Eli5: Why does purple light come from a remote when seen through a camera

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Why?

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Cameras do their best to see and interpret light the same way humans do, but this engineering isn’t perfect. As such, they see some light that we do not, and in this case it’s infrared from the remote’s LED. It thinks it sees red and sends that to the screen.

You have a sensor for red light that when stimulated reads that stimulation as red color which it then displays,

However a sensor without filtering for infrared light may be stimulated by the remote and then making it think it needs to display red.

If you and a colorblind friend are working on a painting and you see something red, if you tell that friend to add it to his painting he might use green which most red things have, however if he looked at the object himself he couldn’t see anything. If he then shows it to someone else who is color blind they may notice this green blob where nothing is normally. The full vision friend who’s seeing red caused the confusion because he couldn’t differentiate what colorblind people (us in terms of infra-red vision) see

The remote uses infrared light to send a signal.

The camera sensors can detect infrared light but they cannot distinguish it from other colours, so it appears as purple light on the image.

It’s actually very useful in some circumstances, and many professional cameras have an IR filter to stop it showing up and ruining your images. But astronomical photography sometimes uses IR so they are basically just normal DSLRs sold without that filter on them.

Even the Raspberry Pi camera has both options, with or without filter. Because the way the sensors are made, they automatically pick up IR light too, but most people will want to use a filter on them to make the image more “natural” to the human eye (i.e. only seeing what the human eye would see)

It’s actually a good way to test if a remote is working, point your camera at it and see if it’s still blinking when you press a button.

Most remote controls work by flashing infrared LED bulbs. The bulbs flash in a sort of very fast morse code pattern, which tells an infrared light sensor on the TV what button you pressed.

Humans can’t see infrared light, but the CMOS sensors in many digital cameras can. These sensors detect the light as a purple color, because it interacts with the chemical structure of the filters and light sensors in the camera in a way that triggers both the red and blue sensors. The light is not actually red, blue, or purple, it just happens to cause the camera sensor to react in the same way it reacts to purple light.

Higher quality digital cameras have a filter to block infrared light from hitting the sensor, so the remote control trick won’t work with every camera. The filter is helpful because there is quite a bit of infrared light in sunlight, so taking pictures outdoors with a camera with no infrared filter may give the picture an unnatural purplish tint.

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0

Why?

In: 1

Cameras do their best to see and interpret light the same way humans do, but this engineering isn’t perfect. As such, they see some light that we do not, and in this case it’s infrared from the remote’s LED. It thinks it sees red and sends that to the screen.

You have a sensor for red light that when stimulated reads that stimulation as red color which it then displays,

However a sensor without filtering for infrared light may be stimulated by the remote and then making it think it needs to display red.

If you and a colorblind friend are working on a painting and you see something red, if you tell that friend to add it to his painting he might use green which most red things have, however if he looked at the object himself he couldn’t see anything. If he then shows it to someone else who is color blind they may notice this green blob where nothing is normally. The full vision friend who’s seeing red caused the confusion because he couldn’t differentiate what colorblind people (us in terms of infra-red vision) see

The remote uses infrared light to send a signal.

The camera sensors can detect infrared light but they cannot distinguish it from other colours, so it appears as purple light on the image.

It’s actually very useful in some circumstances, and many professional cameras have an IR filter to stop it showing up and ruining your images. But astronomical photography sometimes uses IR so they are basically just normal DSLRs sold without that filter on them.

Even the Raspberry Pi camera has both options, with or without filter. Because the way the sensors are made, they automatically pick up IR light too, but most people will want to use a filter on them to make the image more “natural” to the human eye (i.e. only seeing what the human eye would see)

It’s actually a good way to test if a remote is working, point your camera at it and see if it’s still blinking when you press a button.

Most remote controls work by flashing infrared LED bulbs. The bulbs flash in a sort of very fast morse code pattern, which tells an infrared light sensor on the TV what button you pressed.

Humans can’t see infrared light, but the CMOS sensors in many digital cameras can. These sensors detect the light as a purple color, because it interacts with the chemical structure of the filters and light sensors in the camera in a way that triggers both the red and blue sensors. The light is not actually red, blue, or purple, it just happens to cause the camera sensor to react in the same way it reacts to purple light.

Higher quality digital cameras have a filter to block infrared light from hitting the sensor, so the remote control trick won’t work with every camera. The filter is helpful because there is quite a bit of infrared light in sunlight, so taking pictures outdoors with a camera with no infrared filter may give the picture an unnatural purplish tint.