The cat eyeglasses through which Sophia Loren directs a smoldering gaze in a 1960 Rodenstock ad are tinted to serve as sunglasses, but conceal a technical advance as well as providing a mysterious look to the famous actress's eyes. Rodenstock had been a precision lens manufacturer for over 80 years when the advertisement and the sunglasses that it advertised appeared, so it is perhaps unsurprising that it should add another feature to the eyeglasses manufacturing repertoire at that time.
Up until that time, all glasses, whether of a regular type or cat eyeglasses, had been plagued with internal reflections in the lenses. These produced everything from a shimmer that could interfere with vision or cause a headache due to eyestrain, to ghost images that made it difficult to see where objects actually were in certain lights. Though the vision benefits of eyeglasses far outweighed these optical problems, the reflections that appeared in lenses were still a source of annoyance.
Rodenstock's solution: magnesium fluoride coatings
Rodenstock's cat eye glasses of this period bear some of the day's most advanced coatings in a successful bid to quash the obnoxious internal reflections that had bothered eyeglass wearers throughout history up to that point. Magnesium fluoride coatings are transparent, but prevent the double reflections and ghost images that glass lenses create when uncoated.
The glasses produced by the German firm were therefore more comfortable to use than those offered by many competitors, though the use of magnesium fluoride naturally spread like wildfire through the eyeglasses world. Combined with celebrity endorsements and crisp, chic styling, this added freedom from reflections probably helps to explain Rodenstock's explosive success at the time.
Photochromic lenses on Rodenstock cat eyeglasses
Rodenstock lived up to its origins as a precision lens making enterprise by continuing to develop lens technology as the 1960s moved onwards. This was still the time when cat eye glasses were very common, though their popularity was waning slowing as round John Lennon glasses began to come to the fore by swift leaps and bounds.
The major advance that Rodenstock introduced in the late 1960s was the invention and sale of photochromic lenses. These lenses were placed into all types of frames, including cat eyeglasses, due to their potential for making glasses more comfortable to wear outside without impinging on their indoor use.
Photochromic lenses are made by infusing clear glass or plastic lenses with silver chloride, or sometimes another kind of silver halide, in a microcrystalline state. This causes the lenses to darken when exposed to ultraviolet light, returning to a normal colorless state as soon as the ultraviolet light is removed. A quarter hour is the time that full darkening or lightening takes, though most of the darkening or lightening process takes place during the first sixty seconds.
This allowed the creation of dual purpose vintage eye glasses, which would be clear indoors and darken enough to act as sunglasses in bright natural light outdoors. More darkening would occur in strong sunlight than on a cloudy day, making the photochromic lenses automatically self-adjusting to a comfortable level of tinting. Rodenstock's cat eyeglasses, when fitted with photochromic lenses, took the place of separate indoor glasses and sunglasses.
The solution wasn't perfect because the glasses do not darken inside a vehicle, leaving drivers exposed to the full glare of the sun. However, the photochromic vintage eyeglasses offered plenty of utility nevertheless and the use of these special advanced lenses spread.