Splitting Hairs

From The American Horologist magazine, March 1938

Splitting Hairs
By THEODORE B. DRESCHER
Vice President in Charge of Manufacture

To MOST persons, "a hair's breadth" indicates a difference practically non-existent. In the Bausch & Lomb Plant, however, "a hair's breadth" is a very gross quantity indeed. Here accuracy is measured in wave-lengths of light.

The following gives some idea of the tolerances involved in the production of high grade scientific optical instruments.

One micron, 0.001 millimeter, in common measure is 1/25,400 of an inch. In other words, there are more microns to a single inch than there are miles around the earth at the equator.

Or getting closer to home-by actual measurement, hairs from five individuals in an adjoining office ranged from 65 to 97 microns. Imagine splitting a hair into 65 or 97 miniature planks as the case may be.

But in physical measurements and precision optical work, a micron is still too large. Accordingly, it is divided into 1000 parts. This small unit, the millimicron, which is used in specifying the wavelength of light, is used to measure the accuracy of optical surfaces.

In making these tests, light waves themselves are employed through the formation of "Newton's Rings" between the lens or prism surface and the highly accurate quartz test plate. The color and shape of the rings or fringes formed by interference indicate the extent and nature of departure from a true surface. The limit of tolerance in general use is a quarter wavelength of sodium light, or 147.3 millimicrons, and is applied to such products as photographic lenses, microscopes, spectrometers, and telescopes.

For certain applications, even this infinitesimal quantity is too large and the surfaces must be so accurate that when contacted no color is seen. Extreme care must be used in making this test, since such surfaces adhere so firmly that a direct pull of approximately 200 pounds per square inch is required to separate them.

Naturally, such precision in routine production requires great care in the selection of raw materials, the development of special processes, unremitting control and rigid checking at each stage in manufacture, from the making of optical glass to the final mounting.

Mechanical aids are helpful and employed. But, in addition, manual adjustments and manipulation are required.

For this, we must depend upon the experience of the finished craftsmen. Such men pass through long training and various stages of attainment in order to qualify for this work. For hand skill, in all probability, no substitute will ever be found. 

- The Educational Focus.