Thomas, I made a pinholecamera from a filmcan and taped it to my balcony, left it for about four days(eagerly counting the first week and then I forgot about it...).
Actually I made four, first two I put in fixer and this one is a straight scan. The rainbow is actually the sun going up and down and the little gaps are from days when it's been cloudy! See more at http://www.solargraphy.com/
Colin, yes, I have big plans on a grander scale!