From Kerbal Space Program Wiki
Despite its look, the area illuminated by this lamp is circular, not rectangular.
There is another lamp, the Illuminator Mk1, which is much stronger and reaches much further, but the mk-2 has wider light cone and uses less energy.
|“|| The Illuminator Mk2 is an extra-wide beam, low-power Illuminator, for lighting up large swaths of whatever you wish. However, its beam does not shine very far.
— RKEA Inc.
- Initial Release