The Clear Choice: How to Choose the Best Transparent Material
There are many transparent materials out there for you to consider when designing a lens for a lighting application. Take a look at the image above; these different transparent materials all look fairly similar don’t they? You may be asking yourself “What makes glasses and plastics so different? Which material is right for my lens?” To answer these questions, you need to look not only at the material properties but also consider everything from your lens’ operating environment, the transmission requirements of your application, to the durability needs and expected lifetime of your part.
To help make your selection a little bit easier, in this article we compare some of the mechanical, optical, and thermal properties of three common transparent materials—borosilicate glass, polycarbonate, and optical silicone—and discuss how they may perform in different environments.
The table above contains generalized properties that are representative of common borosilicate glass, polycarbonate, or optical silicone materials. Keep in mind that properties will vary depending on the specific type and composition of the material that you select, and can often be tailored to meet the requirements of your application. In this article, we’ll discuss this generalized data, but in the near future, we will publish several detailed comparative studies that examine specific material compositions.
Mechanical Property Comparison
In harsh, demanding environments, lenses and other components are subject to mechanical stresses that demand a material with high strength, rigidity, and durability. Borosilicate glasses and polycarbonates are both strong materials. They both can withstand above 60MPa of tensile pressure before failing; while optical silicone fails at lower pressures of around 10MPa. The rigidity of the three materials, indicated by their Young’s or Elastic Modulus, is varied. Glass is a rigid, elastic material, which means that it will not deform out of shape permanently in application even when loaded with stress or pressure; however under severe conditions, it can fail through instant crack propagation and subsequent shattering. Polycarbonate has a smaller elastic modulus than glass, but it is still fairly rigid while optical silicone is a comparatively more flexible. Both polycarbonate and optical silicone, unlike glass, are plastic materials that can undergo permanent deformation when overloaded. After that point, both materials will fail, with polycarbonate fracturing and silicone tearing.
Oftentimes in an application, your transparent material will need to withstand mechanically or chemically abrasive conditions with minimal surface degradation or transmission loss. Borosilicate glass can withstand mechanical and chemical abrasion while maintaining high levels of light transmission. Uncoated polycarbonate and optical silicone are less resistant to mechanical abrasion, and these polymers have varying levels of chemical resistance. For instance, they can withstand environments including water, humidity, and various alcohols, but they will degrade in many chemicals such as oils, hydrocarbons, and ketones. It’s important to check the chemical compatibility of the material with its operating environment at the beginning of the design phase.
Thermal Property Comparison
It is often necessary to consider thermal properties when choosing a transparent material for your application. For example, if your lens design incorporates a variety of materials, such as coatings, adhesives, sealants, or metal fixtures, you might need to take into account the different coefficients of thermal expansion (CTE). Borosilicate glass, polycarbonate, and optical silicone have differing thermal expansions. Glass expands the least, with a CTE of 43E-7/°C, while silicone has the largest CTE of 2750E-7/°C, which means it will expand much more with temperature changes. You need to be careful not to mismatch materials of significantly different CTE’s, because the differences, when heating or cooling, can induce stress and potential breakage in your lens.
You will also need to know the operating temperature range for your lens. Borosilicate glass can maintain its shape and optical properties over a large temperature range, surviving temperatures above 400°C. Polycarbonate begins to degrade at 145°C while some silicones can be used at temperatures above 200°C. Heating a material above its recommended maximum operating temperature can negatively affect your application in a number of ways. First, the material can soften and slump, which reduces the effectiveness of lens prisms and other optical design features.
Exposure to excess heat can also discolor many plastics, including polycarbonate and silicone. This results in a yellow appearance and a reduction in the transmission of the lens. Be aware of the thermal limits of each material you are considering—and the application’s operating temperature—before making a final selection.
Optical Property Comparison
When you are choosing between transparent materials, you typically have a specific optical performance in mind for your lens, which could include a targeted chromaticity or minimum transmission requirement. In many respects, the three materials considered here are optically similar. Borosilicate glasses, polycarbonates, and optical silicones all exhibit excellent light transmission throughout the visible region. When designed for optimal optical output, the internal transmittance can be near unity and the external transmission values are typically above 90% with any loss being caused by surface reflections. Additionally, specific colorants can be added to each material to adjust the light output and to produce a wide chromaticity range.
The difference between the materials becomes apparent in the ultraviolet region. Polycarbonate is a poor UV-transmitter, with low transmission in the UV-A range (315-400 nm) and no transmission at UV-B (280-315 nm) or UV-C (100-280 nm) wavelengths. Optical silicone transmits well in the UV-A and UV-B regions but has more limited transmission in the UV-C region and can even degrade with UV-C exposure. Borosilicate glass compositions can be designed to either transmit UV light (as far down as UV-C) or to absorb the UV light at targeted cut-off wavelengths, depending on the application.
The optical properties of polymers and glasses can degrade with prolonged UV exposure, especially in the UV-C region. You’re probably familiar with car headlight lenses yellowing over time; in addition to this visual change, transmission is also decreased. UV-C light can be damaging not only to polymeric and glass materials, but to people. In general lighting and some medical phototherapy applications, you may need to use a material that will absorb UV light at specific wavelengths. In contrast, some applications, like UV curing, require high UV-C transmission.
The optical performance of a material can also be affected by abrasion and heat exposure, as discussed in the previous sections. It can be critical, then, to choose a material with good optical, thermal, and mechanical stability to prolong the operating lifetime of a lens in a harsh environment.
A Look at Different Environments and Applications
In the paragraph below, we describe several examples that demonstrate how these materials are commonly used. This is by no means a definitive guide to what materials you should use but serves to illustrate how some materials may be better suited for specific applications.
Borosilicate glasses are the preferred material used in aerospace lighting applications like aircraft wingtips or runway lighting because of their ability to withstand impact and abrasion with minimal degradation. They can be molded into Fresnel lenses for spotlights that direct light and withstand the high temperatures of the bulbs without deforming. And molded optics made from UV transmitting borosilicate glasses can improve UV light distribution and working distance for industrial curing applications.Polycarbonates are commonly used for automotive headlights. Their low weight, low cost, and high transmission in the visible light region make them ideal for this high volume application. Silicones are often used for secondary optics with LED lighting applications. For example, a molded silicone lens can be used in street lighting both to seal the LED circuit board against moisture and to direct the output of the light.
General Guidelines for Selecting the Best Transparent Material
When it comes to transparent materials, there is no one-size-fits-all solution. Every lighting fixture has its own set of requirements and operating environment that will influence your selection. It is important to understand the temperature range, light output, and durability requirements for your optical lens. Once you know these operating conditions, you can choose a material that will best meet your performance needs.
When selecting a material, you should:
- Identify: Outline the operating parameters for your light fixture.
- Prioritize: List properties from most important to least important.
- Analyze: Be aware the advantages and limitations of each material.
- Communicate: Work with a manufacturer as early as possible. They will be able to help you select a material that meets your performance requirements while helping you to optimize your design for manufacturing thus reducing costs.
Having a better understanding of the properties of different transparent materials, including their advantages and limitations, will help you find the right fit for your application. If you would like to learn more about glass, read our three-part series that discusses the thermal, optical, and physical properties of glass. These articles will help you gain a better understanding of the relationship between these properties and their impact on product design.