Use of Anti-Reflective Glass in Lighting Products

David P. Maikowski – Guardian Industries, Corp.

Kevin L. Willmorth – Lumenique, LLC

The Challenge of Using Glass in Lighting

• 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!

GlassGlass

Reflected:1st and 2nd

surface

Transmitted

Absorbed

Light Source

4% Loss

4% Loss

1-2% Loss

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).

2

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

3

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

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

5

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

6

Savings from Lamp Step Down

• 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

7

Based on 3200hrs/year, $0.10/kWh Energy Cost

  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

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.

8

Savings from LED Count Reduction

• 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

9

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

Luminaire Count Reduction

• 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.

10

  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

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.

11

Evaluating Potential Benefits of AR Glass in Lighting

12

• 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

Samples Tested

13

Reference samples included a range of optical designs and light sources to evaluate the impact of the cover glass materials

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%30-90 Incident Angle Zonal lumens⁰ 3239.7 3534.9 9.11% 3585.5 10.67%0-30 Incident Angle Zonal lumens⁰ 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%30-90 Incident Angle Zonal lumens⁰ 6124.0 6286.2 2.65% 6395.3 4.43%0-30 Incident Angle Zonal lumens⁰ 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%30-90 Incident Angle Zonal lumens⁰ 3089.1 3349.2 8.42% 3390.4 9.75%0-30 Incident Angle Zonal lumens⁰ 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%30-90 Incident Angle Zonal lumens⁰ 4451.8 5040.9 13.23% 5187.0 16.51%0-30 Incident Angle Zonal lumens⁰ 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%30-90 Incident Angle Zonal lumens⁰ 3560.8 3733.4 4.85% 3884.0 9.08%0-30 Incident Angle Zonal lumens⁰ 469.7 492.1 4.77% 502.4 6.96%

14

• 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).

Using AR Glass for Lamp Step Reduction

• EXAMPLE 1: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

15

2.72.7

3.63.6

2.52.5 3.03.0

3.73.7

2.52.5

Using AR Glass for Fixture Count Reduction

  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

16

EXAMPLE 2: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

•The additional cost of coated glass over soda-lime glass produces a payback of less than one year.

Using AR Glass to Reach Performance Thresholds

EXAMPLE 3:EXAMPLE 3: The use of DS AR Coating improved luminaire FTE performance from 36 lm/W to 39 lm/W

17

• This improvement required no other changes to the luminaire to transform it from a non-compliant (37lm/W requirement) to compliant product

39393636

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).

.

18

Average illuminance is greater with coated glass, while values at the perimeter of the lighted area remain unchanged

Both plots are based on 20’ luminaire mounting height and 60’ pole spacingIdentical total load of 1001.6W

1.91.9

1.21.2

4.14.1 2.12.1

1.11.1

4.64.6

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).

19