Mirror sphere used to adjust lighting

I watched a documentary recently on merging a CG creature with a real world scene. An actor held a spherical mirrored ball, about 20cm in diameter, in his hands in the centre of the real world scene. They said that the ball was used to adjust the lighting for adding the creature to the scene.

Does anyone know how this is done ?

Sounds like HDRI & Luminance Space.

Interesting. Can HDRI be used for illumination in Blender ?

If someone handed me that piece of video and said, ‘stick a chrome-plated death-droid in place of the actor’, I could simply look at the reflections in the mirrored sphere he was holding to place my lights accurately, with relative brightness and color. I could even grab the pixels from that sphere to create a texture and use it as a skybox. By doing this, I could help ensure that the lighting/shadows/reflections of my CG death-droid matched the lighting, etc. of the scene into which I am placing it.