And for the sake of disclosure I was inspired by this gentleman's work. The "softness" of the image adds an ethereal quality I like.
Step 1: Equipment
An Ultraviolet irradiator, I used my FOD but a compact fluorescent blacklight in a reflector should work also.
A target holder, I used a pair of helping hands.
A convex lens, mine was sourced from a pair of binoculars.
A measuring stick
A fluorescent screen.
A dark room (by which I mean a room you can make dark)
A target ( preferable one which shows strong disparity in appearance under UV irradiation compared to visible light, so as to prevent DIGGer's from dying of apoplexy when you do something they're convinced can't be done.) In this case a yellow lantana.
Step 2: First find the focal length of your lens
Lay the tape out on a flat surface and place the screen at the zero point. Then place the lens at 2N on the tape (with N being any reasonable number to start, say 2 inches) and the light source at 4N. Now slowly move the lens and the light source away from the screen, while keeping the distance ratio the same, until the light source is in focus on the screen. At this point the lens focal length is equal to N.
I assume this is a valid approach since 1/u+1/v=1/f The basic simple lens calculation. Sorry no image, but you can do this empirically as well.