The Observation Room is a component in all test chambers in Portal and Portal 2. They do not serve any purpose in the test chamber, but is it an element of test chambers in the Portal game series. They are rooms separated from the testing chambers with glass, which would have been used by scientists, if not for the events of Lab Rat.
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From a practical point of view, the purpose of the Observation Room is to provide a light source to the test chamber, especially when no other light source would be available. A large Observation Room (2 tiles) is a required element of any puzzle made using the Puzzle Creator, to calculate lighting. Additional smaller Observation Rooms (1 tile) can be optionally placed anywhere the designer wishes.