Tag Archives: cones

Spotting ancient walls at 17,000 MPH

Great Wall of China
Photo by Tianxiaozhang.

“You know, you can see it for miles – goes on for miles, over the hills and everything. But, so does the M6. Do you know what I mean? You can see that for miles. And you go, ‘Great. And that does a job. You can drive on that.’” Thus did an unimpressed Karl Pilkington of An Idiot Abroad describe the Great Wall of China, allegedly the only manmade object visible from space.

Which raises and interesting question: Why can’t you see the British M6 motorway from space? Or can you? For that matter, can you actually see the Great Wall?

Great Wall of China: Only Manmade Object Visible from Space?

Starlight, star bright, first shot I snap tonight

Infrared photo of Webster's Falls
Photo: Marcus Qwertyus/Wiki Commons

Photography is all about light; it’s right there in the name: photo (“light”) + graph (“means of recording”). So how do you shoot in the gloom between the golden hours? Well, you have a few options. You can pop in a flashbulb. You can try your hand at painting with light – that is, fiddling with f-stops and shutter speeds to let more light in over a longer period. Unfortunately, flashbulbs tend to wash out photos, and setting up longer exposures tends to limit your photographic freedom.

Night-vision cameras and attachments get around these problems, either by amplifying existing light or working with a different kind of ambient “light” – aka infrared radiation, either from body heat (thermal IR) or from an active IR illuminator attached to the camera. Today, infrared and ultraviolet cameras also make useful tools for inspections and field work. But how do they work, and what is their history?

How Night-vision Cameras Work