Everything you always wanted to know about Aspheres. Season 3/ Part 2

How to make a nearly perfect Asphere

In order to avoid errors and influences by blocking a semi finished lens (step d) as described in season 2 parts 1 to 6, we must

• Use perfect mandrels without lateral and radial runout, or compensate for their errors
• Avoid differences in thickness and thus wedges in (protective varnish) colors or protective film,
• Avoid measurement uncertainties caused by mechanical dial indicators and Avoid large roundness errors and large roughness on the lens edge cylinder
• Avoid thermal influences when assembling and blocking the semi-finished lenses
• have grinding machines with a workpiece spindle that runs as error-free as possible < 1 micron

This means that we have to

• Compensate for mandrel errors in axial and concentricity as well as out-of-roundness

• Avoid wedge errors due to protective paint or purple tape

• do not measure indirectly on the lens edge cylinder, but directly on the optical surface also in the middle of the lens,

• Must avoid thermal influences and work with adhesive that hardens with UV light.

Solution approach:

To achieve this, we have to clamp the lens with a manipulator and adjust the lens accordingly so that a small annular gap is created between the lens and the ring edge of the mandrel. This gap will be our “compensation” for the mandrel runout along with the thickness variation of the protective varnish or protective film.

These displacements and gaps are expected to be in a statistical range of approximately 5 to 20 micrometers in X (lateral displacement L) and Y (surface slope sigma).

When using a UV adhesive with about 10% shrinkage, the maximum error after fixation is less than 0.5 to 2 microns for L and the tilt error is 0.5 to 2 microns based on the lever arm of the ring edge diameter. In our example, the tilt angle for a lens diameter of 40 mm is 0.04 to 0.16 arc minutes. If this is not satisfactory for your application, we can use an adhesive with less than 10% shrinkage or special lens holders to avoid this shrinkage effect. In our previous manufacturing processes, we have positioned similar parts to very successfully achieve a position with tolerances of 1.5 microns in three dimensions (X, Y, Z) in series production of 2000 parts per month.