There are several sign-conventions. Students should try to follow any one of these. In this package, the new Cartesian sign convention has been used wherever required.
(i) All distance are measured from the pole.
(ii) Distances in the direction of incident rays are taken as positive.
(iii) Distances in the direction of incident rays are taken as negative.
(iv) Distances above the principal axis are taken as positive.
(v) Distances below the principal axis are taken as negative.
(vi) Angles measured from the normal, in anti-clockwise direction are positive, while in clockwise direction are negative.
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