TTHI: Tolerance on Thickness
TTHI is used to tolerance both absolute positions of elements as well as thicknesses of lenses within element groups.
When OpticStudio creates the default tolerances, it is assumed that all variations in thickness affect only that surface and any surfaces in contact with that element. For example, if the first lens in a contact doublet has a +1.0 mm change in thickness, the front and rear vertex of the second lens both shift by +1.0 mm.
However, since OpticStudio defines the position of all surfaces by using an offset from the previous surface, adding 1.0 mm to the surface will shift all subsequent lenses in the system by +1.0 mm. What is more likely to occur in fabrication is that the +1.0 mm offset would be absorbed by the first air space after the lens group. TTHI can handle this case by allowing a "adjustment" surface to be specified. When OpticStudio creates the default tolerances, the adjustment surface is specified as the first air space which follows the surface being toleranced.
To illustrate, imagine a lens where surface 3 was made of BK7, and surface 4 was made of F2, and surface 5 was air. The nominal thicknesses are 3, 4, and 6 mm, respectively. If a TTHI operand was defined by the default tolerance algorithm for surface 3, an adjustment would be defined for surface 5. If the tolerance value was +.1
mm, then during analysis the thicknesses would be changed to 3.1, 4.0, and 5.9, respectively. Thus, the absolute positions of surfaces 6 through the image surface are unaffected by the change in thickness on surface 3.
The adjustment is optional; to disable it, set the adjustment to the same surface number as the tolerance, such as TTHI 3 3. For some lens systems, such as those that are assembled by stacking spacers in a tube, the adjustment may not be desired.
Int1 is used to define the surface number, Int2 in the adjustment surface number, unless Int2 is equal to Int1. Min and max are the extreme errors in lens units.
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