Use of Anti-Reflective Glass in Lighting Products

Use of Anti-Reflective Glass in Lighting Products
Kevin L. Willmorth
Lumenique, LLC
David P. Maikowski
Guardian Industries, Corp.
Abstract
The selection of clear cover materials in lighting requires consideration and balance between optical
transmission, optical interaction with the cover lens and its effect on light distribution, and economic
considerations. The use of clear glass enclosures is rarely utilized as an optical feature and, instead, are
traditionally incorporated to protect the optical system from environmental damage, or to protect human
occupants and combustible building surfaces from failed lamp fragments. The long-time use of glass in
lighting has been tied to optical clarity, mechanical strength for thermal and impact resistance, ease-of-use
and sourcing, and UV resistance vs. other materials.
However, most typical glass enclosure products, interior or exterior, reduce the total luminaire efficiency
from transmissive and reflective losses. Reflective losses include first surface (optical side) reflection
which redirects light back toward the light source, and inter-reflective losses within the lens material.
Transmissive losses can be found through the bulk materials of the glass chemistry itself and its
absorption of visible light energy (e.g., FeOx content). Additionally, the effect of refractive light
redirection imparts a change in light distribution and compounds potential optical losses. The graphic
below shows the efficiency losses commonly seen with standard soda lime float glass:
The ability to minimize these loss factors is key towards optimizing luminaire efficiency. The use of
special glass chemistry (low-iron, etc.) and coatings (Anti-Reflective) can help to substantially reduce
both the reflective and transmissive losses seen when using standard float glass cover lenses.
This paper will review the effect of flat low iron glass with applied Anti-Reflective coatings in lighting
product optical performance and potential efficiency gains over the use of low cost soda-lime iron bearing
glass as well as alternate materials to glass commonly used in lighting products. The study will explore
where the balance between cost and delivered performance produces the most-effective result. Further,
this review will identify where the interaction of optical designs and flat glass lenses create the greatest
need for Anti-Reflective coatings to improve optical performance. To facilitate practical application of the
findings of this review, results are presented in context to standard lighting industry metrics.
Page 1 of 21 Use of Anti-Reflective Glass in Lighting Products
Background of Technology
Anti-Reflective (AR) coatings have been in commercial use for many years in a variety of applications
including vision lenses, display cases, flat panel display cover glasses, plastic lenses, and commercial
facades to reduce the reflection interferences that impede vision clarity and, in many cases, to improve
overall aesthetics. Recently, though, AR coatings have been used more extensively as a functional tool
towards improving energy efficiency in various applications. A couple of the more prominent new
application areas for AR coating are solar energy and Solid-State Lighting (SSL) where efficiency gains
are at an energy cost premium and can create significant competitive advantage and, in turn, can help
accelerate adoption of these new technologies globally.
In solar applications, AR coating are now being used in a many Photovoltaic (PV) module technologies
(crystalline silicon, thin film, etc.) on its cover glass in order to reduce reflection losses from the sun and,
in return, allow more sunlight (broadband range of 0-1,400 nm) to reach the semiconductor material and
translate it more efficiently into electrical energy. The same approach is in use for AR coatings used in
SSL fixtures whereby the goal is to reduce the reflection losses from the SSL light sources in the peak of
the visible range (500-600 nm) and, in turn, translate it to more effectively light the targeted surface area.
AR coatings have a simple core principle of minimizing the interference of light traveling through a given
material’s surface vs. that of its immediate surrounding environment of air. Therefore, the purpose of an
AR coating is to create a layer whereby its refractive index (n) is as close as possible to air (n=1.000293)
and the glass surface (n=1.52) to allow it to filter visible light through with minimal interference (or
reflection losses). AR coatings typically consist of SiO2 and other materials that can be deposited or
surface modified in a variety of methods including:
•
Wet Chemical (Sol-Gel, Acid-Etching, etc.) Deposition through Dipping, Spraying, Curtain
Coating, or Roller
•
Magnetron Sputter Vapor Deposition (MSVD) or “Sputtering”
•
Laminate Sheets
The AR coating discussed in this paper is a 3-layer coating deposited on glass using the MSVD or
“sputtering” process in both Single-Sided (SS) and Double-Sided (DS) configurations. This approach
was selected due to its proven technology, field performance (across many industries and applications) on
long-term mechanical, chemical, and environmental durability; and economic considerations. More
information on this and other AR coatings can be found in the References appendix in this paper.
Background of Testing and Modeling
Testing used for this paper was conducted on five (5) commercially available luminaire devices that were
tested to IESNA LM 79 (Issued: 2008/01/01 Approved Method for the Electrical and Photometric
Measurements of Solid State Lighting Products) for Luminaire Efficacy, Minimum Light Output, Total
Lumen Output, Zonal Lumen Density, Beam Spread for Reflector Lamps, various photometric tables and
plots, and Electrical Measurements at an IES-accredited laboratory. Each luminaire was tested with the
following three lens configurations:
•
Standard Uncoated 3 mm Soda Lime Float Glass
Page 2 of 21 Use of Anti-Reflective Glass in Lighting Products
•
Single-Sided Anti-Reflective (SS AR) Coating on 3 mm Low Iron Glass
•
Double-Sided Anti-Reflective (DS AR) Coating on 3 mm Low Iron Glass
Data files from this testing were then used in the AGi32 and FTE Calculator modeling tools for the
scenarios and case studies covered in this paper. All lumen output data should be assumed to be at 0°, or
NADIR, unless otherwise specified.
Stepped Energy Efficiency and First Cost Value Extraction
In the evaluation of any reflective or transmissive material used in lighting system optical designs,
incremental improvements are easily realized. However, unless these improvements can be further
translated into realized energy savings, in the form of fewer fixtures (lower installed cost), or lower light
source energy demand (energy savings), or both, there is limited value realization. Light sources,
regardless of technology used (LED, HID, fluorescent, incandescent, etc.) are not available in an infinite
array of light outputs. The result is a need for optical optimization to deliver improvements in great
enough steps to allow end-use applications to change source size, or number of luminaires employed.
Figure 1 below illustrates this dynamic. Further, Table 1 indicates the required optical improvement
required to overcome the source lumen loss from each incremental lamp step.
Determining the value available to extract for payback of optical improvements can be calculated from
energy savings alone, as shown in Table 1.
Figure 1 - Light Source Output “Steps”
This graph represents the available range of Metal Halide, Pulse Start lamp outputs in mean lumens. In
order to realize applied energy savings from any system, movement must be made from a higher wattage
to at least one step down in power requirement, without realizing a loss in light output. This requires an
increase in lumen efficiency equal to the drop in lamp lumens as shown in the chart. For example, the
lumen reduction from dropping from a 250W lamp to a 200W lamp requires recovery of the 24%
reduction in lamp lumen potential from gains in system efficiency to effectively payback an increased
investment.
Page 3 of 21 Use of Anti-Reflective Glass in Lighting Products
Table 1 – Value Extracted from Lamp Step Down
400W 350W 320W 250W 200W 175W 150W 100W Annual Energy Cost $ 128.00 $ 112.00 $ 102.00 $ 80.00 $ 64.00 $ 56.00 $ 48.00 $ 32.00 1 Yr Value $ 16.00 $ 10.00 $ 22.00 $ 16.00 $ 8.00 $ 8.00 $ 16.00 Based on 3200hrs/year, $0.10/kWh Energy Cost
Assuming the optical performance improvement produces a step-down in lamp size, the value available
is between $8.00 and $22.00 per luminaire, for each year period to full payback. For example, for a
payback period of 3 years, from a change from a 320W lamp to a 250W lamp, the value available is
$66.00 over that period
One dynamic compounding this “stepping” of performance from lamp changes, is that the most popular
lamps used are 150W, 175W, 250W and 400W. Stepping down to the next available source increment
(100W, 150W, 200W and 350W) requires an improvement of 58%, 17%, 25% and 17% respectively.
These are significant steps to overcome without significant optical design improvements beyond the
simple incorporation of improved cover lens materials. Further, fluorescent and HPS sources offer less
fidelity between incremental steps, producing a courser step (greater difference) between available lamps.
LED-based luminaire products offer the greatest near-term potential for optimizing the use of efficient
transmitting lens materials. Luminaires designed around LED sources utilize arrays of individual LEDs,
presenting the potential of creating fine improvement steps. Figure 2 and Table 3 illustrate this dynamic.
Determining the value available to extract for payback of optical improvements can be calculated from
energy savings alone, as shown in Table 2.
Page 4 of 21 Use of Anti-Reflective Glass in Lighting Products
Figure 2 – Steps Based on Number of LEDs Employed
LED luminaires offer the opportunity to fine tune light output by adjusting the number of individual LEDs
used in the array. This reduces energy in incremental steps with greater fidelity than the courser
conventional light sources offer. This requires an increase in lumen efficiency equal to the drop in LED
lumens as shown in the chart. For example, the lumen reduction from reducing the LED count from 34
to 32 requires recovery of 6% in lumen potential from gains in system efficiency.
Table 2 – Value Extracted from LED Count Reduction
LED Count 34 32 30 28 26 24 22 20 Annual Energy Cost $ 65.21 $ 61.37 $ 57.53 $ 53.70 $ 49.66 $ 46.03 $ 42.24 $ 38.40 Value $ 3.84 $ 3.84 $ 3.83 $ 4.04 $ 3.63 $ 3.79 $ 3.84 Based on 3200hrs/year, $0.10/kWh Energy Cost, 66lm/W system efficacy, 400lumen LEDs
By improving optical performance to reduce LED count, the value available is roughly $3.80 for each
12W (800 lm) of LED power reduced for each year to full payback. For example, for a payback period
of 3 years from increased efficiency to reduce LED count from 28 to 30 the value available is $11.43 over
that period.
Page 5 of 21 Use of Anti-Reflective Glass in Lighting Products
Luminaire Count Reduction Impact
Beyond stepping of individual lamp lumens for improving optical efficiency, is the adjustment in total
luminaires applied to a given application. When arrays of luminaires are applied to produce a desired
illumination result - such as that employed in sports lighting, or large freeway interchanges - the energy
reduction and installed cost reductions compound to increase the value derived from improving optical
performance. Table 3 illustrates the difference in simple lifetime energy use and initial costs from the
reduction of one luminaire in ten.
Table 3 – Value Generated from Fixture Count Reduction
10 Fixtures 9 Fixtures Value First Cost $ 6,400.00 $ 5,760.00 $ 640.00 Annual Energy Cost $ 1,280.00 $ 1,152.00 $ 128.00 Based on 250W MH lamps, 3200 annual operating hours, $0.10/kWh energy cost, and an installed perfixture cost of $640.00, reducing fixtures employed by 10% through improvement in optical
performance generates $768.00 of value payback in the first year, with an additional annual value of
$128.00. This does not fully consider the maintenance cost reduction from eliminating 10% of the
luminaires installed.
Performance Threshold Improvements
A significant exception to the “stepping” dynamic instances exists where an optical system fails to
produce a desired performance result by reducing incremental internal light losses. Reflector designs
often employ angular output distributions that create inefficiencies when overlaid with glass or plastic
covers. The most common losses occur from light being reflected back into the reflector or lamp,
reducing efficiency while creating undesirable distribution effects.
With the growing demand for finite performance thresholds, such as the FTE standard employed by the
EPA for outdoor luminaires to attain Energy Star approval, a difference of just 1% in luminaire
performance can mean a failure to comply. This requires that luminaire manufacturers employ techniques
to optimize product performance in order to meet or exceed established thresholds.
The value of achieving approval of power company, local, state, or national efficiency approval is
realized in increased sales volumes and application opportunities. When this can be accomplished without
product redesign to accommodate changes in lamp wattage or LED count, the value is based solely on the
performance improvement as it pertains to light delivered and/or performance standard achieved. This is
highly subjective, based on individual manufacturer market positioning and sales strategy. When a failing
product can be made a compliant product with a simple change of lens enclosure material, the gain in
sales opportunities will most often be seen as highly valuable.
Page 6 of 21 Use of Anti-Reflective Glass in Lighting Products
Glass Performance Comparisons
While overall transmission is an important metric in comparing transmissive materials, this assumes that
all light that passes through the material share a single incident angle. Luminaire optical designs include a
wide range of optical features that cause light to be directed through the transparent cover material at
many angles. This frequently includes very shallow angles (need to define range of “shallow” angle) that
cause transparent materials to cause specular reflective properties that reduce optical efficiency.
The addition of Anti-Reflective coatings produces an improvement in total transmission, with greater
angular transmission improvement realized at shallow incident angles (source side). Table 4 shows the
characteristics of standard uncoated clear glass and Single-Sided (SS) and Double-Sided (DS) AntiReflective (AR) coatings on low-iron glass.
Table 4 – Coated Glass Comparison
Incident Clear Angle Uncoated SS AR % Gain DS AR % Gain 90 34.46 34.54
0.22%
34.58 0.35% 80 34.46 34.54
0.22%
34.58 0.35% 70 34.46 34.54
0.22%
34.58 0.35% 60 34.46 34.54
0.22%
34.57 0.32% 50 34.45 34.54
0.25%
34.57 0.35% 40 34.40 34.52
0.34%
34.57 0.50% 30 34.33 34.41
0.22%
34.46 0.38% 20 34.09 34.17
0.22%
34.23 0.42% 10 32.42 32.89
1.48%
32.93 1.58% This summary of glass test results shows the effect Anti-Reflective coatings has on angular transmission
as it relates to source incident angle represented in test W/m2 radiance, and % difference from clear
uncoated material. The results indicate that while total transmission is improved, an additional benefit is
the greater improvement realized at shallow incident angles, where clear glass exhibits specular
reflection at the first and second surfaces.
The benefits of coated glass are the composite of its greater total transmission coupled with its significant
improvement at low incident angles. Luminaire testing shows that this generates a significant
improvement in total fixture lumen output, as shown in Table 7.
Page 7 of 21 Use of Anti-Reflective Glass in Lighting Products
Reference: Distribution Patterns of Test Samples Represented in Table 5 and 6
Sample 1 IES Type III Short LED System Sample 2 IES Type II Very Short LED System Sample 3 IES Type II Medium HID Lamp Sample 4 IES Type III Short HID Lamp Sample 5 IES Type II Short Induction Lamp The reference samples include a range of optical designs and light sources to illustrate the
impact of the cover glass materials based on incident angle distributions.
Table 5 – Luminaire Total Lumen Output Test Comparison
Sample 1 ‐ Significant Incident Angles <30⁰ Sample 2 ‐ Mixed Incident Angles >30⁰ Sample 3 ‐ Dominant Incident Angles <30⁰ Sample 4 ‐ Significant Incident Angles <30⁰ Sample 5 ‐ Uniformly Mixed Incident Angles Clear Uncoated SS AR % Gain DS AR % Gain 5984.7
6568.1
9.75% 6702.0
11.99%
6918.7
7091.7
2.50% 7224.5
4.42%
4378.7
4704.4
7.44% 4814.0
9.94%
6972.0
7858.5
12.72% 8213.1
17.80%
4029.4
4224.4
4.84% 4385.2
8.83%
Test results indicate that the greater the dominance of low incident angles (<30°) between the source
and first surface of the glass cover, the greater the gain in total light production (lumens) will be.
Diffuse, or uniformly mixed optical angles products realize the least amount of improvement. These
results are consistent with expectations based on the effect AR coatings have on light transmission as it
relates to source incident angle.
A further comparison of the impact of AR coated glass cover lenses is evident when comparing zonal
light distributions as a percentage of total Luminaire output, as shown in Table 6.
Page 8 of 21 Use of Anti-Reflective Glass in Lighting Products
Table 6 – Luminaire Total Zonal Lumen Output Test Comparison
Clear Uncoated
Sample 1 ‐ Significant Incident Angles <30⁰ 30‐90⁰ Incident Angle Zonal lumens 3239.7
0‐30⁰ Incident Angle Zonal lumens 2746.4
Sample 2 ‐ Mixed Incident Angles 30‐90⁰ Incident Angle Zonal lumens 6124.0
0‐30⁰ Incident Angle Zonal lumens 797.8
Sample 3 ‐ Dominant Incident Angles <30⁰ 30‐90⁰ Incident Angle Zonal lumens 3089.1
0‐30⁰ Incident Angle Zonal lumens 1291.1
Sample 4 ‐ Significant Incident Angles <30⁰ 30‐90⁰ Incident Angle Zonal lumens 4451.8
0‐30⁰ Incident Angle Zonal lumens 2523.4
Sample 5 – Mixed Incident Angles 30‐90⁰ Incident Angle Zonal lumens 3560.8
0‐30⁰ Incident Angle Zonal lumens 469.7
SS AR 3534.9
3030.6
6286.2
808.7
3349.2
1357.4
5040.9
2821.7
3733.4
492.1
% Gain 9.11% 10.35% 2.65% 1.37% 8.42% 5.14% 13.23% 11.82% 4.85% 4.77% DS AR 3585.5 3119.9 6395.3 832.5 3390.4 1425.9 5187.0 3028.9 3884.0 502.4 % Gain 10.67% 13.60% 4.43% 4.35% 9.75% 10.44% 16.51% 20.03% 9.08% 6.96% Based on the incident angle transmission and actual product test samples, the impact of AR coatings
appears to show a strong relationship with the source optics, showing particularly strong results when
applied in products that include a large proportion of low incident angle (<30°) beam direction (high
distribution angles), like that used in outdoor site and roadway lighting. Further, in optical systems with
mixed or diffuse light source distributions generalized gains are also realized from an increase in total
light output, resulting in higher efficiencies, and the potential for reducing light source size (watts or LED
counts), and/or reducing the quantities of luminaires required, while increasing total system efficiencies.
Again, these results were attained with no other modifications (other than swapping out the lens
enclosure) to optical system designs to optimize product performance to take advantage of the increased
transmission at low incident angles. Theoretically, in applications where fixture spacing and optimal
uniformity are desirable, optical systems could be designed with greater emphasis on low incident angles,
using Single- or Double-Sided AR coatings to mitigate losses normally encountered from uncoated glass.
Page 9 of 21 Use of Anti-Reflective Glass in Lighting Products
Cost Reduction Opportunities from Replacement of High-Cost Materials
Another consideration for the use of a low-iron coated glass lens is the material cost difference vs. other
commonly used lens materials such as borosilicate glass and plastics. As shown throughout this study,
the use of low-iron glass chemistry coupled with the use of Anti-Reflective coatings significantly reduces
the light losses traditionally seen with the use of glass. In the past, materials such as borosilicate glass
and plastics (acrylic and polycarbonate) were used to overcome this obstacle. These materials, though,
are typically more expensive and have had significant sourcing and long-term durability issues to
overcome vs. soda-lime glass (standard or low-iron) with AR coatings.
Borosilicate glass offers inert chemistry and strength facilitating thin profiles to attain higher transmission
(94%) than standard uncoated soda-lime glass without requiring full tempering. Its process (drawn or
float) allows for thin glass (< 3 mm) and to be used safely in lighting applications, while its thermal
stability allows the lens to be placed very close to high heat light sources. However, its cost is typically
>600% the cost of standard uncoated soda lime float glass at the same thickness and >30% more
expensive than Double-Sided Anti-Reflective (DS AR) coating on low-iron glass.
Comparatively, a DS AR coating on tempered low-iron glass substrates produces an improvement of 4%
in light transmission (98% when applied to heat treated low iron substrate) over borosilicate, on a thicker
lens, delivering improved durability and toughness at a cost saving of 30%. Further, heat treatment of
low iron soda lime glass produces a stronger lens, with improved impact resistance and preferred
fracturing characteristics (break pattern) than borosilicate.
Plastics are applied to lens applications to realize the benefit of lightweight, durability, and optical
performance. In addition, plastic is used for thermo-formed lens shapes (extruded, pressed or molded).
However, the two most common materials each exhibit compromises that must be considered.
Polycarbonate materials are not inherently UV resistant, and will yellow and cloud quickly when exposed
to daylight or high UV light sources, such as Metal Halide High Intensity Discharge lamps. When
additives are included to produce UV stability, light transmission is significantly reduced. Acrylic
materials are UV resistant, but significantly more brittle than polycarbonate. Additives that increase
flexibility reduce light transmission. Further, regardless of any surface treatment, all plastics suffer from
significant risk of scratching and surface degradation that reduces performance. Finally, the cost
(development prototyping and production) of plastic lens materials is often >200% that of glass
equivalents.
Comparatively, for flat clear lens cover uses (unformed), the use of uncoated, DS AR coated heat treated
low iron glass cover lenses produces the compound benefit of high light transmission, high resistance to
surface degradation and scratching, and durability. These benefits are delivered at a cost saving of 10% to
50% depending on the complexity of the lens part design.
Page 10 of 21 Use of Anti-Reflective Glass in Lighting Products
Case Studies
Example 1 – Lamp Step Reduction with Application of Double-Sided AR Coating on Low-Iron
Glass
The gains realized in Reference Test Sample 4, a 175W MH Type III area light, produced a 17.8%
improvement. This is greater than the required 10% reduction in lumen capacity step from 175W to 150W
lamp, indicating an opportunity to apply Double-Sided AR (DS AR) glass to reduce lamp wattage by one
step.
To verify this, a comparison of the two options (uncoated and DS AR) in an actual area lighting
application produced the results shown in Foot-candle Plot 1 and 2 below:
Footcandle Plot 1 – Uncoated Glass with 175W Lamp
Reference Test Sample 4- Clear Uncoated Glass 175W MH Lamp
20’ Mounting height, 60’ Pole spacing – 1520 watts, 2.93 Fc Avg
Page 11 of 21 Use of Anti-Reflective Glass in Lighting Products
Footcandle Plot 2 – Double-Sided AR Coated Glass with 150W Lamp
Reference Test Sample 4- Double (DS) AR Coated Glass 150W MH Lamp
20’ Mounting height, 60’ Pole spacing – 1320 watts, 3.06 Fc Avg
In this example, the use of DS AR coated glass facilitated stepping down one lamp wattage, from 175W
to 150W. The resulting single fixture energy cost savings of $8.00 per year would support a premium of
$24 in end user value for a full payback in 3 years. When extended over a 12-year life of a typical parking
area luminaire, the total cost savings from energy use would total an additional $72, with no other change
in product design or optical configuration.
Example 2 – Sports or Large Area Flood Lighting Fixture Count Reduction
A typical baseball playing field lighting system comprised of 6 poles utilizing 44 total luminaires. Using a
combination of very narrow to medium distribution angles, aggregate gains in improved candlepower
distribution for Double-Sided (DS) AR glass over clear soda-lime glass is an aggregate 10%. Of this, 8%
is gained from the coating itself, in addition to a gain of 2% from the substrate low iron glass. When
Page 12 of 21 Use of Anti-Reflective Glass in Lighting Products
applied to the total system, a total of 4 fixtures are eliminated, producing the results shown in Table 7
below:
Table 7 – Luminaire Total Zonal Lumen Output Test Comparison
Soda Lime Glass
DS Coated Glass
Fixture Cost $ 177.89
$ 199.89 Fixture count 44
40 Total luminaire cost $ 7,827.16
$ 7,995.60 Annual Energy Cost* $ 2,833.60
$ 2,576.00 Annual savings $ 257.60 Payback period (years) 0.65 *Based on 1610hrs/year, $0.08/kWh Energy Cost and equivalent illuminance levels on all playing surfaces
In this simple layout, the additional cost of coated glass over soda-lime glass produces a payback of less
than one year. This approach is relevant to all lighting applications where a large number of
luminaires are used in arrays targeted in overlapping patterns to attain high illuminance values, or to
reach a greater distance than individual luminaires are capable of.
Example 3 – Achieving a Target Performance Threshold from Change in Glass Specification
The DOE FTE (Fitted Target Efficiency) metric establishes a method for comparing outdoor luminaire
performance based on how well the light pattern delivered fits a given target area, as shown in Figure 1.
Page 13 of 21 Use of Anti-Reflective Glass in Lighting Products
Figure 1 – FTE Metric
Simplified comparison of circular and rectangular distributions of equal area (DOE). The ratio of light
within the target area, and wasted in the peripheral surrounding areas are compared and the luminaires
rated by the efficacy.
In Reference Sample 1, the base luminaire using clear soda lime glass failed to meet the FTE requirement
for EPA Energy Star compliance, required for inclusion in many lighting performance metric standards
for energy saving product applications. Figure 2 shows the luminaire performance with clear soda lime
glass. Figure 3 shows the product’s performance with Double-Sided (DS) AR coating on low-iron glass.
Page 14 of 21 Use of Anti-Reflective Glass in Lighting Products
Figure 2 – FTE with Clear Glass
Using clear glass, this luminaire fails to qualify (36 lm/W) for Energy Star (37 lm/W requirement) by 1
lm/W
Page 15 of 21 Use of Anti-Reflective Glass in Lighting Products
Figure 2 – FTE with Double-Sided AR Coated Glass
Using a Double-Sided AR coating on low-iron glass, this luminaire is transformed from noncompliance to exceeding Energy Star requirements (39 lm/W requirement) by 3 lm/W, an improvement
of 8.3%.
The improvement in FTE performance is evident in comparisons of the sample in application, as shown in
Foot-candle plots 3 and 4. Average illuminance is greater with coated glass, while values at the perimeter
of the lighted area remain unchanged, indicating an improvement in performance of the luminaire in
directing light into the target area. Both plots are based on 20’ luminaire mounting height and 60’ pole
spacing.
Page 16 of 21 Use of Anti-Reflective Glass in Lighting Products
Footcandle Plot 3 – Clear Glass
Page 17 of 21 Use of Anti-Reflective Glass in Lighting Products
Footcandle Plot 4 – Double-Sided AR Coated Glass
With tighter control, the maximum illuminance increased, while the minimum illuminance decreased by a
small amount. The result is a slight increase in the Max:Min ratio, within acceptable recommended
practice.
Page 18 of 21 Use of Anti-Reflective Glass in Lighting Products
Example 4 – Realizing Simple Cost Saving from Replacement of Borosilicate Cover Lens
In this example, we look at the replacement of a borosilicate cover lens required to contain lamp
fragments from lamp failure in a 10” Metal Halide down light (8” cut round lens, heat treated) with
Double-Sided AR coated glass. Table 8 shows the benefits realized by this substitution.
Table 8 – Lens Material Performance and Cost Comparison
Lens Cost Light Transmission Borosilicate Glass DS Coated Glass Change $ 8.96 $ 5.77 ‐$3.19 (35%) 94% 98% +4% In addition to these cost benefits, the DS AR coated low-iron glass produces an increase of >30% in end
mechanical and surface strength. The inherent and advantageous heat treatment properties of soda-lime
glass over borosilicate translate into increased impact resistance for the glass lens and a safer break
pattern for the application.
General Observations
Evaluation of the test samples revealed no significant adverse change in luminaire behavior. In almost
every instance, a small improvement in uniformity within the target lighted area took place when multiple
luminaires are utilized were utilized. However, due to the increased total transmission of coated glass,
brightness increased directly below luminaries. This may cause a decrease in uniformity for luminaire
designs with narrower distribution patterns.
In Reference Test Sample 1, an undesirable increase in BUG (Backlight/Uplight/Glare) rating occurred
when the AR glass was installed and tested. The clear glass product produced a B1-U1-G2 rating, while
the AR glass produced a B2-U1-G2 rating. This was evident in a small increase in high angle illuminance
in a product that was at the upper limit of the Backlight illuminance range, aggravated by the higher
efficiency of the coated glass at high distribution angles (low incident angles).
The single largest factor in consideration of AR glass over clear soda lime for fixture enclosures is the
additional costs involved. Typically, the cost of Double-Sided AR coating on low-iron glass over standard
uncoated soda lime is 4x – 5x. To overcome this cost liability, careful consideration must be given to the
value being delivered, and whether that value can be converted to cost reductions, or increased
marketable performance. Costs aside, every luminaire tested, and several small scale samples with a wide
range of distribution patterns and incident angles, indicate that application of AR coated low iron glass
will generally produce an 8-10% gain in light output, with a potential of up to 18% for luminaires with
internal reflective properties that benefit from reduction on low incident angle re-reflection into the
optical system.
Page 19 of 21 Use of Anti-Reflective Glass in Lighting Products
Conclusion
Development of efficient optical systems that control and deliver light into the desired target zone(s) with
minimal losses is a complex pursuit requiring consideration of light source behavior, intended and
unintended reflective components. The addition of a clear glass covering to protect the optical system and
light source introduces a second transmissive/reflective component that is seldom considered part of the
primary optical design. Often, the impact of a clear cover glass is accepted as a necessary evil. However,
the application of coated low-iron glass significantly reduces the negative impact of glass enclosures,
therefore increasing luminaire efficiency.
As has been stated throughout this study, the use of optimized glass lenses with Anti-Reflective (AR)
coatings can help luminaires become more efficient in both increasing targeted light distribution and
reducing overall energy consumption by reducing component and/or fixture counts. The most-effective
and value-driven use of AR coated glass in commercial lighting is in Solid-State Lighting applications
whereby the count of LEDs can be reduced and, in turn, the value of the substitution of the higherperforming glass lens can be validated within a reasonable payback period. In addition, there is
significant energy and cost savings (including total cost of ownership including maintenance and total
MTBF) when the AR glass is used in lighting applications whereby overall fixture count can be reduced
(e.g., sports lighting, roadway lighting, etc.) while still meeting the specified illumination goals. Last, the
use of AR coatings on glass in lighting allows for better light uniformity spread across the targeted
distribution pattern and reduces the light losses at wide incident angles (> 60 degrees) seen with standard
uncoated glass and, in turn, increases the light output at low-incident angles (<30 degrees).
Future desirable improvements in AR technology include further improvements in total transmission at all
incident angles. With an increase from 8-10% to 15-18%, the range of potential applications includes
virtually all but a few HID applications, and doubles the realized fixture count reductions when applied to
array style large area systems.
Beyond increases in performance, cost reductions over time through technology adoption will extend the
value proposition of coated glass over a broader set of applications. The cost of the AR coating itself can
be expected to decrease over time as it is accepted within the industry and volumes increase. However,
the use of a low iron substrate that comprises a significant portion of the overall material cost. Currently,
the majority global consumer of low-iron glass is the solar power industry which uses it as the base
substrate for Photovoltaic (PV) cover glass as well as for Concentrated Solar Power (CSP) mirrors. As
this market grows, it can be expected that cost reductions in low-iron glass can be realized and, in turn,
with the AR coating on this glass.
Page 20 of 21 Use of Anti-Reflective Glass in Lighting Products
References
Thomsen, Scott V.; Sharma, Pramod; and Lewis, Mark. High Performance and Durable Antireflective
(AR) Coatings. Proceedings from Glass Performance Days. 11-12 June 2009, Tampere, Finland.
Gläser, Hans Joachim. Large Area Glass Coatings. Von Ardenne Anlagentechnik GMBH: Dresden,
Germany
Broadway, David M. et al. U.S. Patent Application US2022/0157703 A1, Temperable three layer
antireflective coating, coated article including temperable three layer antireflective coating, and/or
method of making the same , 30 June 2011.
Biographies
About Kevin L. Willmorth
Kevin has been involved with lighting for 30 years. His experience includes lighting design consulting for
a wide range of projects from residential to large scale commercial and hospitality, product design,
marketing, business strategy, and editorial writing. His past positions include sole proprietor, VP of
Design and Marketing for Kim, Winona, and Visa Lighting, and VP of Design – Lighting for Atlandia
Design. He currently owns Lumenique, LLC focused on the development of performance LED lighting
product, and holds the position of VP Strategic Product Development and Marketing for Zenaro
Americas, and is SSL editorial consultant for all Construction business media publications including
Architectural SSL, Architectural Products, and Illuminate.
About David P. Maikowski:
David is the Global Product Manager for Lighting for Guardian Industries - a diversified global
manufacturing company headquartered in Auburn Hills, Michigan, with leading positions in float glass,
fabricated glass products, fiberglass insulation and other building materials for commercial, residential
and automotive markets. Through its Science & Technology Center, Guardian is at the forefront of
innovation including development of high-performance glass coatings and other advanced products in all
the markets it serves. Guardian, its subsidiaries and affiliates employ 18,000 people and operate facilities
throughout North America, Europe, South America, Asia, Africa and the Middle East. Guardian has
serviced the commercial lighting industry for > 25 years and introduced its first value-added product for
the market, AR Glass for Lighting, in 2010. For more information on Guardian AR Glass for Lighting
and other glass products, visit www.guardian.com or email lighting@guardian.com.
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