Why Does Everything Look Green Through Night Vision Goggles?

istock.com/nightman1965
istock.com/nightman1965

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.

Why Are Sloths So Slow?

Sloths have little problem holding still for nature photographers.
Sloths have little problem holding still for nature photographers.
Geoview/iStock via Getty Images

When it comes to physical activity, few animals have as maligned a reputation as the sloth. The six sloth species, which call Brazil and Panama home, move with no urgency, having seemingly adapted to an existence that allows for a life lived in slow motion. But what makes sloths so sedate? And what horrible, poop-related price must they pay in order to maintain life in the slow lane?

According to HowStuffWorks, the sloth’s limited movements are primarily the result of their diet. Residing mainly in the canopy vines of Central and South American forests, sloths dine out on leaves, fruits, and buds. With virtually no fat or protein, sloths conserve energy by taking a leisurely approach to life. On average, a sloth will climb or travel roughly 125 feet per day. On land, it takes them roughly one minute to move just one foot.

A sloth’s digestive system matches their locomotion. After munching leaves using their lips—they have no incisors—it can take up to a month for their meals to be fully digested. And a sloth's metabolic rate is 40 to 45 percent slower than most mammals' to help compensate for their low caloric intake. With so little fuel to burn, a sloth makes the most of it.

Deliberate movement shouldn’t be confused for weakness, however. Sloths can hang from branches for hours, showing off some impressive stamina. And because they spend most of their time high up in trees, they have no need for rapid movement to evade predators.

There is, however, one major downside to the sloth's leisurely lifestyle. Owing to their meager diet, they typically only have to poop once per week. Like going in a public bathroom, this can be a stressful event, as it means going to the ground and risking detection by predators—which puts their lives on the line. Worse, that slow bowel motility means they’re trying to push out nearly one-third of their body weight in feces at a time. It's something to consider the next time you feel envious of their chill lifestyle.

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Are Any of the Scientific Instruments Left on the Moon By the Apollo Astronauts Still Functional?

Apollo 11 astronaut Neil Armstrong left the first footprint on the Moon on July 20, 1969.
Apollo 11 astronaut Neil Armstrong left the first footprint on the Moon on July 20, 1969.
Heritage Space/Heritage Images/Getty Images

C Stuart Hardwick:

The retroreflectors left as part of the Apollo Lunar Ranging Experiment are still fully functional, though their reflective efficiency has diminished over the years.

This deterioration is actually now delivering valuable data. The deterioration has multiple causes including micrometeorite impacts and dust deposition on the reflector surface, and chemical degradation of the mirror surface on the underside—among other things.

As technology has advanced, ground station sensitivity has been repeatedly upgraded faster than the reflectors have deteriorated. As a result, measurements have gotten better, not worse, and measurements of the degradation itself have, among other things, lent support to the idea that static electric charge gives the moon an ephemeral periodic near-surface pseudo-atmosphere of electrically levitating dust.

No other Apollo experiments on the moon remain functional. All the missions except the first included experiment packages powered by radiothermoelectric generators (RTGs), which operated until they were ordered to shut down on September 30, 1977. This was done to save money, but also because by then the RTGs could no longer power the transmitters or any instruments, and the control room used to maintain contact was needed for other purposes.

Because of fears that some problem might force Apollo 11 to abort back to orbit soon after landing, Apollo 11 deployed a simplified experiment package including a solar-powered seismometer which failed after 21 days.

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

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