Introduction to optical distortions
Tempered glass is generally less flat than standard float glass. Tempering process involves heating and cooling quickly the glass panes moving on rollers, and this causes small wave-like distortions on the glass. The amount of distortion depends highly on glass type and tempering process fine-tuning. On a glass facade, distortions become very visible as other objects with straight lines, such as neighbor buildings, are reflected on the facade. This is illustrated on the picture on the right. We propose here to evaluate the influence of otpical distortions depending on glass type, especially with coated glass.
The test scene
The city tests scene
Standard float glass
We chose to evaluate the effect of each pane independently. Results are shown on the two pictures below, on the left picture, only the outer glass pane was distorted, while the inner pane was distorted on the right picture.
Image rendered with distortions on internal pane
Image rendered with distortions on external pane
- Horizontal and vertical lines are better preserved than oblique and curved lines. This is due to the orientation of the distortion waves, which are nearly horizontal. This is especially visible around the zinc-covered dome roof.
- The dome looks significantly better with the internal pane distortion than with external pane distortion. However, this is mostly due to the screen-printed gradient : as the black dots are not transparent, they hide the reflections of the back pane. The higher part of the dome, where the black dot density is near zero, looks similar on the two images.
- Internal pane distortions look only slightly better than external pane distortions in the non screen-printed areas. The importance of flatness of both panes is similar, they is no strong reason to have different tolerances on flatness.
Coated Glass Simulation
Image rendered with distortions on external pane(coated glass pane)
Image rendered with distortions on internal pane(uncoated glass pane)
- Distortions of the coated outer pane are even more visible than with standard glass.
- On the other hand, distortions of the inner pane (right image) is barely noticeable. This can be explained by the low contribution of the inner pane to overall reflection, due to the higher reflection coefficient of the coated glass.
- With reflective coated glass such as this one, special care should be taken in improving the glass flatness. On some projects with strong aesthetical requirements, use of this tempered coated glass on the outer pane may have to be avoided.
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