Apple has released iMac (Mid 2010) Display Brightness Update 1.0, which the company says “addresses an issue with 21.5-inch iMac (Mid 2010) display brightness.”
Specifically, the update appears to resolve a problem wherein iMacs’ screens would be at full brightness for several seconds or minutes after waking from sleep, then suddenly switch to a lower brightness.
The new release is available through Software Update this download link.