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Author Topic: Why does red plus white (light) equals green?  (Read 9837 times)

paul.fr

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Why does red plus white (light) equals green?
« on: 26/01/2008 08:07:28 »
Yesterday i was in the police control room, one of the CCTV monitors was showing an office that was being heated by an electric fire / heater. The room was painted white and the glow from the fire / heater made the room appear red.
Then a car pulled up to the junction opposite the room, headlights on full beam, shining straight through the window. At this point the CCTV image went green! Why did that happen?
« Last Edit: 09/03/2008 14:49:42 by ukmicky »


 

Offline lightarrow

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Re: Why does red plus white (light) equals green?
« Reply #1 on: 26/01/2008 09:04:25 »
Yesterday i was in the police control room, one of the CCTV monitors was showing an office that was being heated by an electric fire / heater. The room was painted white and the glow from the fire / heater made the room appear red.
Then a car pulled up to the junction opposite the room, headlights on full beam, shining straight through the window. At this point the CCTV image went green! Why did that happen?

If the white light was just a "flash" then that effect should be due to "negative post image" of your eyes, that is this effect:
http://www.thenakedscientists.com/forum/index.php?topic=7347.0

« Last Edit: 26/01/2008 09:15:15 by lightarrow »
 

another_someone

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Re: Why does red plus white (light) equals green?
« Reply #2 on: 26/01/2008 09:48:37 »
When you say the lights came through the window of the room - is that the control room you were sitting in, or the room you were monitoring through the CCTV?

If it was the room you were monitoring, it could be because the CCTV camera was saturated with light before it adjusted to the new light level (not saying this is the reason, only one possible explanation I can think of).

Alberto's suggesting of a physiological response in your eyes is plausible if the light was shining in the room you were sitting in, but does not seem plausible if it relates to the room you were monitoring - in that case, you should be looking at the response of the camera (although arguably, saturation of the camera is in effect not unlike the physiological response of your eyes).
« Last Edit: 26/01/2008 09:51:17 by another_someone »
 

Offline daveshorts

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Re: Why does red plus white (light) equals green?
« Reply #3 on: 26/01/2008 13:12:36 »
It could be a negative post image in the camera itself, because some (posher) cameras do automatic white balancing - this is trying to make a white thing look white electronically, even if it is being illuminated by yellow light from an incandescent bulb. Your eyes do the same thing, but if the room was being lit by a very red heater, this white balanceing is going to be working in overdrive so if something actually white shined in it would look a strange colour.
 

paul.fr

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Re: Why does red plus white (light) equals green?
« Reply #4 on: 26/01/2008 14:06:00 »
or the room you were monitoring through the CCTV?

Yes, it was the room being monitored. Thanks to you all for the answer.
 

another_someone

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Re: Why does red plus white (light) equals green?
« Reply #5 on: 26/01/2008 16:18:27 »
It could be a negative post image in the camera itself, because some (posher) cameras do automatic white balancing - this is trying to make a white thing look white electronically, even if it is being illuminated by yellow light from an incandescent bulb. Your eyes do the same thing, but if the room was being lit by a very red heater, this white balanceing is going to be working in overdrive so if something actually white shined in it would look a strange colour.

But the inverse of red is cyan, not green.

The inverse of green is magenta, so if the glow of the 'red' heater was in fact magenta, that might be an answer.

I suppose if the car headlamp was not pure white, it might have possibly had a more complex result.
« Last Edit: 26/01/2008 16:22:21 by another_someone »
 

lyner

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Re: Why does red plus white (light) equals green?
« Reply #6 on: 26/01/2008 18:46:51 »
Quote
if the glow of the 'red' heater was in fact magenta
That's a bit unlikely, bearing in mind that magenta is the colour we see when red and blue wavelengths are present (it's a non-spectral colour; 'minus green') and a red hot heater will not produce much in the way of blue wavelengths.
I suspect that the automatic gain control on the camera was a bit late in operating and, for a while, the green and blue channels went into overload; temporarily, the green dominated because green is a high luminance colour and I believe many displays have more output in hand from their green phosphors. Even with broadcast quality cameras, the overload condition when a bright light is shone directly at the camera, gives patches of green (and all sorts of zany colours). Whether this is to do with the display or the camera I am not sure.
 

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Re: Why does red plus white (light) equals green?
« Reply #6 on: 26/01/2008 18:46:51 »

 

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