In the past I had briefly wondered how the bright crosses were created when sunlight reflects off double glazed windows onto nearby surfaces. But only very briefly. Not so the author of an article in the December issue of the British Sundial Society Bulletin. Michael Lowne observed the effect, realised it was to do with a dishing of the pane of glass, and proceeded to measure it on a handy window. The centre of his 400mm square unit was 0.8mm lower than the edges.
Anyway, this is the resulting pattern from one of our sitting room windows projected onto the house opposite. The containing circle of light is the reflection from the inner pane, which offers a convex surface to the sunlight, says Michael.