AGG


Volume 26, Issue 3 - May/June 2012

feature

More Than Meets the Eye
A Close Look at Designing with
and Specifying Anti-Reflective Glass

 

Who has ever experienced a scenario such as this: The glass should have been there already. It was due on-site yesterday and the final walk through is today. I made a special trip to the job site as this was another important project for one of my top clients. I looked at the building and could see right in through the storefront where the glass should have been. The brightly lit displays held merchandise ready to be sold.

I called my glass fabricator and huffed, “Where’s the glass?! The store is set to open and we’ve got to close the openings!”

The calm and reassuring voice on the other end said, “Look closer, it’s in there.”

He didn’t know that each lite of anti-reflective glass undergoes an expensive and advanced coating process to provide greater light transmission. The reduction of reflection can make it seem as if there is not any glass between you and the items on display in the store. It is indeed as if the glass isn’t there.

Not a Reflection of You
Invisible storefronts create a unique aesthetic appeal. With reflection reduced by up to more than 16 times when compared to uncoated float glass, product displays pop and architectural designs transcend nature. Stores can blend effortlessly with their surroundings, becoming one with the square. Foot traffic will swell and, as a natural result, your clients will smile.

The type and application of anti-reflective coatings can provide glare-free, glass storefronts with visible light reflection ranging from less than .5 percent up to 4 percent as well as producing little to no noticeable color change. Anti-reflective storefronts are available as monolithic, tempered or laminated and insulating units.

With large formats also available, the many fabrication options give unrivaled flexibility in aesthetically pleasing applications. Many glass fabricators stock and custom process anti-reflective glass to bring life to architectural designer creations. Imagine an environment where beauty flows effortlessly together without the harsh reflections of unforgiving float glass.

Today’s anti-reflective coating technologies produce glass that limits glare and unsightly reflections in numerous unique applications. These high-tech coatings remove glass distractions from picture-frame glass allowing the artwork to leap off the wall and become the focus of attention. Anti-reflective storefronts invite customer attention and welcome passersby to come in and shop. When used in projection systems or displays, the light or visual media smoothly transmit through the glass, capturing the viewer’s attention without double images or visual light-front interference.

Transparent Communication
Restaurants, boutiques and image-conscience retail shops require their carefully designed, visual display elements to be communicated in the most favorable light. Glass provides a weather-tight, physical barrier that still maintains a visual connection between the public and the store. Unfortunately, traditional glass also creates a secondary plane of focus pulling attention away from what is in the store to the reflections on the glass surfaces.

Anti-reflective glass, however, can virtually eliminate the reflection. Traditional glass reflects 8 percent of the visible light. What does this really mean in real life? Only 92 percent of the outside light source reaches the inside of the storefront and provides a maximum potential for surface viewing through the same glass of 84.6 percent since 8 percent of light suffers from internal reflection on the way back out. (Maximum illuminance becomes 100 percent when no glass separates the light source and the object and no glass or substance separates the object and the observer.)

Brightness and Reflection Issues
The relative brightness of typical outside ambient light is 10 to 300 times brighter than inside lighting on overcast and sunny days. The impact that the interior lighting has on noticeable reflectance is negligible in most situations.

The direct-reflection ratio (see box below) allows comparison of visible light-front distraction between differing solutions.This ratio of “reflection annoyance” can be measured and provide a “distractibility index” to compare different solutions. Simply put, the amount of reflection divided by the amount of maximum illuminance gives a ratio that is measured on a scale of 0 to infinity. Zero would indicate that there is no reflectance no matter how much light reaches the objects within the storefront and back out to the observer.Infinity would indicate a perfect mirror where all light is reflected at the glass storefront and no light reaches the objects within the storefront and back out to the observer.

Direct Reflection Ratio
DR = Direct Reflection (% of Source)
MI = Max Illuminance (% of Source)
DIR = DR/MI = Direct Reflection Ratio

As previously discussed, monolithic float glass has visible light reflectance of 8 percent. A maximum of 92 percent of the outside source light reaches the objects within the storefront. Additionally, the brightness of the image of the objects as seen by the outside observer has been reduced by another 8 percent as the light passes again through the glass and another 8 percent of the visible light reflects back into the store.

Now, let’s calculate the ratio for a storefront with uncoated monolithic float glass:

DIR = DR/MI = Direct Reflection

Ratio Direct Reflection Ratio = 8%/84.6% = 9.5%

In this example, the amount of reflection can distract the observer from the object displayed within the storefront. The direct reflection already peaks at a substantial 8 percent. But since the maximum illuminance is only 84.6 percent, the effect of the reflection is 9.5 percent of the value of maximum illuminance. When an object is brightly colored the reflection annoyance is bad enough, but when a darker color object with fine detail and nuances is displayed, the “distractibility index” understates the problem.

Now, let’s calculate the ratio for a storefront that uses monolithic anti-reflective glass as shown in the photo on the right:

DIR = DR/MI = Direct Reflectance Ratio
Direct Interference Ratio = 0.5%/99% = 0.51%

In this example, the lower amount of interference does not distract the observer from the object. The difference in the “distractibility index” can be significant and the ability to see the item on display is increased dramatically. The two examples show a 1900 percent difference in harsh glare and bouncing light. Anti-reflective glass provides this type of benefit. The lower the direct reflection ratio of the anti-reflection glass, the higher the ability of the observer to focus on the objects displayed within the storefront.

Distractibility Index

How to Select Anti-Reflective Glass
There are a number of companies providing anti-reflective glass, each offering a range of capabilities. The following are a few basic criteria to use in selecting anti-reflective glass:
How much “anti-reflection” is needed?
• Keep in mind that normal float glass is 8 percent reflective.
• Anti-reflection glass is available in ranges from 0.5 percent to 4 percent reflection depending on configuration and supplier, and all are labeled anti-reflective.
• Use the distractibility index to compare different glass and different configurations.

How durable of a coating is needed?
• The ability to handle, fabricate, laminate, and temper can vary.
• Is it cleaned easily and can traditional glass cleaners be used?

What type and sizes are best suited for the application?
• Are stock sheets, laminated, or custom fabricated and is tempered glass required?
• What sizes best fit the application and façade?
• Not all anti-reflective glass is available in all types, reflection changes depending on use, and sizes may be limited depending on how the glass is processed.

The long and the short of it is that anti-reflective glass is great when the focus needs to be on the objects within the storefront and not on the glass. Anti-reflective glass provides beauty by being invisible. It is most valuable when noticed the least and allows more of what is being looked at to meet the eye.

Jim Gulnick, is the engineering director for McGrory Glass Inc. in Paulsboro, N.J.


Architects' Guide to Glass & Metal
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