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.