At last day, 21st, May 2012, we enjoyed ring eclipse event in Japan.
Differ from 2010 Total Solar Eclipse, many mass-media and social-media announced not to look sun directly, via lens without correct equipment for escaping warns eyes, and media told how to enjoy eclipse with eclipse glasses and pin-hall projection.
Thanks for their media’s announcement, pin-hall projection is now common sense for Japanese, but I couldn’t find out why ring projected by pin-hall projection. Then, I decide to post this entry.
Thinking why sun-light is thrown for walls through pin-hall, may makes child to be confused, then I try to explain reversely, with “Ray-Tracing” method, which is major way to make photo realistic computer generated graphics on movie.
In Ray-tracing method, starting point is projected wall. Each part of wall receives a single ray from pin-hall in pin-hall projection box. If pin-hall box is not closed, each part of wall receives huge number of rays from everywhere around, but in closed pin-hall box, we only need to consider a ray from pin-hall.
Start to track ray backward straightly from each part with expanding line from part of wall to pin-hall. Rays which starts some parts hit sun, other may hit moon and many hit on anything. With sampling some times, you will get a ring (or 2 lighten area on profile model) projected on wall.