I wrote about the basic information about a Pilot’s Sunglasses in one of my posts titled Sunglasses for Pilots (click here), and then I wrote about the Materials that are available today, and how to pick the correct kind in the post titled Aviators’ Sunglasses Lens Material Options (click here). And I had promised that I will write more about the Extra Features that we need to keep in mind when selecting the best Sunglasses for Pilots, and for that matter, anyone who wants the best eye protection and quality vision.
Here is the list of those extra features that you need to keep in mind as well:
Special coatings can be applied to lens materials for reasons such as those previously mentioned. Crown glass and most plastic lenses require a specific coating to block residual ultraviolet radiation. Plastic and polycarbonate lenses require a scratch-resistant coating to prolong their useful life. The scratch-resistant coating applied to polycarbonate lenses absorb tints and dyes. High-index materials benefit from AR coatings to improve transmissivity due to their high reflective properties.
While AR coats can improve optical clarity, they are extremely porous, attracting water and oils, making the lenses difficult to clean. Lenses with AR coatings should be “sealed” with a smudge- and water-repellant coat that extends the useful life of the AR coat and makes the lenses easier to keep clean. Coatings must be applied correctly, and lenses must be meticulously cleaned for the process to be successful. Coated lenses should be handled with care and not subjected to excessive heat to avoid delamination or crazing.
The choice of tints for sunglasses is practically infinite. The three most common tints are gray, gray-green, and brown, any of which would be an excellent choice for the aviator. Gray (neutral density filter) is recommended because it distorts color the least. Some pilots, however, report that gray-green and brown tints enhance vividness and minimize scattered (blue and violet) light, thus enhancing contrast in hazy conditions. Yellow, amber, and orange (i.e., “Blue Blockers”) tints eliminate short-wavelength light from reaching the wearer’s eyes and reportedly sharpen vision, although no scientific studies support this claim.3 In addition, these tints are known to distort colors, making it difficult to distinguish the color of navigation lights, signals, or color-coded maps and instrument displays. For flying, sunglass lenses should screen out only 70 – 85% of visible light and not appreciably distort color. Tints that block more than 85% of visible light are not recommended for flying due to the possibility of reduced visual acuity, resulting in difficulty seeing instruments and written material inside the cockpit.
Polarized lenses are not recommended for use in the aviation environment. While useful for blocking reflected light from horizontal surfaces such as water or snow, polarization can reduce or eliminate the visibility of instruments that incorporate anti-glare filters. Polarized lenses may also interfere with visibility through an aircraft windscreen by enhancing striations in laminated materials and mask the sparkle of light that reflects off shiny surfaces such as another
aircraft’s wing or windscreen, which can reduce the time a pilot has to react in a “see-and-avoid” traffic situation.
Glass photo chromic lenses (PhotoGray® and PhotoBrown®), like their plastic counterparts (Transitions®), automatically darken when exposed to ultraviolet and become lighter in dim light. Most of the darkening takes place in the first 60 seconds, while lightening may take several minutes. Although most photo chromic lenses can get as dark as regular sunglasses, i.e., 20% light transmittance in direct sunlight, warm temperatures (>70°F) can seriously limit their ability to darken and reduced ultraviolet exposure in a cockpit can further limit their effectiveness. In addition, the faded state of photo chromic glass lenses may not be clear enough to be useful when flying in cloud cover or at night.
The selection of sunglass frames is probably more a matter of personal preference than lens material or tint. The frames of an aviator’s sunglasses, however, must be functional and not interfere with communication headsets or protective breathing equipment. Frame styles that incorporate small lenses may not be practical, since they allow too much visible light and ultraviolet radiation to pass around the edges of the frame. A sunglass frame should be sturdy enough to take some abuse without breaking, yet light enough to be comfortable. An aviator’s sunglasses should fit well so that sudden head movements from turbulence or aerobatic maneuvers do not displace them. Finally, use of a strap is recommended to prevent prescription sunglasses from being accidentally dislodged, or a necklace chain can be used to allow them to be briefly removed and subsequently replaced.