When we set out to fabricate any lens element, our first step is always to review the drawing, ensuring it is in compliance with the parameters specified in the design and tolerance analysis. Only then do we procure the raw materials and our design experience is crucial in this step: glass varies wildly in its properties, some types being soft and easily stained others hard and brittle. Some may be prone to cracking or chipping, or may be difficult to polish. If a glass type is not readily available and is a special order, that factor can increase lead time.
We also look at whether using a higher index material, which is more expensive at the outset, will ultimately save space and improve performance. Using molded vs. cut blanks is another consideration we make, based on the type of glass used and the quantity required. We can procure materials from a number of suppliers (Schott, O’Hara and Hoya, for example). If we find that any substitutions are required, we may need to make a rebalance of the lens design before proceeding.
After this, the generating process is the first step in manufacture; this achieves the specified radius of curvature in the lens. This fast step creates the initial, roughed-in radius of curvature.
Next, there are three stages of grinding the lens surface as we move the lens through rough, medium and fine grinds. As with sanding a piece of wood, this process brings the surface of the glass to an increasingly finer finish. At this point the ground surface of a lens still has a smoky, translucent, rather than transparent, look. As the skilled technician moves through this process the lens is brought closer to its final radius of curvature without exceeding its center thickness tolerance.
Once ground, the lens goes through a polishing stage of a medium and then a fine polish. At this point, the smoky look is gone and the lens becomes transparent; the lens is being brought to its final radius. A master test plate and/or an interferometer will let the technician know when the surface is within tolerance.
Once polished, the lens is cleaned and readied for the next step, centering. This process accomplishes two things: it will first reduce the physical diameter of the optical system, and it will also ensure that the optical axis is in the center of the lens.
After the lens is mounted in a chuck, a technician projects a laser beam through it. As the lens is then rotated, the focal point of the beam also rotates, and the degree of rotation of this focused point of light is measured in arc minutes and seconds. At this point, the technician adjusts the lens until the focal point rotation remains within tolerance. With the lens properly centered, a rotating, diamond-impregnated grinding wheel is next backed into the rotating lens element, reducing it to its desired mechanical diameter.
Coating the lens is the final step in our manufacturing process. An antireflective coating maximizes the energy of light transmitted through the lens. For example, the transmission for two uncoated surfaces of glass (single element) is about 90% (for Nd = 1.5). When the two air-to-glass surfaces are single-layer coated, the transmission increases to about 96%; the application of a multi-layer coating increases the transmission to over 99%. In a multi-element lens assembly, the light loss is cumulative.
In the vacuum deposition method of coating, the lenses are placed into a vacuum chamber and the material, typically MgF2 crystals, are heated to the point of vaporization. The rising vapor then coats the lenses. This is the point at which, depending on the length of time and the thickness of the material deposition, a lens can be peaked or optimized for various wavelengths.
The environment for the coating process must be scrupulously clean, and it is our adherence to the highest standards of production that matters here. The lenses themselves must be entirely free of debris, as any dust or dirt that remains on them will become embedded beneath the coating. The vacuum chamber must itself also be perfectly clean; any dust or dirt in it will be dispersed during the turbulence that the air removal itself creates.
Westech Optical provides most transmissive and metallic reflective coatings for near-ultraviolet through near-infrared. Please contact us if you have questions about these coating designs for your project.