MadSci Network: Engineering

Re: Can you see red light from farthest away? If so, how can I prove it?

Date: Mon Oct 4 11:11:45 1999
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Engineering
ID: 938300937.Eg


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: 

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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