This document discusses the use of anti-reflective glass coatings in lighting products to improve efficiency. Standard glass loses around 9-10% of light due to reflection and absorption. Anti-reflective coatings on low-iron glass can increase light transmission through the glass to 99%, up from 89% for standard glass. This improved optical performance allows for lamp wattage or LED count reductions, fewer luminaires needed, and meeting efficiency thresholds. Case studies show the coatings providing payback periods of less than 2 years through energy savings from these performance gains.

1. Use of Anti-Reflective Glass in
Lighting Products
David P. Maikowski – Guardian Industries, Corp.
Kevin L. Willmorth – Lumenique, LLC

2. The Challenge of Using Glass in Lighting
The historical challenge of using a glass lens in lighting applications has been
with efficiency losses due to light reflection and absorption (shown in red below).
Light Source
Glass
4% Loss
4% Loss
Reflected: Transmitted
1st and 2nd 1-2% Loss
surface
Absorbed
• Glass loses 9-10% of total light transmission by its material properties alone
• Therefore, you must minimize absorption, increase light capture or
transmission, and minimize reflection on the glass to overcome this
challenge and maximize the light going through the glass lens
• How? Through the use of Low Iron glass and Anti-Reflective coatings!
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3. Low-Iron Glass
Standard soda lime float glass contains 0.11 – 0.08% Fe2O3 which allows 2%
of visible light’s energy to be absorbed and lost within the bulk material itself
In contrast, “Low Iron” glass contains only 0.02 – 0.03% Fe2O3 which all but
eliminates the absorption losses in the visible spectrum typically seen with
glass lenses
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4. Anti-Reflective (AR) Coatings
Anti-Reflective (AR) Coatings minimize the interference of light traveling
through a given material’s surface by creating a filtering layer with a refractive
index (n) as close to air (n= 1) and the lens material itself (glass n= 1.52)
AR coatings can reduce glass reflection losses to 0.5% per side and, when
coupled with low-iron glass to reduce absorption losses, can increase
transmission levels of glass lenses from 89% (soda lime float glass) to 99%
(Double-Sided AR on Low Iron glass) in the visible range at NADIR 4

5. Potential Benefit Drivers in Commercial Lighting
• Key enablers to realize significant benefit from
increased light transmission in commercial lighting
applications:
• Increase efficiency (light delivered per watt consumed)
• Reduce light source power level (lamp watts or LED current)
• Reduce luminaire count
• Improve lighting system performance and quality
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6. Lumen Steps Based on Lamp Watts (HID)
The lumen reduction from stepping down 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
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7. Savings from Lamp Step Down
Annual Energy Cost 1 Yr Value
400W $ 128.00
350W $ 112.00 $ 16.00
320W $ 102.00 $ 10.00
250W $ 80.00 $ 22.00
200W $ 64.00 $ 16.00
175W $ 56.00 $ 8.00
150W $ 48.00 $ 8.00
100W $ 32.00 $ 16.00
Based on 3200hrs/year, $0.10/kWh Energy Cost
• Assuming an 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.
• Example: A change from a 320W lamp to a 250W lamp, produces $44 over
two years for payback of costs associated with the retrofit
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8. Lumen Steps Based on Number of LEDs Employed
• LED luminaires require an increase in lumen efficiency equal to the drop in
LED lumens as shown in the chart.
• Example: The lumen reduction from eliminating 2 LEDs requires recovery of
~6-8% in gains in system efficiency.
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9. Savings from LED Count Reduction
LED Count Annual Energy Cost 1 Yr Value
34 $ 65.21
32 $ 61.37 $ 3.84
30 $ 57.53 $ 3.84
28 $ 53.70 $ 3.83
26 $ 49.66 $ 4.04
24 $ 46.03 $ 3.63
22 $ 42.24 $ 3.79
20 $ 38.40 $ 3.84
Based on 3200hrs/year, $0.10/kWh Energy Cost, 66lm/W system efficacy, 400lumen LEDs
• Improving optical performance to reduce LED count results in a savings
of roughly $3.80 for each 12W (800 lm) of LED power per payback
year
• Example: A reduction from 24 to 26 LEDs produces a savings of $8.08 over 2
years to payback any additional costs associated with attaining the higher
optical efficiency
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10. Luminaire Count Reduction
First Cost Annual Energy Cost
10 Fixtures $ 6,400.00 $ 1,280.00
9 Fixtures $ 5,760.00 $ 1,152.00
Savings $ 640.00 $ 128.00
• Based on 250W MH lamps, 3200 annual operating hours, $0.10/kWh
energy cost, and an installed per-fixture 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.
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11. Performance Threshold Enablers
• With the growing demand for finite performance
thresholds, such as the FTE standard suggested by the
EPA for outdoor luminaires, a difference of just 1% in
luminaire performance can mean a failure to comply.
• Achieving approval of power company, local, state, or
national efficiency approval is returned in increased
sales volumes and application opportunities.
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12. Evaluating Potential Benefits of AR Glass in Lighting
• To properly understand and evaluate the feasibility of
using AR glass, 5 products were photometrically tested
• Standard Uncoated 3.1 mm Soda Lime Glass (BASELINE)
• Single-Sided Anti-Reflective (SS AR) Coated 3.1 mm Low Iron Glass
• Double-Sided Anti-Reflective (DS AR) Coated 3.1 mm Low Iron Glass
• All testing was done to LM 79 at an IES-accredited
laboratory
• All photometric data was subsequently used in the
AGI32 and FTE Calculator software tools for evaluation
in typical commercial lighting scenarios and applications
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13. Samples Tested
Reference samples included a range of optical designs and light sources to
evaluate the impact of the cover glass materials
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14. Luminaire Total Lumen Output Test Comparison
Uncoated SS AR % Gain DS AR % Gain
Sample 1 - Significant Incident Angles <30⁰ 5984.7 6568.1 9.75% 6702.0 11.99%
30-90⁰ Incident Angle Zonal lumens 3239.7 3534.9 9.11% 3585.5 10.67%
0-30⁰ Incident Angle Zonal lumens 2746.4 3030.6 10.35% 3119.9 13.60%
Sample 2 - Mixed Incident Angles 6918.7 7091.7 2.50% 7224.5 4.42%
30-90⁰ Incident Angle Zonal lumens 6124.0 6286.2 2.65% 6395.3 4.43%
0-30⁰ Incident Angle Zonal lumens 797.8 808.7 1.37% 832.5 4.35%
Sample 3 - Dominant Incident Angles <30⁰ 4378.7 4704.4 7.44% 4814.0 9.94%
30-90⁰ Incident Angle Zonal lumens 3089.1 3349.2 8.42% 3390.4 9.75%
0-30⁰ Incident Angle Zonal lumens 1291.1 1357.4 5.14% 1425.9 10.44%
Sample 4 - Significant Incident Angles <30⁰ 6972.0 7858.5 12.72% 8213.1 17.80%
30-90⁰ Incident Angle Zonal lumens 4451.8 5040.9 13.23% 5187.0 16.51%
0-30⁰ Incident Angle Zonal lumens 2523.4 2821.7 11.82% 3028.9 20.03%
Sample 5 – Mixed Incident Angles 4029.4 4224.4 4.84% 4385.2 8.83%
30-90⁰ Incident Angle Zonal lumens 3560.8 3733.4 4.85% 3884.0 9.08%
0-30⁰ Incident Angle Zonal lumens 469.7 492.1 4.77% 502.4 6.96%
• 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).
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15. Using AR Glass for Lamp Step Reduction
3.6 3.7
2.7 2.5 3.0 2.5
• EXAMPLE 1: In a parking lot, DS AR coated glass facilitated stepping
down one lamp wattage, from 175W to 150W.
• Energy cost savings of $8 per year would support a premium of $16 for 2 yr
payback with no other product modifications
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16. Using AR Glass for Fixture Count Reduction
EXAMPLE 2: A baseball playing field lighting system comprised of 6 poles
utilizing 44 total luminaires is reduced to 40 luminaires by the 10% optical
efficiency gains realized from DS AR glass
Soda Lime Glass DS AR on Low Iron
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
•The additional cost of coated glass over soda-lime glass produces
a payback of less than one year.
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17. Using AR Glass to Reach Performance Thresholds
EXAMPLE 3: The use of DS AR Coating improved luminaire FTE
performance from 36 lm/W to 39 lm/W
36 39
• This improvement required no other changes to the luminaire to transform it
from a non-compliant (37lm/W requirement) to compliant product
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18. Using AR Glass to Reach Performance Thresholds (2)
The improvement in FTE performance is also evident in foot candle plot
comparisons of soda lime glass (left) and DS AR on low iron glass (right).
.
1.2 1.1
1.9 4.1 2.1 4.6
Both plots are based on 20’ luminaire mounting height and 60’ pole spacing
Identical total load of 1001.6W
Average illuminance is greater with coated glass, while values at the
perimeter of the lighted area remain unchanged
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19. Summary/Conclusions
• Using 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 substituted higher-performing
glass lens can be validated within a reasonable payback period
• There are also 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 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).
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