You are thinking of a diffusion head. Condensers produce collimated light in which it is focusing an image of the top of the bulb down toward the lens with most of the rays traveling in the same direction. If the enlarging lens (or the bulb) is not in the correct position then many of the light rays bypass the lens and the effect is dimness. If you put a diffusing screen above the negative, you have now evened out the light so any lens position is OK but it now is much much dimmer because the rays are scattered about.
Originally Posted by Tim Gray
A clear piece of negative film will allow essentially all collimated rays to pass through in their original direction and is not a diffuser.