Comparison of Visible Light Transmission (VLT) for Thick Structural Glazing

Let's compare the visible light transmission (VLT) for two prominent candidates in structural glazing: thick laminated float glass and thick cast acrylic (PMMA).

Laminated Float Glass

We are considering a very thick, high-performance configuration: four layers of 12mm low-iron glass bonded with three SGP (SentryGlas Plus) interlayers.

• Glass Lites: Low-iron glass (also known as "ultra-clear" or "super white" glass) is specifically designed to minimize the greenish tint inherent in traditional float glass by significantly reducing iron content. This results in individual glass panes with a very high intrinsic VLT, typically around 90-92% even at 12mm thickness.
• Interlayer: SGP (SentryGlas Plus) is a high-performance ionoplast interlayer renowned for its exceptional optical clarity, low yellowness index, and high stiffness. It contributes minimal optical distortion or haze to the overall laminate.

For a substantial laminated glass assembly like 4 x 12mm low-iron glass with 3 SGP interlayers, the total glass thickness is 48mm, plus the thickness of the SGP interlayers. While the individual components are optimized for clarity, the cumulative effect of multiple glass layers and interlayers, along with surface reflections at each interface, will lead to a reduction in the overall VLT.

For such a thick, multi-layered, low-iron SGP laminated glass configuration, you can expect the VLT to be in the range of approximately 75% to 85%. This is still considered excellent transparency given the extreme thickness and structural performance it provides.

Acrylic (cast PMMA)

Clear cast acrylic (PMMA) is known for its outstanding optical clarity. While often quoted as having a VLT "up to 92%" for thinner sheets (e.g., 3mm), it's important to understand how thickness affects this. The VLT for clear PMMA decreases slightly as the material's thickness increases due to internal absorption.

For very thick architectural and structural applications (which can involve many tens of millimeters, or even hundreds of millimeters, of PMMA), the VLT typically remains very high, often above 88-90%. Acrylic still generally offers higher VLT than equivalently thick glass in many standard comparisons, especially when considering very large thicknesses where glass's natural tint becomes more prominent even with "low-iron" formulations.

Conclusion

When comparing these optimized, thick structural glazing options:

• Laminated float glass made with four 12mm layers of low-iron glass and three SGP interlayers will have a VLT in the range of approximately 75% to 85%. This is a highly transparent result for such a robust, multi-layered safety glazing.
• Thick cast acrylic (PMMA), even at substantial structural thicknesses, typically maintains a VLT of above 88-90%.

Therefore, for maximizing visible light transmission in very thick structural glazing, cast acrylic (PMMA) generally offers a higher VLT compared to a multi-layered low-iron SGP laminated glass assembly of comparable structural capability.