by Matt Raithel

HDRI lighting is a method of lighting a scene using the color information within an image to produce light values which illuminate objects in a scene. The advantage of using HDR images versus a regular JPG or PNG is that the HDR image stores light values for multiple exposures in a single image which can give you infinitely more light value combinations than a standard image.

Load a HDR image via the image editor (image1) (crtl+F4). You may need to switch the pull-down menu to accept all images to view any HDR images.

Load the Image World plug-in via the Backdrop (crtl+F5) panel and load the HDR image form the pull-down menu there (image 2).

By preference, I remove all other lighting information (scene lights, ambient lighting) before enabling Radiosity set to Backdrop Only with the rays set to 6 x18 (image 3).

Again, I choose to set Anti-aliasing to Enhanced Low and check the Shading Noise Reduction box in the camera properties, but it is not required (image 3).

Adjust the reflection settings on your sphere, anywhere from 15%-20% yields pretty good results.

Depending on your object complexity, render settings, and processor power the image could take anywhere from 30 seconds to 30 days to render. The goal to find the middle ground where you can get acceptable renders within the allotted time (image 4).

More HDR images can be found on the web by searching Google for "HDR images". However, when downloading from HDR galleries make sure you are using the "save target as" method to ensure you are downloading a HDR image, not the JPEG thumbnail of the HDRI.
The render shown in (image 4) was rendered at 640 x 480 with Radiosity set to backdrop and 8 rays, enhanced low AA, and shading noise reduction on a Dell 2.5 MHz machine with 1 GB of ram and took 12m21s (741.8 seconds).
However, (image 5) utilized the same HDR setup without radiosity and took only 25 seconds on the same machine. The decision to use radiosity should depend on the nature of the project and the time alloted for completion.