As manufacturers have improved the color clarity of their glass ceramic glazing, they have also addressed other critical factors of visual and aesthetic performance, including surface quality, distortion, blurriness, haze, visual light transmission (VLT), and shadow. The relatively new practice of polishing the surface on both sides and edges improves the brilliance and reflectivity of the glass, so it appears clearer, allows more light to pass through, and improves color clarity. This results in an appearance more closely matching the non- or fire-rated intumescent products that may be located nearby. Polishing removes the subtle, dull, ‘orange-peel’ texture that has been a signature of large panes of fire-rated glass ceramics.
Sharpness and brilliance, including the accurate and undistorted definition of objects seen behind the glass, are components of surface quality—a measureable value. Standards covering surface quality include ASTM C1036-11e1, Standard Specification for Flat Glass, and ASTM C1652, Standard Test Method for Measuring Optical Distortion in Flat Glass Products Using Digital Photography of Grids. (A national standard, Federal Specification DD-G-451 Rev D, Glass Float or Plate Sheet Figured, previously covered this variable, but it has been eliminated from use.)
Other standards specifically cover heat-treated flat glass (ASTM C1048, Standard Specification for Heat-strengthened and Fully Tempered Flat Glass) and laminated architectural flat glass (ASTM C1172, Standard Specification for Laminated Architectural Flat Glass).
Distortion, clarity, and sharpness
Similar and related to surface quality is the measure of visual acuity through a glass product. Naked eye or side-by-side comparisons of one piece of glass with another can prove valuable in evaluating visual distortion in windows and doors. A quantitative review can include comparisons of the peak-to-valley depth of the typical sine curve or roll-wave surface of the glass, measured with a three-point or flat-bottom gauge. The results of recent tests by manufacturers show new glass ceramic products have parallel glass surfaces with minimal distortion.
Blurriness, haze, and light transmission
Glass with even a slightly rough surface diffuses the transmitted light and results in partial light transmittance. Smooth, polished glass surfaces provide the sharpest display of objects viewed through the glass.
Haze describes how ‘cloudy’ a view through one or more glass panels appears to be. Its primary cause is a lack of homogeneity, or uniformity within the glass composition as well as non-optimal ceramization process settings. The degree of haze, or ‘haze value,’ is stated as a percentage—the lower the percentage, the greater the transparency of the glass.
The industry’s most impressive haze value for a ceramic glass product is 0.5 percent, although most ceramics on the market have haze values ranging from 0.9 to 2.5 percent—a visually significant differential.
The capacity of light to pass through a glass panel is measured with a spectral photometer. Recent studies and user experience show high VLT levels—which indicate greater amounts of natural daylight within the building’s interior—contribute to the improved comfort of occupants. Higher VLT introduces the potential to use natural daylighting to offset the necessity of electrical lighting.