|MadSci Network: Biophysics|
The blue light problem is something that many companies with lighted or neon signs inadvertently use for marking their stores; they are hard to focus on and to me, annoying! The blue Xmass tree lights are the same! Drives everyone crazy. Well, drives me nuts. Maybe you as well? The problem is simple, because the human eye cannot focus a clear image for that light. This is due to a problem called "chromatic aberration". A lens, even one in a camera or magnifying glass, will gather light reflected by an object and focus it at a certain focal distance after it passes through the lens. If you take Lens, and white piece of paper, you may notice that you can focus the image of your classroom windows (on a sunny day) onto the paper. You have to get the magnifying lens to just the right distance from the paper. (note if you use a magnifying glass the windows will look upside down on the paper) That is the focal length of that lens, where the image comes to a sharp focus. Light coming from one part of the window is focused by the entire lens; some light hits near the outer edge of the lens and is focused onto the paper. Other light enters near the center of the lens and is also focused on the paper. When the image is in focus, and sharp, the light hitting the edge of the lens is focussed to the same point as the light hitting the center of the lens. Thus, you can see a nice small image of your classroom windows. Most high quality man-made lenses, and the lens in your eye, are able to focus all of these different colors of light to the same focal length, so the image seems very sharp. Most lenses, also including the eye's lens, cannot focus blue light hitting the edge of the lens and blue light hitting near the center of the lens to the same spot. This makes the parts of the image in the blue spectrum look fuzzy and never sharp. That is why some company signs and blue Xmass tree lights always look annoying and fuzzy. If you take your magnifying glass and look at some magazine pictures, you can notice that the images you see a the edge of the lens start to have red or blue edges, but they look ok in the center of the image. You may see the same effect looking in a microscope. The objects in the middle look their natural color, and if you move them to the edge of the microscope field of view, you see a colored edge forming on them. This is due to the chromatic aberration. Man made lenses tend to suffer from this in the far red and far blue visible light ranges. The human eye focuses light using your cornea, the aqueous humor and lens of the eye. Light is focused on your retina. Human eye optics do better than man made lenses; they focus the reds just fine, but they still have trouble with the blues. That is why blue lights will never look focused, and that kind of blue is a foolish choice for store signs at night. It is also a foolish choice for cell-phone or car dashboard night lighting for the same reason. Blue light is also why Blue-blocking (yellow) glasses reduce glare and make it easier to see on a sunny bright day. They have yellow lenses, block the blue light, taking out the poorly focussed blue part of the scene, so everything looks sharper. Many Winter Olympic Biathletes (x-country ski and target shooting) wear yellow glasses. Their targets look sharper, and more in focus when they are target shooting during their events. I used to wear clip on yellow lenses when flying aircraft many years ago for the same reason. It made is much easier to spot other planes in the airspace around me, and increases safety. You can find diagrams of the eye at http://eyeresearchnetwork.com Kenneth Mitton, PhD Associate Professor of Biomedical Sciences Eye Research Institute Oakland University Rochester, Michigan
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