Volume 36, Issue 10, October 2001

Roll, Maybe, Roll
Measuring Roller Wave and Optical Distortion in Architectural Glass
by Alex S. Redner and Barbara R. Hoffman

Glass manufacturers can take advantage of new, low-cost, easy-to-use testing instruments that measure roll wave and optical distortion in architectural glass. 

Roll waves are periodic waves typically observed in horizontally tempered glass. They can cause deformation of reflected images, which are visually noticeable in architectural glass. Roll waves cannot be eliminated completely because they are inherent to fully tempered or heat-strengthened glass fabricated on a roller hearth tempering oven. However, roll waves can be measured and controlled to minimize their distorting effects.

When glass is heated, it can be lifted by an off-center roll or can sag slightly between the rolls of tempering furnaces. Waviness in the glass is due primarily to a combination of roll eccentricity and sag, and is also affected by roll spacing and other factors. 

While some companies have developed in-house methods for detecting roll waves testing equipment with objective, quantitative results has not been readily available until recently. 

Figure 1:  The design above illustrates the roll-wave of glass, with a Peak-to-Valley Depth "W" and wavelength "L" Peak-to-Peak.

Measuring Roller-Wave Distortion
The test method proposed recently (Document TD 01 03-00) by the Glass Association of North America (GANA) involves the use of instruments that measure out-of-plane deformation of the glass surface, i.e., the roll wave of the glass (see Figure 1). At press time GANA said the roll-wave subcommittee was planning to meet on September 30 at which time it would approve the document or decide that more work needs to be done. 

Testing involves the use of either a flat-bottom gage or three-point contact gage equipped with a dial indicator, digital micrometer or linear variable differential transformer (LVDT). The gages feature a protruding plunger which measures the peak-to-valley depth of the wave. 

The test is very simple. In essence, the operator places a yardstick/measuring stick or measuring tape on the surface, perpendicular to the roll waves. The roller-wave gage is pushed or pulled along the length of the glass lite. The operator takes note of the gage reading and the position on the tape at the peaks and valleys of the roll wave. The peak-to-valley depth (W) indicates the level of out-of-plane deformation of the roll wave. The peak-to-peak distance is used to calculate the level of optical distortion. 
Various gage types are available. Figure 2 is a flat-bottom gage equipped with a dial indicator. Figure 3 is a three-point contact gage, equipped with a more accurate digital indicator. This model is also available with an LVDT providing electric output and an optional trolley system to pull it across the glass. A number of non-contacting optical measuring instruments are also available, some of which feature PC-based readouts.

Fig 2  Figure 2: An RWG flat bottom gage features a dial indicator.

  Fig 3 Figure 3: The RWG 3-point contact gage includes a digital indicator.

Optical Distortion
Optical distortion is related directly to the depth and length of the roll waves (Distortion = 4p2W/L2). Generally speaking, higher depths (W) and shorter wave lengths (L) will result in the greatest amount of optical distortion. The same relationship exists with fun-house mirrors deep, frequent waves will cause the greatest distortion of reflected images. Numeric tables are supplied with the equipment manuals to help operators convert roll-wave measurements to optical distortion in universally accepted measurement units.
The test method can be implemented by virtually any temperer, since the equipment costs are quite low. The measurements provide tempering operators with a means of monitoring roll wave and optical distortion. Furnace and roll maintenance is essential in minimizing roll waves in flat glass, and excessively high furnace temperatures should be avoided.

Suppliers and purchasers now have a way to measure roll wave and optical distortion, but there are no definitive specifications for roll wave or acceptable levels of optical distortion. Some tempered glass producers are applying their own quality standards, but architects and contractors don't have published standards to specify maximum roller wave or optical distortion. In the future, we can expect to see product specifications that will vary depending on the glass thickness, end use, position and architectural exposure of a product. 


Alex S. Redner and Barbara R. Hoffman serve as president and marketing director respectively for Strainoptic Technologies Inc., based in North Wales, Pa. 


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