LEDs are an energy efficient and long lasting source of light. However, they create hot spots of focused light, which are very intense and bright. Hence, diffusers need to be used for an even distribution of light. This article focuses on the various materials used in light diffusers and on how to choose between them.
By Baishakhi Dutta
The light from LEDs can be harsh to the human eye because of the bright concentrated LED hot spots, which could be jarring when lighting larger spaces. To create effective luminaires using LEDs, lighting manufacturers use multiple LED point sources. But creating a soft glow or reducing glare is important, for which diffusing or light blending technology is used.
Light diffusion is a technique whereby the brightest light near the edges of the bulb is transmitted to the areas of the light box that have the least amount of light. Through this method, an even distribution of light takes place. To achieve this, a diffuser is used. A perfect diffuser is one that creates Lambertian scattering so that the radiance is independent of the angle. With applications ranging from projector screens to commercial lighting, it becomes tough to decide upon which type of diffuser is best suited for you.
For aesthetic and effective optics design, glass and transparent plastics (especially acrylic resins) have been long standing favourites. With the shift towards LEDs, lens and cover producers have focused on developing suitable plastic solutions.
Types of light diffuser solutions
The traditional method of diffusing light is to position a frosted glass in front of the light source. But glass has a very high absorption-coefficient, which blocks a large amount of the LED light output. Apart from that, glass is heavy, increasing the weight of the whole product. It also does not possess the required toughness and is prone to breakage.
Nowadays, manufacturers are adopting new ways to develop cost-effective diffusers entirely from plastic. The types of diffuser solutions that dominate the market these days are:
- Diffusers for panel light tube covers
- PMMA optical diffusers
- Three-sided diffusers
- Transparent diffusers
- Satin diffusers
- Anti-glare diffusers
- Opaque diffusers
- UV stabilised diffusers
Types of diffuser materials and their benefits
LED diffusers generally come with two types of surface finishes—opal (milky) and frosted (translucent). The distribution of light through frosted diffusers is good but the LED light is quite visible. Opal diffusers give better results than frosted options—the light is more uniformly distributed and the LED is well concealed. LED diffusers are available in a variety of forms like UV-tolerant films and rigid sheets. Also, they are manufactured using various high quality plastic materials. The different types of diffuser materials and their benefits are listed below.
Polycarbonate (PC): This is a thermoplastic that has been customised for use in LED parts like reflectors, lenses and, most importantly, diffusers. PC components can be thinner and lighter than glass, and the material offers flexibility in design. Polycarbonate can be moulded in the form of sheets and films which can be an excellent choice for diffusion of LED hot spots.
It is almost unbreakable, hence scoring over glass because of its high strength and elastic coefficient.It can resist extreme temperatures.
- As it is more durable and impact-resistant, it has a longer life span making it cost-effective.
- It provides a soft, diffused light.
- It is very light and thin to handle during manufacturing.
- Can be used to create complex shapes like curved light diffusers.
- Available in UV stabilised grades for outdoor use.
Acrylic or polymethyl methacrylate (PMMA): If you want a diffusion solution with fire-resistant properties, then polycarbonate is the best choice, but if you are looking for optical properties and the highest level of illuminance, acrylic or polymethyl methacrylate (PMMA) is the right choice. PMMA has superior optical properties compared to PC. It offers good light transmission and is easy to fabricate.
- It has superior optical properties and illuminance.
- It has the highest light transmission.
- It is naturally UV stable, and this property can also be enhanced.
- It has excellent weather resistance and good high-temperature performance.
- It provides design flexibility.
- An acrylic plate has excellent weatherability and high surface gloss.
- It is low cost and easy to mould.
Polypropylene: Polypropylene (PP) is a thermoplastic made from a combination of propylene monomers. It is used in a variety of applications like packaging for consumer products, plastic parts for various industries and light diffusers. It is a new material used in the light diffuser industry and needs more time for it to be tested.
- Its toughness is similar to that of PC.
- Its light transmission is equivalent to that of PC, and can reach about 85 per cent.
- Less dust absorption than PC.
- Production operations are relatively simple.
Polystyrene: Polystyrene (PS) is a naturally transparent thermoplastic. It is available in the form of solid plastic as well as rigid foam material. It is used in a variety of consumer products and light diffusion solutions. But this material is controversial as it is slow to biodegrade.
- It is very low on cost.
- It is wear resistant and scratch resistant.
- Its chemical stability is good.
- It has good surface anti-static properties to prevent dust adsorption.
- It provides good optical transmittance and strong refraction.
Selecting the right polymer
When selecting thermoplastics for a light-diffusing cover or lens, transparency becomes a pivotal criterion. In its natural form, the polymer must be highly transparent to minimise the loss of LED output during the light diffusion process. While the number of appropriate engineering polymer solutions are by themselves limited, considering just cost, polycarbonate (PC) and acrylic (polymethyl methacrylate or PMMA) stand out.
Polycarbonate or acrylic: The choice between PC and PMMA is a close call as both are very similar in price, processing convenience and optical properties. The final choice depends on the application requirements. Table 1 compares the properties that manufacturers and developers should consider when selecting between the two polymers.
Prioritising which properties are important for your application will make the selection process easier and more accurate for manufacturers and developers. Acrylic will be a good choice if optical properties are of high priority and the illuminance level (lux units) needs to be as high as possible as well. On the other hand, if the application requires fire resistance because of the high risk of fire, polycarbonate will be the only choice.
Modification of polymers
The good news for designers is that whether they choose acrylics or polycarbonates, they will not be stuck with all the drawbacks of the selection they make. These polymers can be modified to improve their capabilities and reduce their limitations. Here are some modifications possible for acrylic and polycarbonate polymers.
In the case of acrylic, the mechanical properties can be altered. Specifically, the low-impact property can be improved in two ways. In the first method, the lens or cover can be designed with thicker walls. The second option is to use high-impact PMMA grades, in which case, besides the extra costs, there is some tradeoff in terms of lower light transmission.
Polycarbonates are capable of undergoing several types of modifications. For instance, for an ultraviolet (UV)-sensitive product, the poor UV stability of polycarbonates can be largely improved by introducing UV absorbers (UVAs) into the bulk material. UV stability will be directly proportional to the level of UVA used. In case a cover is produced from a PC sheet, UV protection can be enhanced in a cost-effective and efficient manner by using a co-extruded layer of PC enriched with UVA. Scratch resistance can also be improved via special coatings applied in a second-stage process. The automotive industry implements this process heavily in the production of PC lamp covers.
Polycarbonates are, by far, better in terms of fire resistance. Further improvements of flame-retardant properties may be required in some applications, especially when the product must comply with fire standards, for instance, in the European railway standard (EN 45545-2) that covers all the parts used in a train. For such applications, a special grade of PC, which contains flame-resistant additives, should be used by lighting suppliers for the manufacture of LED covers and lenses.