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Why Does Everything Look Green Through Night Vision Goggles?

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The characteristic green tint is by design, for a few reasons. First, device makers have experimented with a few different colors and found that the different shades that make up the monochrome night vision image are most accurately perceived and distinguished when they’re green. In other words, while the night vision images you’ve seen in Silence of the Lambs and Call of Duty might seem a little clunky, green presents a night vision device wearer with the most accurate and user-friendly picture possible. What’s more, because the eye is most sensitive to light wavelengths near 555 nanometers - that is, green - the display can be a little dimmer, which conserves battery power.

Who Invented Night Vision?

The first practical night vision devices were developed in Germany in the mid-1930s and were used by both German tanks and infantry during World War II. U.S. Military scientists had simultaneously developed their own night vision devices that first saw use during WWII and the Korean War.

These “Generation 0” devices used active infrared to brighten up a scene. Soldiers carried an IR illuminator to shoot a beam of near-infrared light that then reflected off objects and bounced back to the lens of their scope and created a visible image of what they were looking at. The illuminators used by the German Nachtjägers, or "night hunters", were about the size of dinner plates and required a large power supply carried on the soldier’s back.

The technology made huge leaps in the following decades, and by the time the U.S. entered the Vietnam War, many troops were outfitted with passive "starlight scopes" that used image-intensifying tubes to amplify available ambient light (usually from the moon and stars, hence the name) and produce an electronic image of a dark area.

This “Generation 1” technology is still around today in the more budget-friendly consumer-grade night vision devices. Military and police forces have upgraded to successive generations of tech with new improvements over the years, but image intensifying night vision - there’s also another flavor, thermal imaging, but image intensification is almost always the kind you see in movies and games - still works on the same basic principles as these early models.

I Can See Clearly Now

The lens or lenses at the end of a night vision scope or pair of goggles gather available light, including some from the lower spectrum of invisible infrared, and focus it on a photocathode on the device’s image intensifier tube, which transforms the photons, or light particles, into electrons.

As the electrons move through the tube, they flow through a microchannel plate, which is a disc with millions of tiny holes, or microchannels, in it. As the electrons strike electrodes on the microchannels, bursts of voltage cause the motion of electrons to increase rapidly, forming a dense clouds of electrons that intensifies the original image.

At the far end of the tube, the electrons hit a screen coated with a phosphor, which is a substance that radiates visible light after being energized. (We talked about phosphors in relation to glow-in-the-dark toys a while ago.) The energy from the electrons excites the phosphor which converts the electrons back into photons. These are in the same alignment as the photons that originally entered the tube, and form the greenish image on the screen inside the viewing lens of the device.

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Big Questions
How Does Autopilot Work on an Airplane?
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How does autopilot work on an airplane?

Joe Shelton:

David Micklewhyte’s answer is a good one. There are essentially a few types of features that different autopilots have. Some autopilots only have some of these features, while the more powerful autopilots do it all.

  • Heading Hold: There’s a small indicator that the pilot can set on the desired heading and the airplane will fly that heading. This feature doesn’t take the need for wind correction to desired routing into account; that’s left to the pilot.
  • Heading and Navigation: In addition to holding a heading, this version will take an electronic navigation input (e.g. GPS or VOR) and will follow (fly) that navigation reference. It’s sort of like an automated car in that it follows the navigator’s input and the pilot monitors.
  • Altitude Hold: Again, in addition to the above, a desired altitude can be set and the aircraft will fly at that altitude. Some autopilots have the capability for the pilot to select a desired altitude and a climb or descent rate and the aircraft will automatically climb or descend to that altitude and then hold the altitude.
  • Instrument Approaches: Autopilots with this capability will fly preprogrammed instrument approaches to the point where the pilot either takes control and lands or has the autopilot execute a missed approach.

The autopilot is a powerful computer that takes input from either the pilot or a navigation device and essentially does what it is told to do. GPS navigators, for example, can have a full flight plan entered from departure to destination, and the autopilot will follow the navigator’s guidance.

These are the majority of the controls on the autopilot installed in my airplane:

HDG Knob = Heading knob (Used to set the desired heading)

AP = Autopilot (Pressing this turns the autopilot on)

FD = Flight Director (A form of navigational display that the pilot uses)

HDG = Heading (Tells the autopilot to fly the heading set by the Heading Knob)

NAV = Tells the autopilot to follow the input from the selected navigator

APR = Tells the autopilot to fly the chosen approach

ALT = Tells the autopilot to manage the altitude, controlled by the following:

VS = Vertical Speed (Tells the autopilot to climb or descend at the chosen rate)

Nose UP / Nose DN = Sets the climb/descent rate in feet per minute

FLC = Flight Level Change (An easy manual way to set the autopilot)

ALT Knob = Used to enter the desired altitude

This post originally appeared on Quora. Click here to view.

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Big Questions
What's the Difference Between Vanilla and French Vanilla Ice Cream?
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While you’re browsing the ice cream aisle, you may find yourself wondering, “What’s so French about French vanilla?” The name may sound a little fancier than just plain ol’ “vanilla,” but it has nothing to do with the origin of the vanilla itself. (Vanilla is a tropical plant that grows near the equator.)

The difference comes down to eggs, as The Kitchn explains. You may have already noticed that French vanilla ice cream tends to have a slightly yellow coloring, while plain vanilla ice cream is more white. That’s because the base of French vanilla ice cream has egg yolks added to it.

The eggs give French vanilla ice cream both a smoother consistency and that subtle yellow color. The taste is a little richer and a little more complex than a regular vanilla, which is made with just milk and cream and is sometimes called “Philadelphia-style vanilla” ice cream.

In an interview with NPR’s All Things Considered in 2010—when Baskin-Robbins decided to eliminate French Vanilla from its ice cream lineup—ice cream industry consultant Bruce Tharp noted that French vanilla ice cream may date back to at least colonial times, when Thomas Jefferson and George Washington both used ice cream recipes that included egg yolks.

Jefferson likely acquired his taste for ice cream during the time he spent in France, and served it to his White House guests several times. His family’s ice cream recipe—which calls for six egg yolks per quart of cream—seems to have originated with his French butler.

But everyone already knew to trust the French with their dairy products, right?

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