  Optical see-through head-mounted displays (OST HMDs) are a major output
medium for Augmented Reality, which have seen significant growth in popularity
and usage among the general public due to the growing release of
consumer-oriented models, such as the Microsoft Hololens. Unlike Virtual
Reality headsets, OST HMDs inherently support the addition of
computer-generated graphics directly into the light path between a user's eyes
and their view of the physical world. As with most Augmented and Virtual
Reality systems, the physical position of an OST HMD is typically determined by
an external or embedded 6-Degree-of-Freedom tracking system. However, in order
to properly render virtual objects, which are perceived as spatially aligned
with the physical environment, it is also necessary to accurately measure the
position of the user's eyes within the tracking system's coordinate frame. For
over 20 years, researchers have proposed various calibration methods to
determine this needed eye position. However, to date, there has not been a
comprehensive overview of these procedures and their requirements. Hence, this
paper surveys the field of calibration methods for OST HMDs. Specifically, it
provides insights into the fundamentals of calibration techniques, and presents
an overview of both manual and automatic approaches, as well as evaluation
methods and metrics. Finally, it also identifies opportunities for future
research. % relative to the tracking coordinate system, and, hence, its
position in 3D space.
