Hello John,
> A glass artisan in Japan contacted me and told me that it is customary there > to put a protective glass or acrylic cover on the outside of stained glass > windows to prevent damage. He said that the covers are usually between 2 > and 10 inches away from the window and wanted to know it the protective > cover would interfere with the gnomon. Well all sorts of red lights went off > in my head about complications such as refraction if the gnomon was attached > outside the protective cover. I couldn't answer his question and told him to > contact Fer! Having looked into the issue of refraction before designing my windowsill sundial, I can report that the refraction in the situation you mentioned should not be a problem. I was worried about the sun's rays being distorted by the window glass before it even hit my sundial. What I found is this - as the ray of light hits the outside surface of the pane of glass in a window it is bent a bit, then it travels through the material at this slightly different angle, then upon exiting past the inside surface of the pane it is bent back to its original angle. This of course assumes a modern pane of transparent material where both the inside and outside surfaces of the pane are parallel (uniform thickness). A hand cast or irregular thickness piece of glass will refract the rays differently, and would be a problem on a stained glass sundial made of that type of glass if the hour lines were on the inside of the window. On my Spectra sundials I do all of the shadow casting activity on the back plane of the glass dialplate to avoid having to deal with the refraction of the light passing through the dialplate. I have placed many sundials over the past year or so, and double or even triple pane glass windows - some with energy/uv coatings - have not confounded the accuracy of any of them. Best Wishes, Jim -
