Coatings

The most important coatings in binoculars are the antireflective coatings on the surfaces of the optical components. An uncoated glass-to-air surface will reflect about 4 percent of the light that is perpendicular to it ("normal incidence"), and even more of the light that is oblique to it. By using interference coatings, this can

Figure 2.15. Single layer film.

be reduced to better than 0.15 percent over a very wide range of the optical spectrum. The coatings are usually optimized for a particular wavelength of light, usually in the range 510 to 550 nm. A single coating of a quarter of the wavelength of light will reflect a small proportion of the incident light. The glass behind it will reflect another small proportion. The path length of the wave reflected off the glass is half (2 x .25) a wavelength greater; the two reflected waves mutually interfere destructively, eliminating the reflection for that particular wavelength (Figures 2.15 and 2.16). At wavelengths significantly distant from the wavelength for which the coating is optimized, interference may be constructive, resulting in more reflected energy than would have occurred in uncoated glass. Additional layers of half- and quarter-wave thickness can reduce reflections at other wavelengths; this is called "multicoating" (Figure 2.17) and "broadband multicoating" (Figure 2.18). Coating is an expensive process, so there are a number of coatings that become uneconomical. It is rare to find more than seven layers on any surface in commercial binoculars.

Binocular coatings are qualitatively described as "coated," "fully multicoated," and so forth. There is no universally agreed meaning to these designations, but they are commonly held to have the following meanings:

• Coated: At least one glass-to-air surface (usually the outer surface of the objective) has a single layer of antireflective coating, usually MgF2; other surfaces are uncoated.

• Fully coated: All glass-to-air surfaces of the lenses (but not the prism hypotenuses) have a layer of antireflective coating.

at 510 nm

Figure 2.16. Coated optics: single layer coating.

• Multi-coated: At least one glass-to-air surface (usually the outer surface of the objective) has two or more layers of antireflective coating. The other surfaces may be single-layer coated or not coated at all.

• Fully multi-coated: All glass-to-air surfaces of the lenses (but possibly not the prism hypotenuses) have two or more layers of antireflective coating.

More recently, some binoculars coatings have been described as "broadband." Again, there is no industry-wide standard—coating can mean anything from three layers upward. Some manufacturers are more forthcoming as to the precise nature of their coatings. For example,the manufacturer of the popular Oberwerk-branded binoculars in the United States (branded as Strathspey in the U.K., Teleskop Service in Germany) provides the following information about its coatings:

• Level I: (Equivalent to Fully Coated). Single layer of MgF2 coating on 16 glass-to-air surfaces: 4 for two objectives, 12 (6 per side) for the three optical elements in each eyepiece. The prisms are not coated.

400 450 500 550 600 650 700

Wavelength (nm)

400 450 500 550 600 650 700

Wavelength (nm)

Figure 2.17. Multi-coated optics: double layer coating.

• Level II: (Equivalent to a blend of Multi-Coated and Fully Multi-Coated). Broadband multi-coatings of 5 to 7 layers on the 4 glass-to-air surfaces of the two objectives, and the 4 surfaces of the eye lenses of the two eyepieces. Single-layer MgF2 coating on all other glass-to-air surfaces, including the hypotenuses of the prisms.

• Level III: Broadband multi-coatings on all the surfaces except the prism hypotenuses, on which there are single-layer MgF 2 coatings.

• Level IV: Broadband multi-coatings on all the surfaces including the prism hypotenuses. (Kunming Optical Instrument Co. Ltd.)

The effect of various coatings can be seen in the reflections of sunlight from objective lenses in Figure 2.19.

1.38

Uncoated glass

Uncoated glass

400 450 500 550 600 Wavelength (nm)

400 450 500 550 600 Wavelength (nm)

Figure 2.18. Broadband multi-coated optics: triple layer coating.

Figure 2.19. Optical coatings. Clockwise from top left: single-layer coated, broadband multicoated, multicoated, uncoated.

Figure 2.19. Optical coatings. Clockwise from top left: single-layer coated, broadband multicoated, multicoated, uncoated.

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