Why glass lenses are the best option for LED Billboard lights
So the topic has come up as to why Holophane uses a glass optical lens for their billboard LED fixtures. Let us explain the pros and cons of using a glass lens.
As many of you in the billboard industry know, for the last 30 years Holophane has pretty much owned the marketplace for one simple reason- the prismatic lens. The prismatic lens provided the industry with the most uniform lighting on the copy. Most of you don’t know this, but Holophane is actually an OEM provider to many of the other LED light fixture manufacturers.
There are three primary materials used to manufacture lenses. These are polycarbonate, acrylic, and borosilicate glass. Holophane uses borosilicate glass.
To determine which application is best for a billboard light fixture we have to examine how well it transmits light, resists impact, it’s surface hardness, clarity, and last but not least the resistance to discoloration.
Borosilicate glass transmits 92% of the lumens, while polycarbonate transmits only 85% when new and drops off significantly as it is exposed to UV, heat, and age. Acrylic is similar to borosilicate glass at 92% when new, but is also has degradation to UV. In order of performance Borosilicate 1, Acrylic 2, Poly 3.
Impact resistance- Acrylic and Polycarbonate have considerably higher impact ratings than glass when you compare similar thickness. What you don’t realize is that you are not comparing apples to apples. Most plastic lenses are less than 1/8″ thick on billboard lights and the Holophane Sign Vue family of billboard LED lights has a wall thickness of 1/2″. This significant difference in mass reduces the plastics material strength advantage. Three way tie in the impact resistance test.
Surface hardness- Borosilicate glass has the advantage of acrylic and polycarbonate when it comes to harness.
Clarity- Borosilicate glass is as good as it gets in this category. Acrylic develops a haze over time and polycarbonate develops a significant haze over time. Many studies show polycarbonate can break down by 20% in 3-4 years even without the exposure to UV light. Just think about how much a UV light a billboard light fixture receives over the course of a year. Without clarity, your ad copy starts to appear dull and dim over time. Another win for borosilicate glass.
Resistance to discoloration- We all know how a billboard looks when the lamps in our HID fixtures start to discolor. Everything starts to appear different and the customers start ringing the phone late at night. Borosilicate glass will not discolor when exposed to UV light, heat, and everyday environmental conditions. Both acrylic and polycarbonate do. Polycarbonate is the loser in the category by far with acrylic bringing in second and once again, borosilicate winning.
So, why is Holophane the only manufacturer offering borosilicate lenses on their billboard LED fixtures designed for outdoor advertising? Two reasons. Cost and ability. A borosilicate lens adds cost to the light fixture. Shockingly enough, our more expensive competitors use acrylic or poly. As far as ability, either they outsource the lens or invest in a foundry.