A Peek Through Glass

Two properties that define optical glass are the refractive index (n) and the index of dispersion (v). The refractive index compares the passage of light through the glass relative to the passage of light through air (n=1). The dispersion value, or Abbe number, tells how the glass scatters light into various wavelengths. Both measures vary with the wavelength of incident light; the "standard" wavelength (d) is 587.56nm. Any optical glass with nd greater than 1.60 and vd greater than 50 — plus those with nd less than 1.60 and vd greater than 55 — are designated crown glass. One crown glass common in sports optics is BK 7, B meaning borosilicate. Another crown glass is BaK 4, a barium-silicon compound. The second type of optical glass is flint. "A binocular or rifle-scope will generally have several types of glass," Zeiss explained optics engineer Walter Schwab. "Not to save cost, but to meet specific needs. Flint glass is typically heavier and more brittle than crown glass, and ill suited to the exposed ends of an instrument. A collecting lens of crown glass transmits light into flint glass, perhaps as the other half of a doublet or achromat. The flint corrects for aberrations."

Fluorite glass is used to limit color dispersion. "A light beam bent by a lens unravels into component colors," said Walter, "each striking the next surface at a different place. The color fringing that results can be reduced by a fluorite lens up front. It's expensive, and typically used as the second in the series, for protection."

Tight tolerances in erector tube manufacture ensure true tracking through a scope's power range.

A lens can be milled to shape in two minutes, but polishing can take eight hours — and plenty of rinse-water. Diamond paste is the common cutting agent. Precise fixtures hold the blanks. Zeiss keeps tolerances to .000l mm. "Surfaces can't be gauged mechanically. Inspectors use light to ensure that optical and mechanical axes coincide," said Walter. "Finished prisms are so smooth and precisely shaped, flat surfaces cling."

Bare lenses lose up to 4 percent of incident light to refraction and reflection at each air/glass surface. In the 1930s a Zeiss engineer named Smakula found that coating a lens with magnesium fluoride changed its refractive index — and light-transmitting ability. After World War II his work was seized upon by other optics makers. Multiple coatings followed. "Figure 80-percent light transmission in a lens system with one coating, 90- to 95-percent through fully multi-coated lenses," explained Walter.

In a "clean room" at the Zeiss factory, he showed me how coating was done, with a tray resembling a paint pallet. "Each of these pockets has a compound that's distributed by an electron beam in a microscopic spray. Finally, volleys of ions pound each layer onto the lens." He added that Zeiss T-star (T*) coatings are not one formula; they're simply the best. "One scope may need different layers than another."

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Binocular prisms are coated too. Roof-prism binoculars have two types of prisms: Schmidt-Pecan and Abbe-Konig. A Schmidt-Pecan prism is shorter but requires a mirror. At Zeiss, the silver mirrors that replaced metal mirrors have given way to dielectric prisms with 74 coatings, for nearly 100-percent reflection. Another prism coating, a Zeiss development circa 1988, corrects for phase shift. After roof prisms split a light beam into halves, wave interference can blur the re-unified image perpendicular to the roof edge. Phase coating improves resolution. "It's relatively inexpensive and easy to apply," said Walter Schwab.

Yet another Zeiss coating is LotuTec, a hydrophobic wash that beads water on outside lenses. LotuTec increases the contact angle of water droplets, so they skitter off instead of smearing. It also makes lenses easier to clean and has a negligible effect on brightness.