the visible spectral region and falls off steeply above 400 nm. If you are designing a system for blue or UV sources, consider sticking with PMMA or COP for your light pipes.
Note that certain materials like COP may turn yellow under UV exposure.
What material should I use if my system operates in a higher temperature environment?
Although PMMA provides good overall performance, its glass transition temperature (Tg) is around 115°C for a working temperature of about 85°C to 105°C (exact values vary by manufacturer and composition; check with your vendor for more information). For a server in a data center, that’s not a problem but some industrial and aerospace applications can expose equipment to more extreme temperatures.
Polycarbonate provides a useful alternative to PMMA. It is more heat tolerant, with a Tg of up to 147°C for a working temperature as high as 130°C.
For very high temperature applications, COP has a working temperature of 135°C and proprietary optical resins likepolyetherimide (PEI) can operate at temperatures of 170°C or higher.
Which material is best for a high-humidity environment?
Light pipes maximize input coupling when placed within 1 mm of the LED. Water absorption can cause dimensional expansion, potentially putting the LED at risk. These conditions should be taken into account when selecting a light pipe material.
PMMA, our go-to material, has a water absorption level of 0.3% per 24 hours of immersion. Polycarbonate is slightly better at 0.2%. For high temperature and high-humidity environments, COP could be a better choice, with a water absorption level of less than 0.01%.
My application involves shock and vibration. Will this affect my choice of material?
Different materials have different physical characteristics. Although PMMA offers some scratch resistance, it tends to be more brittle and can fracture under hard shocks. Polycarbonate and COP are more durable.
When is adding a diffusing material appropriate?
Although LEDs do not emit the same type of collimated output as a laser, they can produce a forward-directed output distribution that could appear as a hotspot. Hotspots may be undesirable for some applications, particularly if the design combines the output of multiple LEDs. In these cases, the light emitted by the light pipe needs to be diffused. This can be accomplished mechanically by texturing the end face. In some cases, however, adding diffusing particles to the plastic or resin prior to molding provides a better effect.
Diffusing materials should be added with care. It’s important to remember that diffusion is scattered light, which equates to optical loss. This reduces the amount of luminous flux exiting the end face of the light pipe. Thus, there’s a balance between creating a homogeneous output and maintaining high enough output levels.
What can colored light pipes do for me?
Although light pipes are typically made of water-clear plastics, they can also be formed of colored materials (red, blue, green, amber, etc.). Colored light pipes can speed identification for assemblers and reduce human error. They may also be used for cosmetic purposes. A colored light pipe should not be used to tint LED output, since the emission of narrowband LEDs is already inherently colored.
What is the best material to use for a light pipe?
As with all things in engineering, there is no one perfect solution, just the best solution for your application. Start by making a list of your system requirements and operating conditions. Do your homework but also tap into the expertise of your vendor. Each type of optical plastic or resin is available on the market in multiple grades, each of which has different properties. Data sheets can be hard to decode (let alone hard to compare from material to material and vendor to vendor). The more informed you are, the more successful your system will be.
The team at Bivar has decades of experience working with light pipes and optical materials. Work with us to determine which light pipe materials will give you the best results.