Overall bow is global curvature across a lite. Local bow (warp) is curvature confined to a section, and roller wave is a repeating ripple left by the tempering furnace’s conveyor rollers. Each has its own visual signature and its own acceptance path: a quick zebra board check can clear low-risk architectural panels, but optical, defence and medical-grade parts need instrumented measurement against a stated standard such as ASTM C1048 or EN 12150.
TL;DR:
- Roller wave shows as parallel ripples aligned with the furnace rollers and can be identified visually with a striped panel test.
- Overall bow results in a smooth, gentle curve across the entire panel, primarily caused by uneven cooling or asymmetric quenching.
- Local warp and bow occur near edges or corners due to heat loss, with improper edge finishing increasing susceptibility.
- Measurement should combine initial visual screening with instrumented optical metrology for precise, repeatable surface surface data in critical applications.
- Specifying measurement method and tolerance together, such as peak-to-valley limits on a set panel size, reduces disputes and ensures the glass meets functional standards.
Table of Contents
- Types of distortion: roller wave, bow, edge lift and lensing
- Causes and process drivers in tempering and IGU assembly
- Measurement and inspection methods: zebra board, micrometers and instrumented metrology
- Standards, specifications and how tolerances are expressed
- Practical mitigation and process controls for reducing bow and warp
- How Precision Glasses approaches distortion control and metrology in practice
- Setting acceptance thresholds without second-guessing every reading
- Getting a distortion specification quoted
- Standards and technical whitepapers to consult
- Sources
- FAQ
Types of distortion: roller wave, bow, edge lift and lensing
Roller wave shows up as a series of parallel ripples running across a tempered lite, spaced at intervals matching the furnace’s roller pitch. It is most visible under raking light or against a reflected grid, which is exactly the principle behind the zebra board test described by Glass Magazine. Overall bow is a smooth curvature across the whole pane, usually caused by uneven cooling during quenching, while local bow or warp affects only part of the lite, often near an edge or corner where heat loss is faster.
- Roller wave: repeating ripples aligned with the direction of travel through the tempering oven, most visible at grazing angles.
- Overall bow: a single, gentle curve across the full panel, typically from asymmetric quench or furnace heating.
- Local bow or warp: curvature limited to a region of the lite, often linked to edge preparation or panel support during heat treatment.
- Edge lift and edge dip: raised or dropped edges relative to the panel’s centre, which complicate gasket seating and frame fit.
- Lensing: an optical magnification effect in laminated or insulated glass units when two surfaces are out of phase, distorting anything viewed through the assembly.
Lensing matters most where glass sits in an optical path rather than a façade. A pane that passes a standard bow check can still fail a functional test if its mating surface in a laminate or an IGU is not parallel to it, because the Buro Happold whitepaper notes that out-of-parallel surfaces in laminated assemblies magnify apparent distortion well beyond what either surface shows on its own.
Causes and process drivers in tempering and IGU assembly
Bow and warp originate at specific points in the process, which makes root-cause diagnosis tractable if the fault pattern is read correctly.
- Thermal gradients and quench profile: uneven heating or an asymmetric quench pulls one face or edge cooler than the rest, producing overall bow.
- Roller condition and sag during heating: glass becomes pliable at high oven temperatures, and sag against the conveyor rollers at this stage is the direct origin of roller wave, as described in the glass flatness case study.
- Panel geometry: thin, large-format panels flex more readily on the rollers than thick, small panels, so susceptibility scales with the aspect ratio and thickness chosen at design stage.
- Edge preparation: a poorly finished or chipped edge changes how heat escapes at the perimeter, encouraging local warp near that edge.
- IGU-specific drivers: trapped gas volume, barometric pressure change after sealing and altitude differences between manufacture and installation cause pillowing, while breather tubes and correct spacer design manage that pressure differential.
Cleaning and roller maintenance sit upstream of nearly every roller wave defect, which is why factories that hold tight tolerances treat furnace housekeeping as a control point rather than routine maintenance.
Measurement and inspection methods: zebra board, micrometers and instrumented metrology
Choosing an inspection method is really a decision about acceptable risk for the application.
- Zebra board: a striped panel viewed in reflection through the glass; distortion bends the stripes and reveals roller wave and gross bow quickly, but it gives no numeric result and depends on operator judgement and lighting, a limitation the Glass Magazine piece flags directly.
- Handheld micrometers and rolling profilometry: a dial or digital gauge rolled across the surface captures peak-to-valley roller wave numerically, suited to spot checks on the factory floor rather than full-panel mapping.
- Instrumented optical metrology: laser scanning or interferometric systems map the full surface and produce a traceable, repeatable dataset, the level of evidence needed for optical, medical or defence components.
Pro Tip: Reserve the zebra board for a first-pass screen, and require an instrumented report referenced to a named standard whenever the part sits in an optical path or a regulated assembly.
Our own measurement methods guide covers how these techniques are combined in practice for parts with tight functional tolerances.
Standards, specifications and how tolerances are expressed
Specification language is where most disputes start, so naming the standard and the measurement method together closes the gap between what a buyer expects and what a fabricator delivers.
- ASTM C1048: the US specification for heat-strengthened and fully tempered flat glass, including bow definitions fabricators work to.
- ASTM C1651: covers measurement of flatness for tempered glass, giving procedural detail behind the bow figures quoted in C1048.
- EN 12150: the European standard for thermally toughened soda lime silicate safety glass, referenced alongside ASTM in cross-border specifications, as summarised in the BSI technical preview.
- Illustrative tolerance example: technical whitepapers cite a roller wave control target around ±0.15 millimetres peak-to-valley on large architectural panes, with figures dependent on panel size and thickness rather than fixed across all products, per the Buro Happold whitepaper.
The practical takeaway is to specify method and limit together, for example peak-to-valley of 0.15 millimetres measured by rolling profilometer on a stated panel size, rather than quoting a bare number with no test method attached. Our tolerances guide walks through translating a standard’s clause into a procurement acceptance line.
Practical mitigation and process controls for reducing bow and warp
Controlling distortion is largely a matter of discipline at a handful of process steps rather than exotic equipment.
- Pre-treatment checks: inspect rollers for wear and confirm panels are clean before they enter the furnace, since debris on a roller transfers directly into the glass surface.
- Quench and loading controls: orient panels consistently, sequence furnace loading to avoid uneven heat exposure, and follow documented furnace best practice for the glass thickness in question.
- IGU assembly controls: fit breather tubes where altitude or transport pressure changes are expected, manage gas fill carefully, and use spacer and seal practices that accommodate normal pillowing rather than fighting it.
- Inspection gates: verify incoming stock, take post-temper metrology readings before the batch moves on, and run a final pre-assembly acceptance check against the agreed specification.
Pro Tip: Treat post-temper metrology as a gate, not a record: hold the batch until the reading is in hand, not after it ships.
CNC panel processing on the cutting line also affects downstream shape control, an operational detail covered in Anderson Australia’s panel processing guidance. Our own manufacturing workflow overview sets out where these gates sit across a full production run.
How Precision Glasses approaches distortion control and metrology in practice
Distortion control is an important part of fabrication for medical device, defence, aerospace, automotive dashboard, lighting and electronics applications, with instrumented inspection and batch-level traceability often used behind orders. For parts with a functional optical path, that combination of tight tolerances and documented export compliance matters more than a low price on commodity tempered stock. A specification sent for quotation should state panel size, thickness, functional tolerance and the measurement method expected on the inspection report, details covered further in our glass metrology primer.
Setting acceptance thresholds without second-guessing every reading
Architectural glazing tolerates visible roller wave that would fail an optical window outright, so the acceptance rule has to match the application, not a generic industry number. A site mock-up under the lighting conditions the glass will actually see settles most borderline architectural cases faster than another round of measurement. Escalate to instrumented remeasurement only when a part sits in an optical, sealed or regulated assembly where lensing or trapped stress has a functional consequence.
— Alexandra
Getting a distortion specification quoted
If you are commissioning glass with a tight functional tolerance, list the panel dimensions, thickness, orientation in service and the acceptable peak-to-valley limit with the measurement method you expect on the report.
Toughened glass, laminated glass and coated glass fabrication alongside CNC machining and edge finishing typically involve instrumented inspection and traceable documentation integrated into the production process rather than added as an afterthought.
Send your panel sizes, thickness and functional tolerance through our services page to request a tolerance review or a quotation for your next order.
Standards and technical whitepapers to consult
Use ASTM C1048, ASTM C1651 and EN 12150 as the baseline reference set, and tighten the numeric limits from technical whitepapers where the application demands stricter flatness than the standard’s minimum.
Sources
- Identifying and measuring roller wave distortion — Glass Magazine
- Digital prototyping of architectural glazing — Buro Happold
- Making glass flatness a standard — technical case study
FAQ
What are the downsides of tempered glass?
Tempered glass cannot be cut or drilled after treatment, and it carries a higher risk of roller wave and bow than annealed glass because of the heating and rapid quench involved in toughening. These distortions can affect appearance and, in optical assemblies, cause lensing.
What is curved glass called?
Curved glass is typically called bent or curved glass, produced by shaping the pane deliberately during heating rather than as an unwanted distortion. Bow and warp, by contrast, are unintended curvature that falls outside a flat panel’s specified tolerance.
What is the recommended thickness for a glass balustrade?
Balustrade glass thickness depends on the applicable local building regulation and the loading the barrier must resist, so the figure should come from the relevant standard or a structural engineer rather than a general rule. Ask your glass supplier to confirm the correct specification for your project’s regulatory context.
What are the five types of glass?
Definitions vary by industry, but common categories referenced in fabrication include float glass, tempered (toughened) glass, laminated glass, coated glass and insulated glass units. Each addresses a different combination of strength, safety, optical or thermal performance.
