The Paraxial Trace command lists the paraxial ray height, the paraxial ray angle relative to the optical axis, and the paraxial ray angle of incidence with the surface for the chief ray and marginal ray. The marginal ray information has the “a” subscript and the chief ray information has the “b” subscript.
It is important to note that these are paraxial quantities. By calling them paraxial, we are assuming that the rays have been raytraced in a tiny “pencil” of rays around the optical axis. The rays in this tiny pencil have infinitesimal Y heights and angles. In this limit small angles we can assume that U = sin( u ), U = tan( u ), and 1 = cos( u ) where u is in radians. “Since these infinitesimal values have finite relative magnitudes, we may use any finite numbers to represent paraxial quantities, but we must remember to assume that each number is to be multiplied by a very small factor such as 10-50, so that a paraxial angle written as 2.156878 does not mean 2.156878 radians but 2.156878 x 10-50 radians.”1
1.Kingslake, Rudolf, Lens Design Fundamentals, San Diego, Academic Press, 1978
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