Calendar Introduction to Advanced Topics in
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Quick Tip –Tolerancing Radius of Curvature CODE V has four types of tolerances on radius of curvature: DLR (delta radius), DLC (delta curvature), DLS (delta sag at clear aperture), and DLF (delta fringes). Some engineers may prefer to specify the tolerance as a percentage of radius, such as 0.1%. The can be easily entered for a specific surface using the Macro-PLUS command: DLR Sk (RDY Sk)*0.001 Or, in the GUI, by entering the parametric expression " = (RDY Sk)*0.001" in the DLR tolerance value cell when defining the tolerance as shown below:
To modify a range of radius tolerances, a macro loop could be used, such as: for ^j #2 #3 where #1 is passed to the macro as the desired radius percentage, and #2 and #3 are the beginning and ending surface numbers of the desired surface range. We have chosen to not implement a percentage radius tolerance directly in CODE V, since the accepted engineering standard is to use only physically measurable units on drawings so the lens fabricator does not have to calculate any dimensions (and perhaps calculate them wrongly!). The real effect of a radius tolerance is the OPD change due to the resulting sag error, and sag change is really the critical tolerance for the optician making a test plate. Because of this, percentage radius tolerances are only really useful when the resulting sag errors at the clear aperture are in a desirable (i.e., measurable) range. For a fixed percentage radius tolerance, the equivalent sag tolerance decreases as the clear aperture decreases. The minimum sag tolerance that is measurable is roughly ¼ wave with an ordinary interferometer, which establishes a lower limit on the clear aperture over which a given percentage radius tolerance is feasible, or equivalently, establishes a lower limit on the percentage radius tolerance for a given clear aperture. The manner in which the radius is measured (mechanical scale or fringe-counting optical scale on an optical bench) also affects the minimum achievable radius tolerance – this may be an issue with very short radii. When using a DLR tolerance with default or user-entered limits, CODE V's tolerance option (TOR) compares the maximum DLR value with the value obtained from the maximum DLS and uses the smaller value. It does this independently for each surface (the minimum DLR is taken directly from the default or user-entered limits). Thus, the default maximum DLR (which starts at 1.0e10) will be based on the maximum DLS. If you want to use a larger maximum DLR that this allows, increase the maximum limit on DLS appropriately. Radius errors generally introduce primarily focus error. If a defocus compensator is used, such as DLZ SI, you may find that the upper limit on radius tolerances is determined by compensation excursion limits or by acceptable focal length error. You can increase the former by re-entering the compensator with a larger value (the input value is used as the compensator's upper limit). To monitor the focal length change, request TOR'’s distortion output (DST command), and look for the “Scale change (fraction)” in the TOR output, which is the probable fractional error in focal length or magnification (e.g., 0.005 = 0.5%). For more details on the various radius tolerances supported in CODE V, including advice on when to use each, please refer to page 8-43 of the CODE V Reference Manual, installed as part of the CODE V electronic document library. Updates to ORA's Customer Service Web Site The following additions have been recently made to the CODE V Support section of www.oraservice.com:
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