Help with making a pinhole camera?

I'm trying to make a pinhole camera thingy for looking at the sun, and it's too cloudy to test it at the moment so I've been using a super bright lamp. When I look at the lamp using the camera, I get an image of a blurry-edged arc which I am assuming is the lamp filament. How do I stop it from being blurry edged? I want a sharper image.