If we try to see light emitted from TV remote we can't see it. However, if we try to photograph it using an electronic camera, you can see the light in the picture. Why is this?
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1$\begingroup$ This is really a biology question, not a physics question. $\endgroup$– Kyle KanosCommented Aug 29, 2015 at 0:32
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$\begingroup$ @KyleKanos if the camera uses a CMOS chip, then it will see into the near-ir. $\endgroup$– JohnSCommented Apr 18, 2018 at 21:39
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4 Answers
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The quick answer to your question is this: TV remotes typically operate in the infra red (IR) and our eyes are not sensitive to IR but camera sensors are.
I guess you are intrigued by the fact that when you operate a TV remote facing a camera, you can typically see a red light on the IR diode of the remote in the camera screen, whereas with naked eyes you see nothing.
To understand this I will explain a little bit about the different mechanisms used by our eyes and cameras to detect color. Our eyes detect color by the cone cells. There are 3 kinds of them and they respond to different regions in the electromagnetic spectrum. As you can see from this image,
a typical human cone cell cannot detect beyond 700 nm and hence our eyes can't detect IR. The image sensors in a camera can only count the number of incident photons (that is they can only measure light intensity). To detect color, they are mounted on top with color filters (such as e.g. Bayer filters). Now, as you can anticipate, these filters do not strictly block IR and hence IR gets detected by the camera. Typically the IR passes mostly through the red filter and hence the camera detects IR as red.
Good quality cameras employ good IR filters and in those you will not be able to see the TV remote light.
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A common wavelength for an IR emitter for a remote control is 980 nm.
The human eye can only detect wavelengths up to about 725 nm efficiently, so we don't see the beam from the IR remote. (In the case of an 850 nm IR signal you may see some residual emission within the human sensitivity range).
Cameras, on the other hand, typically are made with silicon sensors, either CCDs or CMOS image sensors. In either case, the silicon chip is naturally sensitive to wavelengths up to about 1100 nm, which gives a photon energy just slightly below the silicon band gap energy. Therefore the camera sensor does see beams from typical IR remotes.
Many cameras have IR filters that are meant to block IR from affecting the images made. However, no filter is perfect, and one that still lets a bit of IR through (but not enough to disturb a typical image) is cheaper than one that would completely block a strong IR beam like from a remote.
answered Aug 28, 2015 at 4:51
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Some remotes, in a dark room, I can actually see the IR from a remote control. The really bright ones on camera, I can see a very dim red light from the IR LEDs. Not sure if thats because of spillover into visible, or if I'm just extra sensitive to that light.
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there is a very small portion of visible on the electromagnetic spectrum compared to the others. gamma rays and radio rays are big on the spectrum compared to the visible amount. IR light is closer to the visible part, but still not in it, but its so close, that the camera sensor can detect it.
