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Going Observing: The Red Light District

May 17, 13 Going Observing: The Red Light District

If you’ve ever been to an observatory, you may have noticed that astronomers are pretty picky about the presence of lights. The reason is that bright lights can dramatically reduce your night vision.

In fact, when I open up the University’s optical observatory to the public, I insist that everyone either keeps their cell phones off or in their pockets. Even the light from small electronic devices such as these can affect your ability to adjust to the darkness.

Luckily, if the ability to see in the dark is important, the use of red light can provide needed illumination without messing with your night vision. The reason is that your eye uses two different types of structures to see: rods and cones.

The cones detect both color and brightness, and are used for seeing in high light intensity situations, such as during the day. The rods, on the other hand, are very sensitive to brightness, but cannot differentiate color; they literally see the world in shades of gray. Rods also have the unique feature of being blind to the color red.

So, in low-light situations, say when the sole source is the moon and stars, your rods give you your sole ability to see. If you need more light, however, the solution is to shine a dim red light. The red light has little affect the rods (maintaining your ability to make out dark features around you), yet will give you a little extra illumination to see what you are doing. The effectiveness of red light is so significant that many car manufacturers use red lights to illuminate dashboards at night.

The key, though, is to keep the red light intensity low. Because the red light is stimulating your cones, your vision is susceptible to image-burn; you essentially “see” a residual image even once the light is turned off. This, of course, fades, but it can be a slow transition to getting your full night vision back.

It is still better than using white lights, though, as the shorter wavelengths of light will overwhelm the rods, and it takes about 15 – 30 minutes for the rods to adjust properly to the darkness, even if the use of white light is very short.

This week, though, the use of red lights has a secondary purpose. The particular observatory where I am currently observing is the VERITAS (Very Energetic Radiation Imaging Telescope Array System) Array in southern Arizona. I’ll detail this experiment in my next post, but for now, simply know that the telescopes have specially designed cameras that are very sensitive to ultraviolet light.

In fact, their efficiency at detecting different wavelengths of light is a sharply peaked curve, centered at about 320 nanometers. The result is that the longer the wavelength (i.e. the redder the light) the less sensitive the cameras are to it.

So, as we walk around the observatory at night, the use of red lights not only helps us to see in the darkness, it also reduces the risk of us damaging the cameras. (I should note that while their sensitivity to red light is reduced, it is not zero. Because the instruments are so sensitive, even shining red light directly on the cameras is a bad idea when they are turned on.) If we, instead, strolled around the observatory using blue/ultraviolet flashlights, the scattered light would be enough to cause problems, even if we never shined them directly at the cameras.

In short, the use of red light can give you the ability to see what you are doing in the dark, without losing your ability to see once the lights are once again shut off.

Image Credit: Kiril Stanchev / Shutterstock

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