Sign conventions - centered systems

Sign conventions

The proper signs for curvatures, thicknesses and indices are determined easily for systems that do not contain tilted surfaces. Then the following rules apply:

Sign conventions for centered systems

RADIUS OF CURVATURE	The radius of curvature, or curvature of a surface is positive if the center of curvature lies to the right of the surface.
THICKNESS	The thickness separating two surfaces is positive if the next surface lies to the right of the current surface; otherwise it is negative.
REFRACTIVE INDEX	OSLO expects all refractive indices to be provided with positive signs. Reflecting surfaces are specified explicitly by the designation, rfl.

A simple example illustrates the sign conventions used for setting up optical systems. Consider a glass bead in which light enters from the left, reflects from the back edge, and then emerges from the same surface that it entered, as shown in the figure. The correct surface data for this system is shown below:

*LENS DATA
Glass bead
SRF  RADIUS    THICKNESS   APERTURE RADIUS    GLASS
  0    --      1.0000e+20    1.0000e+14          AIR

  1  5.000000   10.000000      4.999999 A        BK7
  2 -5.000000  -10.000000      4.999999      REFLECT
  3  5.000000      --          4.999999          AIR

  4     --         --          4.999999 S

For systems that contain tilted and decentered surfaces, or systems that are specified in global coordinates (OSLO SIX), the difference between left and right is not always apparent, so a better model is needed. A better model is to think of each surface as being described in a right-handed local coordinate system whose z-axis is the axis of symmetry.

Next Page | Previous Page | Chapter Summary | Table of Contents