Visible light, also known as "white light", makes up the relatively short range of light waves that humans are able to see within the full electromagnetic spectrum. While light frequencies are measured in Hertz (Hz - cycles per second), light waves are measured by their distance from the beginning of one wave to the end of that same wave. The distance unit of measurement is usually nanometers (nm), or one billionth of a meter (1/1,000,000,000). The two terms, frequency and wavelength, are inversely proportional -- the higher the frequency (Hz), the shorter the wavelength (nm), and vice versa.
When our visible light is separated or broken out, as if through a prism, the resulting image is of a rainbow pattern. We see the entire Blue range (Blue, Indigo, Violet) at one end, and Red at the other. In the middle of the pattern we find Yellow and Green which combine to make up a wide range that is the easiest for our eyes to see and process. Some lenses, such as the G-15 lens from Edge, have been specifically engineered to enhance this range while slightly muting others, for better long-term viewing and comfort.
If we go beyond the Red end, we first reach Infrared (IR), an invisible range for us. Infra-red can be divided into two zones, Near-IR and Far-IR. Both IR ranges present potentially harmful hazards for us, which is why we must protect ourselves from IR light during welding and lampworking applications, as the use of common tools in such tasks will emit IR light. Light welding and cutting jobs will require glasses or goggles with a minimum of Shade 3.0, while heavier, more intense jobs would require no less than a Shade 8. Common is the use of Shade 10-14 for superior protection from IR exposure.
Going beyond Blue and Violet, we get to Ultraviolet (UV), another invisible range. There are actually three zones or ranges of ultraviolet light, UV-A, UV-B, and UV-C. Each presents its own burn hazards with exposure, particularly overexposure, to our skin and eyes. This is why we use sunscreen on our skin and why we should wear only eyewear that blocks UV light. Fortunately, all polycarbonate lenses naturally block at least 99.9% of UV light.
Researchers have known for years that the exposure to blue light can affect us in a variety of ways. Because the range of "Blue light", which includes Violet (and Indigo, often considered part of Violet), has the shortest wavelengths of those we can see, it scatters more easily, making it more difficult for our eyes to focus on objects and colors in that range. With prolonged exposure, our eyes will fatigue more quickly than if viewing in, say, the Yellow-Green range. Largely for this reason, lenses that efficiently block blue light from our view have increased in popularity over the years. Such lenses can not only reduce eye fatigue, but can also enhance clarity and depth perception. As such, these lenses, which are typically some shade of Copper or Brown, also make for excellent driving sunglasses.
Another drawback to blue light exposure, is the suppression of Melatonin. Melatonin is a natural hormone that our brain releases to make us tired and help us sleep. Stopping, delaying, or reducing its release will make falling asleep and/or staying asleep much more difficult. With the evolution of electronic gadgets that many of us rely on daily, sleep deprivation has risen among our population. Flat screen TV's and monitors (which have all increased in size), pads and tablets, and even our larger smartphones all emit a higher percentage of blue light than we're used to from regular light. The solution is to either remove the source or wear eyewear that blocks the blue light that's emitted from the source.
Whatever your application is or whatever your need may be for lenses, one thing to consider is the luminous transmittance for a lens. Also known as Visible Light Transmission (VLT), it is given as a percentage (%) of the available visible/white light. Clear lenses typically allow about 90% light transmission, where darker lenses, such as that of typical Gray sunglasses, allow around 10-18%. Knowing the light transmission of a lens can help you choose the right color, lightness or darkness for your needs.
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