Vision and Special Senses
Many creatures see in the dark much better than humans do. Some can even see in utter lightlessness. Other creatures get along in the dark by using other senses: uncanny hearing, sensitivity to vibrations and air movement, or an acute sense of smell.
A creature’s vision determines how it is affected by obscured squares (see “Concealment". Unless otherwise noted, a creature has normal vision. Some creatures have special senses that are noted in their race entries or stat blocks. For example, a human typically has normal vision, whereas an elf has low-light vision.
Radius: If a creature has a special sense followed by a number, that number is the sense’s radius. Beyond that radius, a creature relies on its normal senses. For instance, if a creature has blindsight 6, the blindsight extends only 6 squares in every direction. If no radius is specified, the special sense extends as far as the creature can see.
Published in Rules Compendium, page(s) 167.