Home Headlines For better light transmission, choose the right additives for light diffusers

For better light transmission, choose the right additives for light diffusers

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Learn more about the additives used in light diffusers. The right choice of material will ensure the product remains light in weight, eliminates hot spots and doesn’t result in eye strain.

By Deepshikha Shukla

LEDs are durable, energy-efficient light sources. However, designing LEDs can be challenging due to the creation of ‘hot spots’ of focused light. The glare of the LEDs can be harsh to our eyes, leading to discomfort. The glare of high-intensity lighting in the offices where people need to concentrate on their screens often leads to eye fatigue, headaches and stress. Replacing old, damaged light diffusers reduces this discomfort.

Understanding the properties of additives for plastic light diffusers enables LED lighting developers to choose the proper additives and production techniques to achieve the desired beam patterns. One of the main challenges in LED design is to create uniform diffused light while minimising the loss of light intensity. The diffusing agent should also be lightweight, and sustain both high and low temperature conditions. Finally, it should be economical to manufacture in high volumes.

Polycarbonate light diffusers
Acrylic film for light diffusion
Polycarbonate light diffusers


Materials for light diffusers

Special grades of acrylic sheets, polycarbonate sheets, and polycarbonate films are engineered to diffuse LED hot spots without sacrificing light transmission (LT). Acrylic and polycarbonate are lightweight, tough, plastic sheet materials with outstanding optical properties. These characteristics make them good choices for lighting applications, particularly for mass transit and aerospace, where durability, strength, and lightness are required. Other lighting applications are lighting fixtures, commercial lighting, menu boards, backlit signs, retail merchandising, lenses, and LED light diffusion.

Acrylic offers better light transmission, diffusion of LED hot spots, and easy fabrication into complex structures. Acrylic light diffusers are available in several different thicknesses. Their consistent thickness and uniformity of diffusion, lightness, shatterproof quality, impact- and scratch-resistance, as well as being UV stable and chemically inert structures make them perfect for outdoor applications including illuminated signs. UV stable acrylic will not turn yellow with age and helps reduce LED ‘hot spots’.

Polycarbonate (PC) is tougher, more durable, can operate at a higher temperature, and has low flammability. Polycarbonate sheets and film have good light transmission, diffuse LED hot spots well, and offer impact resistance along with good heat resistance. They can be cold formed into complex shapes and are available in UV stabilised grades that are engineered for outdoor use. Special grades of polycarbonate are modified to meet high anti-flammability requirements for critical applications like aircraft. These are also used for machine vision systems, for visual inspection cameras, and automated parts inspection lighting.

Choice of material
Polycarbonates and acrylics are very similar in price, in the way they are manufactured and with respect to their optical properties. Thus, a comparison needs to be done based on the application’s requirements. If the light is required for indoor use, the mechanical properties are not high priority; so acrylic is the best choice. If the illuminance level is required to be as high as possible, acrylic may be the right choice again. On the other hand, if the application requires fire-resistant properties because of building standards or the risk of fire, polycarbonate is the only choice.

Polycarbonate light diffusers

The best part is that designers do not need to compromise and suffer the drawbacks of their selection. Whatever is chosen, it can be modified as per the requirement. In the case of acrylic, the mechanical properties and impact properties can be modified. With polycarbonates, the UV stability can be largely improved by the introduction of UV absorbers into the bulk material. Scratch resistance can also be addressed. These improvements are particularly useful in the production of PC lamp covers for the automotive industry.

Polycarbonate light diffusers

Optical properties
Reflections occur when light bounces from a surface to the eye. If the surface is very smooth, like a mirror, the reflected light can form a very clear image. This is known as regular or specular reflection. Conversely, when light is reflected from a rough surface, the waves of the light are reflected in many different directions, so a clear image does not form. This is called diffuse reflection. The transmission of light occurs when light passes completely through matter. As light is transmitted, it may pass straight, it may be refracted or it may get scattered as it passes through.

Diffusion occurs when light passes through transparent media that contains small transparent particles. When the particles are encountered, the light may be refracted or scattered or both. The result is a dispersion of the transmitted light, which is also the mechanism of the light diffusion system.

The extent of light diffusion is based on many parameters. Light diffuser (LD) systems contain material that is translucent to light, and their transparency corresponds to the polymer. To increase the light diffusion properties, the LD additive concentration in the polymer has to increase. But this will reduce the intensity of the light emitted from a surface. Therefore, with any type of LD additive, the concentration must be optimised to account for the trade-off between LD and LT (light transparency).

Measuring transmission and diffusion parameters
Manufacturers and developers employ different methods to measure diffusion quality. Two parameters determine light diffusion performance—LT level and LD level. It is quite challenging to achieve a system with both a high LD level and high LT level, which is as close as possible to the source light level. Therefore, while testing the optical performance of the system, both LT and LD need to be measured.

ASTM D-1003 is a standard test method to measure haze and the LT of transparent plastics. A light transmission meter or haze meter is typically used to measure the light transmission of a plastic film or sheet. But this method is limited to the product development stage and cannot be used to measure the luminance level of a complete lighting system.

Haze is defined as the amount of light that is subject to wide-angle scattering, at angles greater than 2.5° from the normal. Haze measurements are also conducted according to ASTM D-1003. For medium-to-high diffusion levels, haze can be a good indication of the system’s performance. However, for highly diffusing systems, the haze value reaches a maximum, and at that level, the meter cannot distinguish between different levels of light diffusion.

Haze measurement using the ASTM D1003-00 standard test method (Courtesy: www.researchgate.net)

One of the most proficient methods to quantify light-diffusion performance is the half-value angle (HVA) test. This is based on measuring the light intensity at different angles to the incident light that is vertical to the object surface. The HVA measurements are conducted with a goniometer. With appropriate detectors, the goniometer measures the light scattering intensity at different angles. The advantage of the HVA test is its sensitivity, which enables it to distinguish between systems with high light diffusion levels.

The light diffusion power (LDP) test is also a good way to characterise diffusion properties. The LDP methodology is similar to the HVA method—the main difference being that the sample is illuminated at an incidence angle of 45°, not 90°. Among manufacturers of sheets, profiles, and lenses, the most common and practised test is the hot spot test. This is a quality test that does not rely on any formal standard. It is the most straightforward test that directly indicates the diffusion system’s performance.

Luminit is one of the micro LED light manufacturers that offers an extreme angle (100 degrees) for use on portable and wearable displays. The goal of testing is to have an effective distribution of LED dots over a wide area that features exceptional uniformity and high translucency.

Cost and scope
Cost is an important parameter while selecting an LD system. LD additives with higher purity, improved thermal stability, and narrow particle size distribution will cost more. But such additives will perform much better in terms of light-diffusion and luminance levels. High-quality materials for LED lighting applications depend on the choice of LD additive systems.

As the lighting market moves increasingly towards LED technology, the requirement for high light diffusion means that we need to develop suitable plastic solutions. LED efficiency losses can be minimised through a highly transparent polymer during the light diffusion action.

Government regulations and consumer preferences for more environment-friendly, power saving products have led to manufacturers developing solutions that offer new levels of energy efficiency. As a result, the demand for light diffusion systems with high-quality performance will continue to grow.

1 COMMENT

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