I think you are misunderstanding what High-Dynamic-Range Imaging (HDRI) actually is. Perhaps you are confusing it with PRT (I'm guessing this due to the title of the thread). Whereas PRT is used to simulate the lighting and shading in your scene, HDR allows you to display a wider range of brighnesses. This increased range of luminace makes things look a lot more realistic.

There are a few short algorithms associated with HDRI, such as tone mapping. To read about it, check out these pages:

HDR Lighting
HDR Lighting Sample