Diffusers are an important part of many lighting applications, especially LED powered ones. Unlike incandescent and CCFL light sources LEDs are directional and do not have built-in glare control. Without diffusion the light from LEDs is usually too bright and unpleasant and each separate LED is visible. This translates into glare, narrow beam angles and the so called "scalloping" - the visible beam separation resulting from the numerous discrete light sources.
Besides hiding the individual light sources, providing more even surface luminance, reducing glare and enabling a much more pleasant light, diffusers can also double as light wavelength converters (as opposed to filters) for increasing CRIs and precise color rendering while preserving high efficiency.
Hiding individual LEDs with a low-angle diffuser
LEDs mimicking a CCFL tube with the help of a 3D tube diffuser
How Diffusers Work
Diffusion is achieved using several methods:Bulk/Die Additive Diffusers
Inks, dies or other light-absorbing chemicals are added to the substrate to create a complex combination of refraction, reflection and diffraction.
The most notable downside of this method is that a large part of the light from the source is absorbed by the diffuser, which translates into low efficiencies.
Holographic Diffusers
The most popular method, which relies on surface structures of various shapes to diffract light.
Where this method falls short, is that higher quality diffusion requires finer surface structures. The finer the surface structures, the easier they are to get damaged mechanically or chemically.
Volumetric Diffusers
By far the most advanced method relies on precisely suspended particles within the substrate that guide light through refraction in a controlled fashion.
This type of diffusers are not only generally more efficient but are also more mechanically and chemically resistant due to their volumetric nature.
Diffuser performance, efficiency and trade-offs
Diffuser performance is generally measured by its FWHM (full width half maximum) diffusion angle. The FWHM diffusion angle is the angle at which half the intensity of collimated light (e.g. laser) passing through the diffuser is observed.
FWHM of a 60-degree diffusion angle - half intensity is reached at -30° and +30°
Diffusion angle is not the same, however, as final beam angle - this is only the case if the light source emits collimated light. As most light sources emit incoherent light the final beam angle will be calculated by the square root of the sum of squares of the light source beam angle and diffusion angle [FBA = √(LSBA² + DA²)]. For example most LEDs have a beam angle of 120° so the combination with a 60-degree diffuser would result in a final beam angle of 134.1°.
In general higher diffusion angles result in better light source hiding and more uniform light, however this means more diffusion agents in the substrate that absorb a larger part of the light and create more backscatter, resulting in lower overall efficiency.
This trade-off is inherent in all of the diffusion methods, mentioned above. Therefore, it is important for an OEM to choose a diffuser that will hide the light source well enough with maximum efficiency (lowest diffusion angle of the most efficient diffusion method).
For more information on Fusion Optix diffusers and their advantages visit our diffusers page.
