Inverting Normalmaps

A normalmap is made up of several rgb images.

In this example, the normal map on the left won't produce the intended result in Doom 3 but the problem is corrected very easily by inverting the green channel as depicted by the normal map on the right.

As for each channel itself and what you should be looking for.

The red channel represents the horizontal displacement. It should appear as if it's being illuminated from the right.

The green channel represents the vertical displacement. For Doom 3 engine based games, it should appear as if it's being illuminated from the bottom.

Depending on the application you used to render your normal map, you may need to invert this channel in an image editing application.

The blue channel is, from the perspective of an artist, unimportant so you don't really need to concern yourself with it.