principal plane

[′prin·sə·pəl ′plān] (mathematics) For a quadric surface, a plane that passes through the midpoints of all the chords perpendicular to it. (optics) Two planes perpendicular to the optical axis such that objects in one plane form images in the other with a lateral magnification of unity. The vertical plane passing through the internal perspective center and containing the perpendicular from that center to the plane of a tilted photograph. principal section

principal plane

A vertical plane that contains the principal point of an oblique photograph, the perspective center of the lens, and the ground nadir. It is the vertical plane containing the optical axis.

principal plane

prin·ci·pal plane

the theoretic plane of a compound lens system. See: principal point.

ray

In geometrical optics, a straight line representing the direction of propagation of light.
axial ray A ray that is coincident with the axis of an optical system.
chief ray A ray joining an object point to the centre of the entrance pupil of an optical system (Fig. R2). See pencil of light.
emergent ray A ray of light in image space either after reflection (reflected ray) or after refraction (refracted ray).
extraordinary ray See birefringence.
incident ray A ray of light in object space that strikes a reflecting or refracting surface.
marginal ray A ray joining the axial point of an object to the edge or margin of an aperture or pupil (Fig. R2).
ordinary ray See birefringence.
paraxial ray A light ray that forms an angle of incidence so small that its value in radians is almost equal to its sine or its tangent. (i.e. sin θ = θ or tan θ = θ. These are approximate expressions referred to as the paraxial approximation (or the gaussian approximation). See paraxial optics; paraxial region; gaussian theory.
principal ray A ray joining the extreme off-axis object point to the centre of the entrance pupil or aperture (Fig. R2).
ray tracing Technique used in optical computation consisting of tracing the paths of light rays through an optical system by graphical methods or by using formulae. Nowadays, computer methods are used. See sign convention.

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Fig. R2 Rays of light incident to the eye (E, centre of the entrance pupil of the eye)

Table R1 Differences between the sine and the tangent values of various angles (in degrees and radians). The error is calculated between the sine value and the value in radians and between the value in radians and the tangent value
angle (deg)	angle (rad)	sine value	tangent value	error (%) sine error	error (%) tangent error
0.5	0.008 727	0.008 727	0.008 727	0.00	0.00
1	0.017 453	0.017 452	0.017 455	0.01	0.01
2	0.034 907	0.034 899	0.034 921	0.02	0.04
3	0.052 360	0.052 336	0.052 408	0.05	0.09
4	0.069 813	0.069 756	0.069 927	0.08	0.16
5	0.087 266	0.087 156	0.087 489	0.13	0.25
6	0.104 720	0.104 528	0.105 104	0.18	0.37
7	0.122 173	0.121 869	0.122 785	0.25	0.50
8	0.139 626	0.139 173	0.140 541	0.33	0.65
10	0.174 533	0.173 648	0.176 327	0.51	1.03
15	0.261 799	0.258 819	0.267 949	1.15	2.35
520	0.349 066	0.342 020	0.363 970	2.06	4.27
30	0.523 599	0.500 000	0.577 350	4.72	10.27