It's light that brings the shadow puppet alive on the screen. Traditionally, Asian puppeteers use coconut oil lamps on bamboo frames between the puppeteer's face and the screen. It's romantic, atmospheric, and the fire marshal's worst nightmare. American puppeteers have had to find something different. There are several factors to look at when choosing your light - intensity, spread, color, and angle.
Intensity is the power of the light, usually measured in watts.
That's a tough question to answer exactly. For shadow puppets, there must be more light coming from a controlled source behind the screen than in front. The best way to find out what's enough is to experiment in the space you'll perform. Bright light on the screen is easy to see even at a distance, but can be harsh closer up. Dim light is harder to see, but can give a very intimate feel.