Perception of Precision, revisited

Photonics West (PW) is a trade show held in San Francisco each January. The  event is organized by International Society of Optics and Photonics (SPIE). I have been exhibiting there for over 25 years and, each year, I find new components and  technologies, but the one thing that I come away with is the fact that the Contact Lens industry is very much a precision optics industry. This is borne out by the manufacturing and inspection methods utilized in making lathe turned contact lenses.

At PW, people come to the booth and ask about the quality of lenses that our machines can produce. They often ask about "scratch & dig and figure". Scratch & dig refers to scratches and pits on the surface of the lens. A scratch is defined by a groove of a certain length and width. A dig is a pit of a certain diameter. It can be caused by grinding compound or from an inclusion or void in the material. These defects can be limited to a specified size that will not adversely affect the performance of the lens. 

An example is a multi-element optical system like in a camera lens. A 30/20 specification  is one 30 micron wide scratch over half the surface and one 20 micron pit. I was surprised to hear that this is a common level of quality. The scratch and dig affect the scatter of the light. Of course, we would never accept a scratch and dig of that size on a contact lens.         
Figure refers to the actual precision of the radius. This controls the optical performance  and is usually measured in 1/20th-1/10th the wavelength of the transmitted light. This metric holds true for reflective optics as well. So, for a visible light wavelength of around 600 nm, a 30 mm radius lens with a 1/10 wave figure is within 600 nm/10= 60 nm. That is .00006 mm.  There are probably not very many contact lenses made to that precision, especially after prolonged polishing. I commented to a precision optics fabricator that a 30/20 scratch and dig spec was big for a contact lens.

He said "That's no problem, you can polish that out in a few minutes." So, I was thinking, where do lathe turned contact lenses fit in the optical range of precision? To evaluate this we can look at the traditional methods of fabrication.Precision optics are generated by a rotating grinding wheel in contact with a rotating work piece. The rim of the grinding wheel is positioned over the center of the work piece. When the wheel is inclined relative to the axis of the work piece, it grinds a sphere, concave or convex. With an accurate setup, a very precise sphere can be generated. The final precision is determined by pitch polishing under the watchful eye of an expert optician. Fraction wave precision can be attained with this method. Any radius 
can be made.

Contact lenses are turned on a lathe with a single point tool. The precision is primarily determined by the lathe spindle and stability and tool setup. With a superior surface finish and figure, little polishing is required. Again, any radius can be made. Spectacle lenses are generated with a rotating grinding wheel with the non-rotating work piece passed across it. Setting up for a precise sphere is not inherently as good as the precision optics type of generator. But the advantage of the spectacle lens generator is that a toric (cylinder) surface can be generated. The drawback is that these surfaces need to be ground or lapped using a fixed lap covered with sand paper from a tool inventory. This tool actually sets the radius. Then it is polished with a fabric faced 
lap. The laps are made in fractions of diopter increments. These increments are good enough for ophthalmic applications.

With the move to free-form optics in all three industries, the generating machine precision has improved to the extent that it is no longer necessary to correct the optics by polishing. In fact that is just what should not be done. If polishing is required at all, it should be kept to a minimum. 
I find it interesting, that in all three industries, generator precision has driven the quality and availability of lens designs to new levels. Lens manufacturing is moving to a 
greater dependence on the generator/lathe (and related tooling/blocking) and to more of a buffing operation for final surface quality.
While some lathe turned contact lenses are close to the 1/10 wave in figure, they are certainly well within the scratch &dig specification. So, you are making Precision Optics.