How Night-vision Cameras Work


Showtime After Dark

In the darkness between the golden hours, photographers find themselves with a few options to choose from. There's always the good old flashbulb or the painting-with-light technique of fiddling with f-stops and shutter speeds to let more light in for longer. The problem is, flashbulbs can wash out detail, and relying on longer exposures can really put a dent in your flexibility.

Night-vision cameras and attachments get around these problems by amplifying existing light or working with a different ambient "light" -- infrared (IR) radiation, either from body heat (thermal IR) or from an active IR illuminator attached to the camera. These tools help make surveillance cameras and nanny cams possible -- to say nothing of the applications spies and private eyes might find for them -- but they're just as useful for photo bugs seeking to see the world through eyes of real bugs.

If you think you can just slap some infrared film into your camera, however, I've got good news and bad news. First, the bad news: IR film is sensitive in the near IR spectrum, not the thermal band, so unless you equip an IR light-emitting diode (LED) or some other IR source, it won't be much help after sundown.

The good news? You might want to click away during the day anyway. Nature comes alive in new ways when photographed in IR, because chlorophyll reflects in the near-infrared (NIR) spectrum. Along similar lines, many flowers assume new splendor when captured by cameras tweaked to photograph UV; their pollen and petals fluoresce in the ultraviolet spectrum.

By the way, did you know that digital cameras come with NIR sensitivity right out of the box? It's true. In fact, manufacturers have to build in a special filter to block the IR channel [source: Chen]. Otherwise, it could cause problems, such as autofocus confusion, soft images -- or unintentional peeping through IR-transparent clothing. Some IR still gets through, so you can shoot IR snaps simply by blocking all non-IR radiation with a filter and taking a very long exposure.

If you don't mind cracking open your camera, you can also remove the blocker entirely. Some shops will do this for you. Then, you can either replace the IR filter with one that removes visible light (for an IR camera) or a transparent filter (in which case you'll be able to shoot color, or IR, depending on the filter you put on your lens).

With film or digital cameras, you can always kit-bash an IR flash. Just place a piece of polyester IR filter on your flash and you're good to go.

Author's Note: How Night-vision Cameras Work

Growing up, I was captivated by various kinds of vision, from the world-tinting wonder of cheap transparent plastic to the insectlike, compound-eye effect of kaleidoscopes. I wandered rooms of funhouse mirrors, clicked through View-Masters and collected and constructed optical gadgets small and large, from cheap microscopes and telescopes to pocket-sized periscopes.

My foray into night vision began with an ill-advised high school trek into an unfamiliar part of town to find a security retailer. My friend and I were full of beans and on a mission: he, to check out the "spy" gear, and me to look through a night-vision scope. Had we given the matter any thought, we would have assumed that the sales staff would take one look at our teenaged faces and give us the boot. To my surprise, though, one of the staff took me to a darkened demo room and let me peer through one of the shop's several-thousand-dollar scopes.

Once I'd gotten over the fear of breaking the thing, I was swept up in the marvel of seeing my own hand in pitch-blackness. I still recall it vividly: The lack of parallax and the odd, invisible-flashlight greenness of it combined to make me feel oddly disembodied.

I first encountered daytime UV and IR photography while writing another HowStuffWorks article, How to Capture Winter Scenes in Photography. As I surfed around the Web looking for experts on the subject, I came across landscapes that looked to be covered in frost and rime, but were actually the product of IR reflecting brilliantly off chlorophyll-packed leaves and grass. Ever since then, I have been obsessed with the idea of experimenting with IR and UV photography.

I think the desire to see the world differently -- both literally and metaphorically -- is a natural tendency, and a useful one, for artists, scientists ... anyone, really. Now, if you'll excuse me, I need to find a pawnshop that sells digital cameras on the cheap; I have some IR blockers to hack.

Related Articles

Sources

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