Tom Kreyche wrote: > . . . > Does anyone have ideas on building simulated sources of sunlight? . . .
I read this thread of messages about your question just last Wednesday. I built a solar simulator 2 years ago using a Fresnel lens just like the suggestion of W. Maddox. What was very coincidental is that I had just come in from my workshop where I am building another, smaller and improved. Using a slide projector would give you only a beam as wide as the lens diameter (although quite bright). For a large diameter at low cost the Fresnel lens is used. I used one from Edmond Scientific that was designed for an overhead projector and is 10 by 10 inches square with a focal length of about 12 inches but expensive. A much less expensive source is at most good office supply stores - plastic page magnifiers! They cost around $2 and give you an aperture of about 10 by 7 inches at focal lengths of 11 to 20 inches. At that cost it is worthwhile to shop around and buy several different brands. Test them for focal length and clarity by imaging a distant scene. Inspect for a good cast from the mold with no ripples. Although they are not as rigid, there are some that are as good as the overhead projector types. When I built the first simulator I was not trying to " test the ability of different gnomon designs to cast shadows". Consequently, I tried Thursday to use my old simulator for that purpose, or as Bill Maddox said, for sciagraphics. It is very difficult! I placed objects in the beam and attempted to find the distance from a screen at which the central umbra disappears. I consistently placed the ball of a florist pin or thin strips too far away. I had a little better success in sunlight, but I still got angles for the sun that were too small just as I had done for the simulator. Judging the quality of shadows is hard, especially on a small scale, even using a hand magnifier. For a light source, consider the high intensity lights using the small 12 volt halogen bulbs. The filaments of such bulbs are about .5 cm long and 1.5 mm wide. On my old simulator I had turned the bulb to point the filament at the lens. With a focal length of about 11 inches this should yield a divergence of only 1/4 of a degree. Turning about half way to perpendicular to the lens should give about the 1/2 degree. However, the divergence will now be larger in one direction (same as direction of the filament) than perpendicular to it. If you want ideas for other refinements let me know. Claude Hartman Los Alamitos, CA (yes, the sun still shines!)
