A second Bright View Technologies white paper, with deeper technical detail and a series of LED luminaire simulations

1.
PRACTICAL INFORMATION FOR THE USE OF
ACEL™ LIGHT MANAGEMENT FILMS IN LED
LUMINAIRE DESIGN

2. Practical Information for the Use of ACEL Light Management Films in LED Luminaire Design
Table of Contents
Table of Contents ...................................................................................................................... 2
Introduction to Bright View Technologies ACEL Family of Light Management Films................ 3
Contents of This Document....................................................................................................... 4
Reference Designs ................................................................................................................ 4
“Bulb Hiding” Studies ............................................................................................................. 4
Comparative Diffusion Studies .............................................................................................. 4
Executive Summary of the Studies........................................................................................ 4
The Product Designs................................................................................................................. 5
Common Design Principles ................................................................................................... 5
Common Comparative Analysis ............................................................................................ 5
Circular Downlights................................................................................................................ 6
Rectangular Downlights....................................................................................................... 11
Linear Bulbs......................................................................................................................... 15
Bulb-Hiding Geometry with Diffusion Structures ..................................................................... 18
Round Downlight Secondary Optic Comparisons ................................................................... 22
Square Downlight Secondary Optic Comparisons .................................................................. 25
Summary and Discussion ....................................................................................................... 27
Notes on Material Selections .................................................................................................. 27
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3. Practical Information for the Use of ACEL Light Management Films in LED Luminaire Design
Introduction to Bright View Technologies ACEL Family of Light
Management Films
Since its founding in 2002, Bright View Technologies, Inc. has developed a powerful technology for the
creation and replication of large-area micron-sized structures using soft tooling and rolled goods
production processes. The flexibility and capability of Bright View’s platform has allowed the company
to create a rich and deep portfolio of products for lighting, display and security. One of the most
strategic of these products is the ACEL family of Light Management films, created for use in the LED-
based solid state lighting market.
Bright View’s production platform has allowed for the creation of a new generation of diffusion
structures that offer excellent flexibility in both light distribution and diffusion. ACEL Light Management
Films produce deterministic patterns through refraction, rather than scattering –delivering greater
precision and efficiency than previous technologies. Unlike conventional diffusion materials, Bright
View’s ACEL Light Management Films can be “tuned” to the properties of illumination subsystems to
achieve optimum mixing, appearance, and efficiency.
Bright View’s development of ACEL Light Management Films was driven by 4 major criteria, common to
all LED-based luminaries. The film must:
Depixellate (hide) the individual LEDs
Create a homogeneous, pleasing “glow”
Extract light efficiently from the luminaire
Provide optimum shaping of the far-field light pattern
Following the release of Bright View’s first whitepaper on LED-lighting, “Light Management Films in
LED-Based Luminaire Design 1 ,” we received many requests for more concrete information and design
information to illustrate the design principles we advocate. A Bright View led team has developed a
series of LED-based luminaire designs and studies in response to these requests.
1
Hirsh & Wood, “Light Management Films in LED-based Luminaire Design”, 2008, Bright View Technologies, Inc.,
http://www.brightviewtechnologies.com/downloads/LightManagementFilmsInLEDLuminaireDesign.pdf
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4. Contents of This Document
We set out to illustrate the important principles of efficient LED luminaire design and to provide a series
of practical reference designs using current LED component technology and ACEL Light Management
Films.
Because direct-lit multi-LED designs are the most common way to deploy LED illumination and
because this application is universal in its requirement for task-optimized diffusion films, we created a
series of luminaire and bulb designs which deploy a variety of LEDs in a variety of geometries.
Reference Designs
We created three basic designs (circle, square and linear) and for each basic design created two
variants with each with a different kind of LED. The designs and the numbers of LEDs they use are
summarized in the table below.
12” T8 Linear
6” Circular
LED-Type Rectangular Fluorescent
Downlight
Downlight Tube
Low Power - - 89
High
16 64 31
Brightness
High Power 3 16 -
We modeled the performance of each of these luminaires with respect to efficiency, mixing, light
distribution and general photometric performance using LTI Optics, Photopia V3.0 simulation software 2 .
“Bulb Hiding” Studies
In addition to laying out these basic geometries in recirculating optical cavities, for two of the designs
we studied the effectiveness of LED-hiding as a function of the size and spacing of the optical cavity.
Comparative Diffusion Studies
For both the circular and rectangular downlights we ran simulations with several different diffusion
materials, to confirm the superior capabilities of the Bright View ACEL films in these applications.
Executive Summary of the Studies
There are three high level conclusions to be made from these studies.
2
Special thanks to the team at LTI Optics (www.ltioptics.com), a premiere software development company in the field of
luminaire design and simulation. LTI Optics is the developer of Photopia (http://www.lighting-
technologies.com/Products/Photopia/Photopia.htm), the market leading software devoted to photometric simulation and
evaluation of luminaires.
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5. Every kind of material that diffuses (or scatters) light imposes some performance penalty in
system efficiency. But the micron-sized structures of Bright View’s ACEL Light Management
Films offer the smallest loss combined with the best bulb hiding (diffusion).
ACEL films are substantially more efficient than any of the conventional lighting diffusers we
evaluated.
In both observation and simulation we have shown the optimal “bulb hiding” distance between
LEDs and ACEL films is always 1:1 proportional to the spacing between LEDs.
Finally, in addition to the educational aspects of these designs, they are also modern, high
efficiency designs which can be used in the creation of a number of popular sizes of
luminaires3.
The Product Designs
Common Design Principles
For all of these designs, we used arrays of LED to achieve fluxes of between 35 and 55
lumens/watt.
The objective of these designs is always to create a high efficiency luminaire without glare or hot
spots from individual LEDs. The luminaire has a bright, smooth glow.
All of the LED arrays are set in mixing chambers lined with a high efficiency diffuse white reflector 4 ,
with a total diffuse reflectivity of greater than 95%. Because ACEL films are tuned for these
reflective cavities this design will always deliver superior results to cavities that are lined with a
specular reflector or no reflector at all 5 .
The size and depth of the chamber is always optimized to be as small as possible for the given
array of LED, but also to be sufficiently large to completely obscure the individual LEDs.
ACEL Light Management Films rest on top of a clear acrylic or polycarbonate lens, but a glass lens
could be used just as easily. This is largely a safety consideration, so the luminaires can conform
to UL94 fire safety requirements.
Because ACEL films have structure on both surfaces, the films cannot be laminated in place.
Rather they must be held in place through some kind of shape-aligned key and/or some form of
perimeter clamping. The details of securing the film in place are not specified in these designs.
Common Comparative Analysis
For four of the six designs (both of the LED arrays for the circular and rectangular designs) we included
the results of simulations with five different secondary-optic materials, including two configurations with
3
The designs we developed have been used solely for optical simulation. Mechanical and thermal considerations have not
been taken up as part of these exercises.
4
In our simulations we used a diffuse white reflector from Furukawa because this film was already in the Photopia materials
database. Other films, such as the Dupont Diffuse Light Reflector, or similar offerings from Toray and others will give
similar results.
5
For a detailed discussion of these principles see Hirsh and Wood, Op. Cit.
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6. Bright View ACEL films and two using commonly available diffusion materials. The table below
summarizes the materials we analyzed in the designs.
Secondary Optics, Direct Lit LED Luminaires Diffusion Materials
Clear Acrylic Cover (3mm thick)
Acrylic Cover + BVT-LED-ACEL-C01
(circularly symmetric light management diffusion film)
Acrylic Cover + BVT-LED-ACEL-C01 + BVT-LCD-ACE-1F
(LCD backlight diffusion film, 1.5 gain)
Cyro Acrylite 8mm ZDF Plus 6
ALP Plastics, Lumeio 3mm White 7
In every case Bright View’s ACEL films used with this class of recirculating design, always allows for
system efficiencies of about 90%, with some of the designs showing efficiencies above 93%.
In the third case, using two BVT films, we wanted to illustrate the kinds of light field manipulation that is
available to the luminaire designer using micro-optic Light Management Films. Combining a BVT ACEL
film with a BVT gain film modifies the light field by producing a greater degree of collimation from the
luminaire. Though the efficiency of the two-film combination shows no advantage over a conventional
diffuser, the concentration of on-axis gain combined with excellent bulb hiding is a significantly different
and superior illumination result.
Circular Downlight Designs
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http://www.cyro.com/methacrylates/us/
7
http://www.alplighting.com/pdf/Lumieo.pdf
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11. Rectangular Downlight Designs
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15. Linear Bulb Designs
The goal for these linear designs is finding appropriate sizes and structures to accomplish excellent
bulb hiding with a given separation between an array of LEDs. In all of these designs the ACEL film
surrounds the array of LEDs with 320° of circumference. A photograph of a variety of prototypes
enclosures built at Bright View are shown below.
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Bulb‐Hiding Geometry with Diffusion Structures
The principle technique we advocate for effective direct-lit luminaire design is the use of high quality,
high efficiency optical diffusion film - Bright View’s ACEL Light Management Films - used in
conjunction with a highly efficient diffuse reflector, creating an appropriate optical mixing chamber.
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19. The most common follow-on question we receive is “what distance should the film be from the bulbs?”
It is still a complex question because the answer involves
1. Number of LEDs
2. interLED spacing
3. The size and shape to the mixing chamber
4. Chamber reflectivity
5. Viewing/emission Angle of the LEDs
6. Surface treatments of optical film(s)
7. Distance from the LEDs to the back surface of the diffusion film
8. The brightness of the LEDs
The general principles we have observed are drawn from two studies we used to explore these topics
and are applicable to both low and high power LEDs.
LEDs should always be regularly spaced in the mixing chamber; and
The distance between the diffusion film and the LEDs should be about the same as the inter-LED
spacing on the circuit board. There is, roughly, a 1:1 ratio between LED spacing and LED-to-film
distance.
We performed two simulations to explore the principles of bulb hiding. In the first we took 6” circular
downlight, with 16 high brightness LEDs and, holding everything else constant, changed the size of the
mixing chamber - that is the distance between the LEDs and the ACEL film. The simulation in
Photopia allow us to “look up” at the luminaire from below.
The results of the simulation for this design show a 0.95” distance between LEDs and film does a good
job of hiding the bulbs and allows for a compact design for the high-brightness LEDs spaced on a 1.1”
diagonal. Larger chambers offer incrementally better mixing, but the difference is not appreciable.
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20.
Similarly, a distance on 1.75” does an optimal job of bulb-hiding in the luminaire design with 3 high-
power LEDs spaced 1.7” from each other.
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21.
The second simulation was to experiment with diffusion cylinders of different diameters, covering a
linear array of LEDs intended to perform as a LED-based substitute for a T8 linear fluorescent tube.
Many of these implementations do not include any form of diffusion film and the glare from these
devices can be quite unattractive. If we relax the constraint that a tube which emulates the behavior of
a linear fluorescent tube must be the same size as the fluorescent tube, then it becomes possible to
create a cylindrical diffusion structure that does a near-optimal job of mixing an array of LEDs.
In this study we created an array of 31 LEDs spaced 1.5 inches apart. Using a circular diffuser we
created a series of covering cylinders of varying diameters.
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22.
Round Downlight Secondary Optic Comparisons
For each design and for each diffusion material we present a summary performance data, the image of
the illumination pattern on the floor directly underneath the luminaire, an image of the luminaire viewed
from below, and a polar plot of the photometric distribution of the luminaire.
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25. Square Downlight Secondary Optic Comparisons
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27. Summary and Discussion
The important conclusions to be made from these studies are:
Every kind of film that diffuses (or scatters) light imposes some performance penalty in system
efficiency. But the micro-optic structures of Bright View’s ACEL Light Management Films offer
the smallest loss combined with the best bulb hiding (diffusion).
ACEL films are substantially more efficient than either of the conventional lighting diffusers we
evaluated.
The addition of a second, microlens-based gain/diffusion film used in conjunction with the ACEL
diffuser produced a narrower, higher flux beam. The loss in efficacy imposed by this second
film is equivalent in function (and loss) to the tall lower reflectors that are sometimes used to
focus the beam and limit the beam-angle of conventional downlights.
For this set of designs, we were able to achieve greater system efficacy with our rectangular
designs than with our circular designs.
Achieving high system efficiency requires efficient optical cavity design, including the use of
high efficiency diffusion films and high efficiency diffuse reflectors.
Notes on Material Selection s
In the creation of these simulations we have used specific devices and materials from a variety of manufacturers
so that we could create exacting models with simulations that will have real correspondence in the physical world.
We tried to use LEDs from a variety of manufacturers. The selection of any one material, other than our own
ACEL Light management films, does not constitute any kind of endorsement on the part of Bright View
Technologies or LTI Optics. In fact several of the material selections were governed by whatever materials were
already a part of the Photopia standard materials library.
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