Both lenses are plano-vex, so it is reasonable to assume that the flat face of each lens will be the mounting face, i.e. the face that is held against the lens mount. Let us also assume that the 200 mm spacing between the lenses can be adjusted during assembly, and in use. This would allow the beam expander to be used at different wavelengths, for example. This has some important consequences.

Consider the lens formed by surfaces 2 and 3. As surface 3 is flat, it will be the one placed in the lens mount. As a result, surface 2 is wedged with respect to surface 3 by whatever the TIRX, TIRY on surface 2 is. We should not specify a TIRX, TIRY on surface 3 as well: these should be deleted.

Similarly for the lens formed by surfaces 4 and 5: surface 4 is flat and is held against the mount. Surface 5 is then wedged with respect to surface 4: which means that the TIRX, TIRY on surface 4 should be removed.

Thickness tolerances affect not only the surface being toleranced, but also other surfaces. 'Thickness' defines the z-shift between components, and we must consider how the mounting arrangement makes thickness tolerances accumulate. As the first lens is mounted on its rear face, any extra thickness gained during tolerancing will mean that the lens grows 'backwards', and the total length of the optical system will increase.



Now the TTHI (thickness) tolerance allows you to specify an 'adjustment surface' for thickness variations. For example, if the lens were mounted on its front face, extra thickness added during tolerancing would make the lens protrude further into the 200 mm air gap between lenses, which would make this thickness slightly shorter. ZEMAX accounts for this with the 'Adjust' setting:



The Default Tolerances tool assumes that all glass thicknesses are compensated by the next following air thickness: which basically means it assumes the lens is mounted on its front face. The adjustment is optional; to disable it, set the adjustment to the same surface number as the tolerance, such as TTHI 2 2. In this case, neither glass thickness is compensated by any other thickness, and so we disable compensation in this example.

Furthermore the thickness of surface 1 and surface 5 do not matter, as these simply show light entering and leaving the optical system. These tolerances may be deleted.

The thickness of surface 3 (the 200 mm air gap between lenses) will be used as a compensator, in that it can be adjusted to minimize the wavefront error either duting assembly or in use. It should not be toleranced, so the TTHI on surface 3 can be deleted. Instead, a compensator on the thickness of surface 3 should be defined instead:

COMP 3 0 0 -.2 +.2

This defines a thickness compensator of range -0.2 to +0.2 mm.

The file is saved as 'Beam Expander Ready for Tolerancing.zmx' in the zip file at the end of this article.