Air mass numbers have traditionally been obtained by techniques that use height as the integration
variable. This introduces an inherent singularity at the horizon, and ad hoc solutions
have been invented to cope with it. A survey of the possible options including integration by height,
zenith angle, and horizontal distance or path length is presented. Ray tracing by path length is shown to
avoid singularities both at the horizon and in the zenith. A fourth-order Runge–Kutta numerical
integration scheme is presented, which treats refraction and air mass as path integrals. The latter may
optionally be split out into separate contributions of the atmosphere's constituents.
© 2008 Optical Society of America
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