Light Diffusion and Light Diffuser Types

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.

What are Light Guides

What Are Light Guides

Light Guides are a clever method of directing light in applications where direct light from the light source can not be used due to physical, economic, aesthetic or other restrictions.

Light guides (also called light pipes) are a major component in edge-lit LCD display back lights where there is a CCFL or LED light source at one or two opposite edges of a light guide, which extracts the light towards the viewer. This approach allows for the use of less powerful light sources while maintaining high screen uniformity and very thin display profiles.

Basic single-edge-lit light guide that extracts light from one side

Rectangular display guide with visible extraction side

With the growing popularity of Solid State Lighting (LEDs and OLEDs), light guides are becoming increasingly sought for the creation of ultra-thin sleek and stylish lighting and signage fixtures.

Light guide that extracts light from both sides

Fusion Optix Direct/Indirect fixture utilizing 2-side light-extracting guide. LED light sources are hidden in the cylindrical bezels along the edges of the fixture.

How Light Guides Work

Light guides rely on the combination of total internal reflection (TIR) and a light extraction method to create a flat thin evenly emitting surface. For TIR to occur light has to strike the inner surface of the guide at an angle smaller than a specific critical angle. All light rays that strike the surface at angles higher than the critical angle are transmitted. 

What sets apart light guides from mere plastic rectangles is that light guides greatly increase the chance of light being transmitted through the extraction sides. This can be achieved in two ways:

Critical Angle Transmission

In CAT numerous imperfections are introduced at the extraction surface that increase the chance of light reaching critical angle. Depending on the light source, light guide dimensions and other factors, special imperfection patterns and shapes are developed to ensure maximum light extraction.

Frustrated Internal Reflection

In FIR a scattering material is applied to the extraction surface. The scattering material has different optical density from both the light guide and air, which results in light being transmitted from the guide to the scattering material.

Designing Efficient Light Guides

There are several factors that influence light guide efficiency including mixing distance, edge coupling, light scattering and reflectance. You can read more about light guides and efficient light guide designs in our downloadable Light Control and Efficacy using Light Guides and Diffusers presentation at www.fusionoptix.com.

Have you checked out the Fusion Optix's redesigned website yet?

Fusion Optix is thrilled to announce that we have launched our new website this Tuesday! We have redesigned it from the ground up to provide visitors with streamlined navigation, more detailed information and rich educational material. Here are some of the new features: Built-in search Convenient navigation menu Easy Inquiry form - accessible from anywhere on the site Streamlined content structure

And it will not stop there. We are dedicated to constantly improving ourselves, our products and our services. Stay tuned for continuously updated content, educational materials, videos and more. Visit us now at http://fusionoptix.com !

Top 5 Advantages of using LED Color Mixing to Create White Light (Part 3 of 3)

Additive Color Mixing

     Beyond wavelength conversion technologies such as chip level, volumetric, or remote conversion technologies, the third way to create white light using LEDs is by color mixing.  Like an RGB pixel of any standard video monitor, color mixing uses the principles of additive color to combine two or more colored LEDs to create white light.  By far it is the most electronically complex method of creating white light, as it depends on some type of control of the LED RMS current through either Pulse Width Modulation (PWM) or Amplitude Modulation which dims LEDs of different color combinations (such as red, green, blue, mint green, amber, even yellow or cool white) at appropriate intensity levels to achieve white light of a desired color temperature.  As with any lighting system, there will always be advantages and challenges.

                

Top 5 advantages of using LED Color Mixing to Create White Light:

     1.  Color uniformity 

     Capitalized upon by Cree through their TrueWhite™ technology, LEDs of different color are actively controlled via dimming through either some type of optical, thermal, or electrical feedback system.  This helps increase color uniformity from fixture to fixture.

     2. Easily customizable color temperature

     Where chip level or remote conversion techniques require different chemistries to achieve different CCTs such as 2700K, 3000K or 4100K, LED color mixing often requires nothing more than a simple firmware change to set the CCT of the system.  This introduces a higher level of simplicity on the manufacturing side, which helps lower the overall cost of the system.

           3. Advanced controllability during dimming

     The PWM controls used in an LED color mixing system enable the system to be inherently more controllable during dimming in application- which opens the door for better color performance in a dimmed down state.  A common complaint observed with dimmed LED systems is the even further absence of a red component, which gives dimmed LED lighting a faint, ghostly appearance, far from the warmer dimmed tone of an incandescent bulb which is more generally preferred.  By designing for increasing levels of red and/or amber LED light while dimming, the system can more accurately approximate the performance of an incandescent bulb.

     4.  Lower LED cost

     Since issues such as color uniformity and dimming can be effectively solved using the control techniques described above, LED binning becomes less of a concern, which enables the usage of less expensive and larger bins.  This lower LED cost, however, may be offset by higher costs in electronics and firmware.

          5. High system efficacy and CRI

     Luminous efficacy and CRI can be significantly increased using LED color mixing with the use of red and green LEDs.  Since the value of a “lumen” is based off the human eye sensitivity curve and CRI is dependent on the blackbody spectrum of Tungsten, adding strong green and red components can give a significant boost to lumen and CRI performance.

     As can be seen, LED color mixing in general does enable improved color controllability and potentially superior CRI and efficacy performance as well compared to the other two methods we have discussed.  Of course, the types of systems mentioned above do present some challenges.  A color mixing system will invariably require an array of LEDs which can limit beam control options, and will most certainly require an efficient diffuser to reduce shadowing on the application surface.  The inclusion of some mixing chamber or mixing distance into any system using this technology is often desirable as well.  Additionally, care must be taken during the design to account for varying lumen maintenance of different color LEDs.  Since Red LEDs often have a much longer lifetime and shallower lumen maintenance curve than their cooler counterparts, care must be taken to balance system color performance from a programming perspective as well as heat sinking to make sure this higher lumen maintenance is accounted for so the system does not experience a red shift over the course of its life.

     Since all three conversion methods to create white light discussed in this series- chip level (either volumetric or true chip level), remote, or color mixing- all offer their advantages and disadvantages, each may lend itself more pertinently to one application over another.  It is ultimately up to the designer to determine which method to choose whether it is choosing the appropriate light engine for a given fixture, or choosing the appropriate fixture for a given application.

Creating White Light Using LEDs

Part 2) Optical Effects of True Chip-Level Conversion vs. Volumetric Conversion

When designing a secondary optic for use with LEDs, the method of chip-level phosphor conversion has more influence on optical performance than many would think.  There are basically 2 methods of chip level conversion.  In what I call true chip-level conversion, the phosphor is deposited directly on the LED die, as in the case of the OSRAM Golden Dragon Plus (Fig 1).

Fig 1: OSRAM’s Golden Dragon Plus exhibits true chip-level conversion, where the phosphor is deposited directly on the die

        The other most widely used method for phosphor application and conversion is known as volumetric conversion such as in the Nichia 183A (Fig 2).  In volumetric conversion, the phosphor effectively floods the chip package and the blue die rests underneath a sort of pool or film of phosphor.

Fig 2: Nichia’s 183A Series is an example of an LED using volumetric phosphor conversion, where the blue die sits beneath a die film

        While either method can be argued to be equally efficient and effective at converting blue light to white light as phosphors are intended to do, each presents a different set of challenges for secondary optic design.  

        An LED utilizing true chip-level conversion better approximates a point source of light compared to an LED which uses the volumetric method.  This has a major influence in optical design applications where a point source is essential to light quality and efficiency.  For example, most parabolic optics will require a point light source to maintain efficient beam control.  When using a volumetrically converted chip, shadowing and dark/light rings are often observed due to its diffuse nature, where the larger emitting area creates superfluous reflections within the optic.  Another example of the importance of focal point location occurs in refractory optics, such as batwing style lenses which perform very extreme light bending and have a very low tolerance on focal point location, such that much of the light escaping from a volumetrically converted chip enters the optic at incorrect incident angles, resulting in color separation and undesirable Fresnel scattering losses which can result in lower optical efficiency.

        There are indeed times when a volumetrically converted LED is desirable, however.  In light guides, the larger diffuse surface of the LED widens the area of injection, and effectively narrows the spacing between LEDs which will promote a shallower mixing area thus enabling larger emitting areas for the light guide.  This can lead to thinner and more attractive bezel design in, for example, LCD panels.  Also, volumetric conversion can be a reason to choose a particular LED when the LEDs are placed very closely behind a diffuser lens which requires higher backlighting uniformity.  As can be expected, the extra few millimeters of emitting surface area provided by a volumetric conversion can help with uniformity at the surface, especially in the case of very high clarity diffusers.     

          In the case of reflective optics where flat and angled reflectors are used to direct and control the light, chip-level phosphor conversion method perhaps matters a little less.  A key point to remember in a situation such as this is that highly specular surfaces can be glary and distracting to people in the space.  However, when flat reflector are used, the concept of a focal point usually does not come into play and for the most part light exiting from an LED or array of LEDs has already achieved a far-field characteristic by the time it first reflects off a flat optic.

          Designing for high optical efficiency is one of the most cost effective ways of maximizing the efficacy of an LED lighting system.  By paying attention to the type of optics needed for the application, and selecting the right LEDs to be used with those optics based on knowledge of the phosphor conversion method, designers can realize easy and significant gains in system efficiency.

          Fusion Optix has years of experience in selecting the right LEDs to meld with our innovative AirOptics(tm) secondary optics, LED modules, and light diffusers to help our OEM customers select the right LEDs for their applcaition.  Visit our website at www.fusionoptix.com, call us at (781) 995 0805, or email us at info@fusionoptix to learn more!

What is the difference between luminance vs. illuminance?

What is the difference between luminance vs. illuminance?

 When talking about light, one of the most confusing terms will be luminance and illuminance.  They are more or less used incorrectly by people, sometimes even by the people in the industry.

Luminance

Luminance is the density of luminous intensity in a given direction and falls within a given solid angle.

It is measured in cd/m2.

Luminance is often used to characterize the emission from a diffuse surface. It indicates how much luminous power will be perceived by eye when viewing the surface from a particular angle. Luminance remains the same regardless of the distance from the light source.

Luminance is the light that is coming out of a surface.
Photo Credit: Fusion Optix's Display

Illuminance

Illuminance is the density of photons which fall within a given surface area. It is measured in lux, or footcandle(fc).

Illuminance can be measured with a lux meter. For a given light source, the closer to a light source the illuminated area is, the higher the Illuminance value.

Illuminance is the light falling to a surface.

Illuminance is used in lighting applications to measure the amount of light reaching an object such as a wall. ...Read more

Creating White Light using LEDs
By Mike Georgalis, LC


Different methods to create white light include chip-level conversion, color mixing, and remote conversion utilizing Fusion Optix ColorTune™ Technology

When it comes to creating white light using LEDs, there are many different methods, all of which have their advantages and disadvantages.  First, the most common and traditional is chip level conversion, where the converting material is integrated directly onto the blue LED die or fills up the LED package volumetrically.  Another method is color mixing using red and white (or mint colored) LEDs, where the LEDs are dimmed and mixed to create the desired color temperature.  A third method is converting the blue light of a diode far away from the chip, which utilizes a mixing chamber and remote optic integrating a converting Phosphor, Fluorescent Dye, Quantum Dot, or Fusion Optix’ proprietary ColorTune™ technology.

This series of Fusion Optix Blog postings will focus on the characteristics of each of these methods to help engineers and designers choose the right conversion method for their system.

Creating White Light Using LEDs Part 1:

Using Remote Wavelength Conversion Optics

How Remote Phosphors, Dyes, Quantum Dots, and Fusion Optix ColorTune™ Technology Impacts Lighting System Performance.

ColorTune™ Technology provides new color control and customization options for the LED lighting system designer

without ColorTune™	with ColorTune™
Example of use of ColorTune™ Technology in 60 deg diffusion lens for use in royal blue LED (450nm) pumped 6" LED downlight application

In a remote wavelength conversion optic, discrete blue LEDs are mounted in an array, and directly illuminate the inside surface of a lens which contains the conversion material.  There are many different types of conversion methods available on the market today including Phosphors, Fluorescent Dyes, Quantum Dots, and Fusion Optix’ proprietary ColorTune™ technology.  Typically, the lens is mounted as part of a mixing chamber several centimeters away and from the LEDs to provide uniform light at the surface.  This method results in a number of optical and thermal effects of which designers should be aware.

1)      Thermally, there will be heat generated on the lens from the wavelength conversion.  Designers should be sure to select materials with high conversion efficacies to minimize this heat which can cause rapid deterioration of the lens substrate or the conversion material itself.  This results in the need for creative heat sinking of the lens- which is often a challenge since lenses are not often made form highly thermal conductive material, and they do not frequently have a very large or tight thermal interface with heat sinking materials.

2)      Remote conversion optics often result in large, diffuse emitting areas, which can pose challenges in down lighting and spot lighting for tight beam control.  Just as there will be losses from the conversion, a designer should be aware of high losses from using a large diffuse source where a point source should be i.e. in a parabolic lens.

3)      Remote conversion methods can offer advantages in binning and color uniformity- which can reduce the overall cost of a system.  Especially when used to control white light exiting from chip level LEDs, using wavelength conversion to more finely tune output during production can allow OEMs to purchase larger, and therefore cheaper, bins of white LEDs 

4)      Aesthetically, many remote conversion technologies have a yellow, green, or red tint when the light engine is off (as opposed to the normal white diffuser or just fully visible CFL or incandescent light source), an unfamiliar effect that some lighting designers have found hard to swallow.  This is a matter of taste, but I am not sold on this is a sole reason to move away from remote conversion technology.

Fusion Optix has developed industry leading wavelength conversion technologies in our ColorTune™ family of optical components.  To take advantage of our optical expertise and highly efficient systems using ColorTune™ optic, visit http://www.fusionoptix.com/solutions/lighting/components/colortune.htm or email us at sales@fusiopnoptix.com.

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