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