how night vision cameras work when our eyes need light to see
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Luckily night vision cameras don’t have human eyeballs in them.
Camera sensors do require light to detect an image, but they can be more sensitive than the human eye. A “starlight scope” for example may have increased sensitivity and a large lens to gather more light than falls into a human pupil. This is all collected and displayed to the user.
Another way is to use light the human eye can’t see. A UV lamp, like a big invisible flashlight, can be used to illuminate the scene so the camera can work.
Most night vision cameras see the infra-red spectrum of light, which humans don’t see, but instead feel as heat.
They have cameras on the front that detect infrared light. They translate levels of infrared to levels of green on screens inside them so the human eye can see the translated image.
Sometimes they emit infrared light to provide illumination for the cameras.
Some night vision systems work by amplifying any available visible light. They take in light which would be below the threshold for us to effectively make out details, and increase its intensity.
There are multiple ways for cameras to see what our eyes can’t. Other comments have covered these, but I think there may be some confusion between the methods.
I’m going to group them into three groups, but there can be some combinations. The term “night vision” is probably most often applied to the first type, and still commonly applied to the second type. The third type, is more correctly referred to as “thermal cameras” but also “inferred cameras”.
1) Light multiplication cameras take in light from the moon, the stars, or other dim light sources, and multiply the intensity to where our eyes can see things. These cameras require some light to operate. Advanced versions of these are often used by the military.
2) An infrared light my be included with a camera to provide light in low light conditions. This is very common with security cameras. These infrared lights put out light at a frequency just below what our eyes can detect, but camera sensors can generally detect it easily. In fact, many (most?) “normal” cameras actually have a filter to remove this infrared light or the image wouldn’t look right to us. These infrared lights put out light in what is know as the “near infrared” spectrum because it is “near” to the light we can see.
3) Thermal cameras are often considered to not require any “light” at all. They do require electromagnetic radiation, but the radiation they detect is usually in the “far infrared” spectrum. “Far” because it is a long ways away from the light that we can see. Every object (at above absolute zero) literally glows. The frequency of the electromagnetic radiation released is dependent on temperature. At normal temperatures this glow is in the far infrared range. If the temperature is hot enough, we can see the glow — as in “the metal was red hot”. Anyway, thermal cameras can detect this glow that is put off by all objects, so no other light source is necessary. A disadvantage to thermal cameras is that the detail is usually not as clear as we are accustomed to with normal cameras. Thermal cameras are used a lot of places, in the military, in search and rescue, by industry to check equipment temperatures, and in many other places. Note thermal cameras work equally well in daylight as they do in the dark which is one reason they aren’t really “night vision”, but some people may refer to them by this name.
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