Figure 2. Single-point diamond turning of aspheric components is performed
one part at a time on a lathe.
once the equipment is properly configured, it is possible to produce many parts without having to make major adjustments. As
a result, diamond turning is most cost-effective when used with
medium- to high-production volumes.
The use of diamond turning also opens up the possibility of
directly producing a diffraction grating profile on the surface of
an optic. This can be a particularly powerful tool for the designer,
enabling the correction of various wavefront errors as well as
making it possible to achromatize an element using only a single
However, just as in the case of aspheric surfaces themselves,
there is no universal standard on how diffractive surfaces are
mathematically described. Thus, it is important to communicate
all the relevant parameters of the grating to the manufacturer.
These include the diffractive coefficients, the refractive index
of the materials preceding and following the grating surface, the
diffractive order, the profile direction and the number of zones
Fabrication cost factors
The single-point diamond-turning fabrication technology used
to produce aspheres has certain distinct characteristics, advantages and limitations. Understanding these can help the buyer to
obtain components that perform well, yet still minimize costs.
Of these factors, one of the most important is the different cutting characteristics of various commonly used materials. For example, silicon is difficult to machine and wears cutting tools very
rapidly. A tool might cut literally just a few silicon parts before it
must be changed out or sharpened, which requires time and therefore incurs cost. CaF2 also wears tools rapidly and typically might
yield a dozen or so parts before replacement. In contrast, ZnS and
ZnSe are easier to machine, and it’s not uncommon to produce
two to three times as many parts of the same size with a single
tool. Germanium is perhaps the material for the infrared that is
softest and easiest to machine; processing hundreds of parts
might be possible before tool replacement is required.
Diamond turning generally produces a rougher surface finish
than traditional polishing techniques, so it is most commonly
employed for infrared optics where a given roughness has less
of a performance impact. However, roughness is still an impor-