Tolerancing

Tolerancing

The air space in the previous design can be expected to be critical, since it controls the ray height on the overcorrecting surface. To get an idea of the tolerance on thickness 2, you can use the User-defined tolerance routine in OSLO. User-defined tolerancing uses whatever current error function is defined. In OSLO PRO and OSLO SIX, you can also set up the error function using the normal internally defined operands.OSLO PRO and OSLO SIX have additional routines for Change-table tolerancing and MTF/Wavefront tolerancing. The lens being toleranced here is the lasrdb2 solution, not the lasrdb3 solution (although the two are similar).

To run a tolerance analysis on the lens, use a copy of the lens, not the original file. Change all the variables back to direct specification except the thickness of the image surface, which will be used as a compensator during tolerancing.
Click Options >> Update Tolerance Data to open the tolerance data spreadsheet. The only tolerance of present interest is the second thickness. The ISO 10110 default value for this thickness is 0.2 mm, which is much too loose for the present system. Change the tolerance to 0.05 mm, and close the spreadsheet.

Click Options >> User-Defined Tolerancing >> Setup operands. The *opsettol star command will be executed, and the operands spreadsheet will open. For the tolerance operand, enter ocm2, and enter the name pkval opd, as shown.

To compute the effect of a .05 mm tolerance on the peak-to-valley opd, click Options >> User-Defined Tolerancing >> Sensitivity. An options list will pop up, as shown below. Select Air space, and click OK. After a short time, the text window should contain an analysis similar to the following.

*TOLERANCE SENSITIVITY ANALYSIS
MERIT FUNCTION FOR NOMINAL SYSTEM:    0.110161

AIR SPACE TOLERANCE
                     MERIT FUNCTION CHANGE           COMPENSATED CHANGE
SRF   TOLERANCE    PLUS PERT    MINUS PERT       PLUS PERT    MINUS PERT
  2     0.05         1.284532      1.292483        0.308596      0.284308

STATISTICAL SUMMARY
                       UNCOMPENSATED    COMPENSATED
WORST CASE CHANGE        1.292483        0.308596
STANDARD DEVIATION
     RSS                  1.292483        0.308596
     UNIFORM              0.746216        0.178168
     GAUSSIAN             0.568451        0.135724

COMPENSATOR STATISTICS
COMP           MEAN        STD DEV        MAX
TH     5     0.000183     0.029274     0.029457

The analysis shows the effects of a .05 tolerance on the merit function, which represents the peak-to-valley opd. You may be interested in the tolerance that can be allowed to produce a given change, say .15, which would bring the system to (approximately) the diffraction limit. Click Options >> User-Defined Tolerancing >> Inverse Sensitivity. Select Air Space, then enter .15 in response to the prompt for allowed change. The text window will then show that the allowed tolerance is about 16 microns if the focus is not adjusted, or 34 microns if the focus is used to compensate for a spacing error.

*INVERSE SENSITIVITY ANALYSIS
MERIT FUNCTION FOR NOMINAL SYSTEM:    0.110161
ALLOWED CHANGE IN ERROR FUNCTION:     0.150000

AIR SPACE TOLERANCE
                ALLOWED TOLERANCE
SRF     UNCOMPENSATED    COMPENSATED
  2         0.016983        0.034557

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