Burning Bright in the Forests of the Night - How Night-vision Cameras Work

Burning Bright in the Forests of the Night

Nature abounds with creatures capable of seeing a world hidden from us, a land of ultraviolet-patterned flowers, thermal signals and bright nights. Night-vision technologies borrow tricks from these nocturnal and deep-sea denizens, boosting extant light or peering into infrared realms.

When it comes to seeing in low light, the eyes have it, thanks to a slew of specialized adaptations, including larger eyeballs, bigger lenses and pupils that open extra-wide -- all of which add up to more light entering the eye. Some dark dwellers' retinas bristle with many more rods (highly light-sensitive cells) than cones (detail and color receptors). Others lack cones altogether or feature rods specially built to maximize light-gathering.

The retinas of cats, cows and other animals feature a kind of mirror called a tapetum lucidum. This reflector bounces light back through the light-sensing cells of the retina, increasing the "brightness" of the image by boosting the nerve signal to the brain. Some say that this is why cats often have creepy glowing eyes in photographs (but I'm pretty sure it's actually because they're gazing into your immortal soul).

Some animals perceive wavelengths of energy that human eyes cannot. People see the world trichromatically, building a color image by combining signals from three kinds of light sensors, each tuned to a different wavelength within the 390-750 nanometer band: short (blue), medium (green) and long (red), with signal receptions peaking at 445 nanometers, 535 nanometers and 565 nanometers, respectively.

Bees are also trichromats, but one of their light-sensing cell types is tuned to the ultraviolet spectrum (peaking at 360 nanometers) [source: Meyer]. Some flowers and pollens display stunning patterns in this range; it's nature's club stamp, viewable only under black light, and it grabs bees' attention [sources: ASU; Deriso].

Rattlesnakes sport sensory pits capable of "seeing" radiation in the thermal infrared spectrum, which means they can spot a warm-blooded animal based on its body heat [source: NASA]. Predator, indeed.

When humans began developing night-vision technology, we adopted two of these approaches: light amplification, also called light intensification, and thermal infrared. We've also dabbled in near-infrared.

Humans radiate most strongly in the infrared in the 10-micrometer band, so thermal cameras typically operate in a range of around 3-30 micrometers [source: Morovision]. Within this wavelength bracket, warm objects -- such as vehicle engines, fires or people -- stand out against background heat, particularly at night -- a fact the U.S. Army would soon take advantage of.

The infrared portion of the electromagnetic spectrum lies nestled between visible light and microwaves. Near-infrared (NIR) is found on the scale just after the color red and has wavelengths on the scale of cells (0.7-2.5 micrometers). We send and receive electronic signals wirelessly with NIR in TV remotes, for example, but we can also use it to illuminate areas in a way that is imperceptible unless you look through special scopes or lenses. This is the basis for active NIR night vision [source: NASA].

At the far end, thermal IR hews nearer to microwaves, with wavelengths the size of a pinhead (3-100 micrometers). As the name suggests, warm objects radiate in this wavelength and vice versa -- which explains how thermal IR lamps keep restaurant food warm [source: NASA].

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