High-Quality Optical Lenses Only Come From High-Precision Test Plates
Lens test plates are literally the fiduciary shaping reference critical to all types of optical element production, constituting a part of Tamron’s historical roots as a leading manufacturer of optics for various applications.
Fabrication of test plates was started at the Omiya headquarters plant about half a century ago in 1966. It was later transferred to Namioka Plant (at the time) in 1969.
Processing of the test plates requires extremely high precision beyond that of typical lens elements performed with automated polishing machines, and is accredited to legacy craftsmanship nurtured by a group of skilled workers using conventional Oscar polishing machines. Those machines are meticulously maintained on a daily basis for full use at Current Aomori Factory (Namioka Site) today, inherited from the days of their first installation at the Omiya plant, which proudly demonstrates the heritage of craftsmanship.
When master test plates, comprising of a concave and convex shaped pair, are placed together, a characteristic interference pattern called Newton fringes can be observed. The seemingly simple and non-distorted rainbow-like fringes exhibit the integrity of the surface profile accuracy without any slope errors on both sides. Those properties of the master test plates represent a valuable legacy, which Tamron takes pride in as one of the few manufacturers of test plates around the globe.
Heritage of Tamron Test Plates
Two types of test plates are in place: a master and a replicated one for on-premise use. Plant test plates are used daily for quality inspection of elements on production lines. Naturally, they are susceptible to unavoidable abuse such as scratching and chipping, while the masters are to be preserved separately with utmost care as the reference. As many as 2000 pairs of master test plates are retained as a portfolio of Tamron Test Plates, ranging from 1R to 4000R, in special storage at the Aomori Factory (Namioka Site). The overwhelming number and shapes of the master test plates are a blessed heritage of the ceaseless dedication of the expert workers to date, and are treasured as Tamron heirlooms.
Serial Production of Test Plates to Satisfy Diversified Requirements
An Oscar-type polishing machine is the equipment used to formulate a given curvature onto glass elements with a specifically shaped polishing dish. Since the profile of the polishing dish correlates the R (radius) of a test plate, subtle manipulation of the machine and the dish is required to produce the element profile as designated by repeated visual inspection and manual adjustments. Currently at Aomori Factory (Namioka Site) some ten expert workers are assigned for test plate production provided with custom skill sets in order to satisfy critical requirements and delivery schedule. Because the precision of a test plate directly determines the degree of precision of a finished lens unit, there is no room for compromise in their work. A number of high-precision test plates are also manufactured to meet various kinds of requirements that Tamron receives from optical designers across the industry.
Highest Precision Assured for All Test Plates
Test plates are identified by an index R (radius) of the curvature (denoted as R). A curvature refers to a slope of a surface profile of a spherical lens. A smaller value of R corresponds to a sharper curvature, and a larger to a more moderate, eventually coming closer to an optical flat. When producing a test plate, a glass element is polished to fit into a required R by delicate manipulation of the polishing dish. Without thorough process controls in place while polishing, certain defects come into play, typically characterized as slope errors, astigmatism and surface roughness. As none of those defects can be discerned by visual inspection, corrections are made by qualifying the surface integrity with designated test plates, cautiously observing the Newton fringes. Ultimate precision of a test plate as produced is qualified with an interferometer, referring to the wavelength λ. All the test plates by Tamron are delivered with a guaranteed high-precision of λ/10*.
＊λ/10 signifies a surface profile integrity of the test plate, being free from any geometrical irregularities exceeding 1/10 of the wavelength as measured with an interferometer (approx. 60nm (nanometer)).
Persistent Transfer of Craftsmanship at Tamron
Production of a test plate mandates skill sets that would take as long as ten years of time to acquire. By virtue of one-on-one discipline, exercising visual and tactile inspection and performing corrections as needed, such skill sets are to be transferred. Evidently, patience and persistence as an individual character are essential to be groomed as an expert. Strong motivation to clear the final goal is equally important. Only those capable of clearing those mental hurdles can be qualified to challenge demanding processing jobs that mandate ultimate high precision. “Meister”-grade expert workers at Tamron engaged in test plate production, having gone through visual and tactile training to deliver the surface quality of λ/10, are capable of processing the highest level of profile accuracy and surface quality of any form of lens elements.
Determination to Turn Impossible to Possible
Defying a challenge to overcome a technical limit drives technology to an even higher level. A small-diameter test plate with an extremely sharp curvature (less than 1R) needs to be wholly hand-crafted simply because it is beyond the realm of processing capability of Oscar-type polishing machines. A record 0.6R test plate has been accomplished by a specially skilled expert, while a precision ofλ/15, which exceeds a standard criterion ofλ/10, has been demonstrated by several skilled workers. Technical challenge to culminate a sub-1R curvature orλ/15 precision goes far exceeding human recognition capabilities, and belongs to a dimension only substantiated by the interferometer. Pursuit for such a region of processing technique comes only from a mind to turn the paradigm around.
Embodiment of Technology Confidence and Pride
Chamfering is a secondary process applied to moderate sharp edges of a work piece for the purpose of preventing chipping and potential injury that might be caused by inappropriate handling. For Tamron test plates, such chamfering is applied, limited to absolutely necessary locations with minimum width. The wider the chamfering becomes, the more degradation in surface quality is caused at the very corner regions of the test plate. A definitive minimum signifies another facet of expertise at Tamron. Genuine beauty of a test plate, flawlessly finished with meticulous care is an embodiment of technology confidence and pride of the company.
