I’ve seen how you can test remotes by pointing them at your phone’s camera; when you press a button on the remote the phone screen shows a purplish light coming out of the remote that is invisible to the human eye. The signal coming from the remote is infrared though, making it closer to reds than to blues and purples, so why does it look purple on a phone screen?
In: Technology
A camera has light-sensitive areas that count the amount of light that hits them. They can detect all visible and even part of the infrared light, perhaps UV light to I am not sure.
If you just have a sensor you can get a black and white image
You create a color image you put a filter in from of the areas. so only light of a color range can pass tough. You have read, blue, and green filters. Any light that passes through the filter will be interpreted as a light of that color.
The filter is not perfect and some IR light will leak through them The result is that they leak through in a way that that the red, green and blue light from the screen looks purple to us.
Purple on a screen is if you have read and blue light but no green, so for some reason the green filter blocks the IR better than the red and blue.
The way to solve this is to put a IR filter in front of the camera so it blocks IR light but lets visible light through. The most camera has some IR filter that blocks more or less of the IR light.
The camera is picking up the harmonics of the light. It is much easier to think of it in terms of sound as this is how most people experience waves. Visible light fits pretty tightly in the space of a single octave. So if red is a C on a keyboard you would go up with orange being a D, yellow is E and so on to violet which would be a B. The next tone would be a C in a higher octave but our eyes is not able to see that far. Similarly the infrared spectrum would be notes that is lower then the C that is red but are again outside of our spectrum. However digital cameras have a strange quirk where they are able to detect infrared but will detect them as an octave higher. So infrared gets detected as blue light just as you detect a low C as the same note as a high C just higher.
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