Omega Optical Donates Historical Equipment to UMass Amherst
This spring, Omega Optical decided to say goodbye to some historical equipment. The Ayer site, formerly Optometrics, had been storing two Strong Ruling Engines for years, and it was time to find them a new home. Dave Ventola, Director of Engineering- Diffractive Optics at Omega, reached out to the UMass Amherst Physics Department to see if they would be interested in taking the machines in and giving them new life. He was connected with Christopher Ertl, Technical Specialist for the department, who enthusiastically agreed to come to Ayer for a demonstration of the machines, along with a history lesson on their origin.
Ruling engines have a long history, and indeed, it seems every ruling engine has a story behind it. Since the advent of diffraction gratings in the 19th century, the chase was on for the perfect machine to produce them. The quality and resolving ability of any diffraction grating is directly related to the accuracy of the placement and straightness of its grooves. With the spacing between grooves for a UV region grating as small as 400 nm or less, mechanical precision takes on a whole new meaning.
As technology and metrology improved over the decades, the mechanical accuracy and precision of early ruling engines steadily improved. Ruling engine designs tend to be as varied and unique as the designers themselves, with each design striving to improve on the state of the art at the time. The advent of interferometric closed loop positioning correction was a relatively late addition to the technology that took precision to the next level, but the mechanical precision of the engine is still a necessary prerequisite for the best result.
John Strong (1905-1992) was a famous scientist and inventor well known for his work in optics and astrophysics. In the early 1950s, during his professorship at Johns Hopkins University, he designed a rather unconventional ruling engine. While most engine designs at the time used a single precision screw to index the ruling substrate between grooves and the ruling diamond traversed the substrate to create the groove, the Strong design utilized twin precision screws, independently driven, to index the ruling diamond between grooves, while the ruling substrate traversed. The diamond carriage was attached to a bridge spanning the screws riding on half-nuts. The lapping process that he developed to average errors in the screw threads virtually eliminated periodic errors that were commonly present in other designs and would cause troublesome ghost orders in the diffracted spectrum of the grating.
These particular ruling engines were in operation at Farrand Optical in Valhalla, N.Y., ruling master gratings for the UV, Vis, and NIR monochromators that they produced. The optical replication of the master gratings was done at PTR Optics in Waltham, MA. PTR Optics (later Optometrics Corp) purchased the engines from Farrand when the facility closed in 1987. The engines and their temperature-controlled enclosures were moved to PTR’s facility in Massachusetts, but were not immediately reassembled.
In 1989, PTR Optics changed its name to Optometrics Corporation and moved into a new purpose-built building in Ayer, MA. The engines and enclosures were reconstructed and made operational with some improvements to allow for variable grating sizes. Both ruling engines were made functional, but due to constraints in the basic design, only one ruling engine had a practical application. As was the common practice at the time, indexing was accomplished with a pawl drive system, where the groove spacing is fixed by the pitch of the indexing screw(s) and the number of teeth on the pawl wheel. With a screw pitch of 40 threads/inch, and tooth counts of 360 and 720 teeth/rev, the corresponding groove frequencies were fixed at 14,400 grooves/inch (567 grooves/mm) and 28,800 grooves/inch (1134 grooves/mm) respectively. The twin screw design made it impractical to attempt to redesign the indexing system to accommodate variable spacing, so the course pitch engine was never used for production purposes.
The finer pitch engine, however, found a purpose in life for the production of products. Optometrics produced ruled wire grid polarizers for infrared applications. V grooves were ruled in zinc selenide and calcium fluoride to act as the support structure for the grazing metal coating, which formed the wire grid of the polarizer. The 1134/mm pitch machine was perfect for this application, and produced all of the ruled wire grid polarizer products for the company for 25 years until the line was sunsetted after the advent of their holographic wire grid line, which was capable of much finer groove spacings and improved spectral performance.
John Strong ended his esteemed career as a professor at UMass Amherst, so it only seemed fitting that the engines were donated there to preserve an important piece of scientific history. The fine pitch engine is still in operational condition and will be used for educational purposes, and may someday rule another grating!
Article by David Ventola and Mary Chandler
