What to observe – The luminance of the objects above changes over time. Judge the brightness variation of the two large squares (the ‘surrounds’) and the two central disks.
Possible observations – Brightness of all objects changes, sometimes in manners difficult to describe. Beautiful effects can occur when the surround luminance passes that of the disk, and when the two surrounds are momentarily of identical luminance.
Comment – The luminance of the two gray disks stays constant at all times. Their seeming change is the classic simultaneous contrast..
Notice that for the physical stimulus property the term luminance is used, for the subjective experience the term brightness.