|MadSci Network: Engineering|
Greetings: The human eye can detect lightwaves that range from the red wavelengths to the violet wavelengths. The red waves are about 2 times longer than the violet waves. By using lasers we can generate over a thousand million (trillion) different wavelengths (colors); however, our eyes only can sense relatively few different colors. These colors in order of decreasing wavelength are Red, Orange, Yellow, Green, Blue, Lavende,r Indigo, and Violet (the colors of the rainbow). A good way to remember the order of the colors is to use the first letter of each color to make a boys name: ROY G BLIV. Human eyes sense lightwaves with sensors called rods and cones. The cones sense color and the rods, which are the most sensitive sensors, sense faint light but not color. Our eyes sense light slightly differently in darkness than in daylight. In day light our eyes are most sensitive to green light, they are slightly less sensitive to yellow and blue light, they are only one half as sensitive to orange and lavender light and they are only one tenth as sensitive to red and violet light. Thus for viewing at a distance, a 100 watt green light bulb can be seen about 3 times farther than a red or violet bulb of the same light power. In darkness the eyes maximum sensitivity shifts toward yellow-green wavelengths with the other colors staying at about the same sensitivity as stated above. Thus red is the worst color for seeing at a distance. You can observe this on a clear dark night from a hilltop looking at distant traffic signals. The yellow light will be brightest, the green light (which is slightly blue) will be seen almost as bright and the red light will be quite dim. For the same reason, on newer cars with yellow turn indicators, you will notice that they are much brighter than the red turn indicators on older cars. So why are red lights used? In daylight the sun brightly lights the world that we see which often is filled with trees, grass and green things. Thus the contrast of seeing a green light and perhaps a yellow light against a bright yellow green background is difficult. In nature there are very few red colors except for flowers and so red stands out against the natural daylight background. In the nighttime this is not true and the background is less of a problem. In the city where I live we now use yellow fire engines because they can be seen much better in dim light. We still have a few old red fire engines and they are almost black in dim light. The color that is best used for signaling will depend on the distance that it must be seen, the background behind the light and the time of day that it will be used (day, night, dawn, or dusk. You will notice that airport runways are lined with bluish green lights and back in 1969 when we conducted a laser communications experiments with the first astronauts on the moon, we choose a green argon ion laser beam, because it would be the most visible color for the astronauts to observe from that great distance. Best regards, Your Mad Scientist Adrian Popa.
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