Architects spend countless hours refining elevations, aligning mullions, and tuning glass selections so a façade reads as clean, intentional, and cohesive. Yet once the building is up, wavy reflections or “funhouse” distortions can undermine all that work.
Viracon’s new Enhanced Glass Flatness Specification, measured in millidiopters (mD), is designed to close that gap between the rendering and the built reality—without adding cost to the project.
This article breaks down what’s changed, why it matters, and how you can easily incorporate millidiopter-based flatness into your specs.
What Is Optical Distortion—and Why Do You See It?
Optical distortion is unwanted curvature in the glass that shows up as:
- Ripples in reflections
- Bent or “wavy” verticals and horizontals
- A subtle “funhouse mirror” effect as you move past the façade
Most of this distortion is created during heat treating. As glass passes through the furnace on rollers and is heated/cooled, it can sag or bow. That curvature is what your eye reads as distortion.
Viracon’s process focuses on reducing those distortions at the source:
- Tight control of furnace temperature and conveyor movement
- Heat‑treating first, then coating (instead of heating coated glass), which helps preserve flatness and visual clarity
For architects and owners, this isn’t just a technical nuance. It’s the difference between a façade that looks crisp and intentional versus one that appears slightly warped or “cheap‑looking” in certain light.
Design question to consider:
On your recent projects, where have you noticed glass distortion show up most—mock‑ups, punch‑list walks, or after occupancy?
From Roller Wave to Millidiopters: A New Way to Specify Flatness
Historically, the industry has relied on roller wave as a measure of glass flatness. Roller wave is:
- A single, linear measurement taken across a line on the glass
- Useful, but limited—it does not capture the full optical behavior across the entire surface
Viracon has moved to millidiopters (mD) as the primary flatness metric:
- An mD measures the curvature of the glass surface, which directly corresponds to what the human eye sees
- Instead of sampling a line, Viracon measures every square inch of the glass surface
To make this meaningful in practice, Viracon’s enhanced spec introduces three mD measurements for each lite:
- mDA – All Surface Area:
Measures distortion across the full glass surface.
- mDL – Leading Edge:
The first ~13" of glass entering the furnace (edge in the direction of travel).
- mDT – Trailing Edge:
The last ~13" of glass exiting the furnace.
This matters because edge zones are inherently higher risk for distortion. In a traditional single-number approach, those edge issues can get “averaged away” by a large center area that looks better. Viracon’s three-part metric prevents that.
Design question to consider:
If you’ve called out roller wave in your specs before, did it actually help you evaluate the as‑built façade, or did it feel too abstract once the glass was installed?
Viracon’s New Flatness Standard: What the Numbers Actually Mean
Viracon now publishes defined mD limits for common thicknesses (6–12 mm), across mDA, mDL, and mDT. Each value is written like this:
- 95/100, 95/90, 95/135, etc.
Here’s how to read that:
- The “95” is the 95th percentile:
95% of all mD readings on the measured region (full surface or edge zones) must be below the stated limit.
- The second number (e.g., 100, 90, 135) is the maximum allowed mD value for that region.
So, for example, mDA 95/90 means:
- 95% of all surface readings are below 90 mD
- Only the worst 5% of readings are allowed to exceed 90
Some competitors quote specs at 90% instead of 95%. That may sound similar, but in practice it means they’re discarding twice as much bad data—and often that 10% includes the most visible edge distortion.
Viracon’s enhanced spec:
- Applies to all substrate tints on the exterior lite
- Uses different limits for different thicknesses because thinner glass is more prone to distortion during heating
- Applies to all sizes, without size-based carve-outs
Design question to consider:
Does having a quantifiable, whole-surface flatness spec like this feel more useful than legacy roller wave language in your master spec?
Why Thicker Exterior Glass and mD Go Hand in Hand
Viracon’s tighter mD limits for 8–12 mm glass do more than improve aesthetics:
- Higher load resistance and less risk of “pillowing” under changing weather and pressure
- Potentially better acoustic performance (STC/OITC) in some makeups
- In some configurations, improved solar/optical performance
- A more robust exterior lite that can help deter casual vandalism
In other words, specifying thicker outboard glass with defined millidiopter limits isn’t just an aesthetic upgrade—it can also support performance and durability.
No Premium, No Deduct: This Is the New Standard
One of the most important aspects of Viracon’s enhanced flatness spec is commercially simple:
- No added cost: There is no upcharge to access this mD-based flatness standard.
- No deduct: Because this is the new standard offering, there is no “credit” for not using it on a given project.
Viracon’s position is that glass flatness is too closely tied to design integrity and building value to be treated as a niche premium. This is about:
- Raising the baseline for optical quality
- Giving architects, owners, and façade consultants clear, measurable criteria that match what the eye sees
Owner lens:
It assures your building will be critiqued against something that represents visible performance, not just legacy manufacturing tolerances.
Benefits for Architects, Owners, and Tenants
For architects and specifiers
- Design integrity protected: Glass that better matches your renderings—clean lines, sharp reflections, uniform elevations.
- Clearer specifications: Millidiopters give you a quantitative tool to define flatness, beyond general ASTM bow/warp language.
- Reduced disputes: Less gray area around “acceptable appearance” after installation.
For owners and developers
- Stronger aesthetic story: Flatter glass elevates curb appeal and the perceived quality of the asset.
- Value and leasing: Cleaner façades and better views can support rents, absorption, and resale value.
- Lower risk of complaints and re-work: Fewer “this doesn’t look like what we paid for” conversations.
For occupants
- Better views: Clearer, less wavy outlooks to the outside.
- Perception of quality: Interior environments that feel intentional, not distorted by rippling façades.
Design question to consider:
If you could get measurably flatter glass at no added cost, is there any reason not to require it on your higher-profile projects?
How to Specify Millidiopters on Your Next Project
If your current specs only reference:
- ASTM C‑1048
- Generic heat‑treat requirements
- Or loosely defined roller wave limits
…you can strengthen your façade package with a few targeted updates.
Practical steps:
- Call out millidiopter-based flatness for the exterior lite:
- Reference Viracon’s published mDA, mDL, and mDT limits for the relevant thicknesses.
- Align high-visibility areas with tighter expectations:
- Campus gateways
- Healthcare and research
- Cultural and civic buildings
- Class A office and high-end residential
- Use mock-ups strategically:
- Include visual evaluation of reflections and distortion against the mD spec.
- Use the mock-up to align expectations between architect, owner, and contractor.
Viracon has updated its recommended architectural glass specification (2.7‑3113) to include millidiopter language, making it easier for architects and spec writers to incorporate.
Design question to consider:
Where in your practice does this belong—your standard master spec, a “high-visibility façade” addendum, or a client-specific upgrade path?
Measurement, QA, and Transparency
Behind the scenes, Viracon’s enhanced spec is powered by advanced digital measurement:
- Glass exiting the heat‑treat line passes under an Osprey scanning device.
- The system creates a topographical map of the entire lite and calculates millidiopters for:
- All surface area (mDA)
- Leading edge (mDL)
- Trailing edge (mDT)
- Pass/fail is determined against the 95th percentile limits in Viracon’s spec.
Key practical points:
- Every square inch of the glass surface is evaluated.
- mD captures all forms of surface deviation, including what used to be described individually as bow, local bow, edge kink, “pocketing,” and roller wave.
- Viracon can provide data reports for project mock‑up orders and invites stakeholders to plant tours to see the process and methodology in person.
Sustainability and Waste Considerations
A natural question is whether tighter flatness tolerances increase waste.
- The vast majority of global warming potential (GWP) in architectural glass comes from the float glass process, not from heat treating.
- If a lite does get rejected during heat‑treating, uncoated glass can be returned to the float manufacturer’s recycle stream.
- Viracon has been producing to these tighter internal tolerances for some time and does not expect an increase in waste as a result of publishing the mD spec.
Why This Shift Matters for Design Reputation
Not all fabricators are created equal, and flatness is not automatic—it must be designed for, controlled, and specified.
For architects, this new standard is an opportunity to:
- Align your specs with the way you and your clients actually experience glass.
- Reduce the risk that a completed project’s façade looks compromised by unintended distortion.
- Demonstrate to owners that you are proactively managing both performance and appearance down to the level of optical quality.
Ultimately, teaming with a fabricator that produces to defined mD tolerances helps ensure that:
- Your buildings are photographed the way you intended.
- Your name is attached to façades that look as sharp in person as they do in the portfolio.
Closing Thoughts
Viracon’s Enhanced Glass Flatness Specification isn’t just a new number on a data sheet. It’s a shift toward measuring and guaranteeing what the eye actually sees.
For architects, owners, consultants, and contractors, this creates a clearer path to:
- Flatter, higher-quality façades
- Better tenant experiences
- Stronger project narratives and long-term asset value
If you’d like help reviewing a current or upcoming project for where millidiopter-based flatness might add value, Martineau & Co can walk through your elevations and specs with you and talk through practical options for integrating this standard