This what is in the rules for model aircraft which multirotor is included dating back to 2007. This is only one paragraph of those rules.
By definition, a model aircraft must be “flown within visual line of sight of the person operating the aircraft.” Public Law 112-95, section 336(c)(2). (1) Based on the plain language of the statute, the FAA interprets this requirement to mean that: (1) The aircraft must be visible at all times to the operator; (2) that the operator must use his or her own natural vision (which includes vision corrected by standard eyeglasses or contact lenses) to observe the aircraft; and (3) people other than the operator may not be used in lieu of the operator for maintaining visual line of sight. Under the criteria above, visual line of sight would mean that the operator has an unobstructed view of the model aircraft. To ensure that the operator has the best view of the aircraft, the statutory requirement would preclude the use of vision-enhancing devices, such as binoculars, night vision goggles, powered vision magnifying devices, and goggles designed to provide a “first-person view” from the model. (2) Such devices would limit the operator's field of view thereby reducing his or her ability to see-and-avoid other aircraft in the area. Additionally, some of these devices could dramatically increase the distance at which an operator could see the aircraft, rendering the statutory visual-line-of-sight requirements meaningless. Finally, based on the plain language of the statute, which says that aircraft must be “flown within the visual line of sight of the
person operating the aircraft,” an operator could not rely on another person to satisfy the visual line of sight requirement.
See id. (emphasis added). While the statute would not preclude using an observer to augment the safety of the operation, the operator must be able to view the aircraft at all times.
Regulations.gov