Many modern video cameras have a “night” mode, which allows grayish video of close subjects to be taken, even in utter darkness. This feature means that such video cameras can be used as infrared (IR) detectors–they can pick up infrared light that is invisible to the human eye.
The video camera I’m using today is a Sony DCR-SR45. It’s not a high end camera, and I purchased it used, but it has “Nightshot Plus,” which I’m interested in today.
Here’s how the night mode works on these cameras:
1) An infrared light on the front of the camera illuminates the subject with infrared light. You might see the near-infrared output of this light as a faint red glow if you look directly at the video camera when it’s in night mode.
2) The infrared sensor in the camera picks up on the reflected IR light to render the picture. If you’ve ever used such a camera, then you know the video it produces when you’re using night mode is grayed out and looks something like the following:
So what can we do with this IR detector? Well, one of the easiest things we can do is check if our TV remotes are working. Most TV remotes work by broadcasting an IR signal to the TV. To the naked eye, this IR light is completely invisible, but not to our IR detector. Check out the side-by-side video captures of a TV remote without and with night mode:
If you want to try it for yourself, just point the TV remote at your video camera (which should be in night mode) and press a button on the remote. If you’re watching the screen on the video camera, you should see the little bulb on the tip of the remote turn into a rapidly flashing light.
To the naked eye, the infrared light from the TV remote should be completely invisible, but even video cameras without night mode may detect some infrared light (see the left picture above). Turn it into night mode, however, and the IR emission from the remote becomes practically blinding.
Another interesting use for your IR detector is to tell from a distance whether a heat source is radiating thermal energy in the form of IR radiation. Recall from physics class that heat is transferred three ways: conduction, convection, and radiation. If you run really hot water through a faucet and take a look at it through a video camera on night mode, you won’t notice anything unusual. This is because the hot water isn’t transferring much heat through IR radiation. If you hold your hand close to the stream of hot water you won’t feel much. Touch it, however, and the heat from the water is conducted into your hand. Now take a look at a radiative heater through your IR detector, and you’ll see something interesting. In regular video, the heat source in the heater appears orange but it’s well-defined. Turn on night mode, and the heat source turns into a bright diffuse glow. It’s particularly interesting, to watch such a heater warm up through a video in night mode. When the heating tubes (or coils) are cold they look well defined (but monochrome) in the video. As they heat up, they become enveloped in a diffuse, white glow.
If you’re the nefarious type, you may find our IR detector very useful. Security cameras with nightvision capabilities usually illuminate their field of view with infrared LEDs. To the naked eye, this illumination is invisible, but point a video camera with night mode toward a house with nightvision security cameras, and those security cameras stand out like spotlights.
One thing I haven’t tried yet is to point the video camera in night mode toward a circling police helicopter to see if they’re using an infrared spotlight to illuminate the ground. Some day I’ll check it out. I’ve always been curious whether or not our local police chopper uses IR technology to track suspects.