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Author Topic: The changing colours of white light!  (Read 3935 times)

paul.fr

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The changing colours of white light!
« on: 06/03/2007 23:20:25 »
whilst at work today i was thinking hard (honest) and staring up at the flourescent lights.

After a few moments i looked away and noticed that i could see a large light spot on the wall, the colour of the light was not white, it was green! with a reddish outline!

So i again stared at the light to see if i could repeat the "vision", and i did! In doing this again i noticed something else. When i was staring at the flourscent light, even before i looked away it began to look a light green.

Why is that?


 

another_someone

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The changing colours of white light!
« Reply #1 on: 06/03/2007 23:36:12 »
Ordinary fluorescent lights are green (you can get daylight balanced ones, but I assume we are not talking about those) - they are one of the most difficult lighting conditions to photograph under, because not only are they green, they don't have a smooth spectrum, so some colours are simply absent (ordinary incandescent lamps are yellow, but they don't have any colour missing, so you can either adjust the colour by adding a blue filter, or with modern digital photography, you can adjust the colour balance on the computer - but if a colour is actually missing, you cannot add what is not there).

There are two things that might be happening when you look at, and then look away from, the fluorescent lamp.

When you look at the florescent light, your eyes (and brain) try to compensate for the distorted colour of the light, so they try and pretend the image is less green, but stronger in the red.  When you subsequently look at a scene that is lit by a different light source, it may take your eyes and brain a short period of time to work out that it has to compensate for the different light source, and during that time it may see the colour as too red.

The other possibility that I consider might be happening is that if you look too intently into the light (possibly not that healthy a thing to do), is that the light detectors in your eyes might become saturated (a little like snow blindness, but only effecting some colours, because the light itself is restricted to some colours).
 

Offline lightarrow

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The changing colours of white light!
« Reply #2 on: 07/03/2007 19:05:09 »
whilst at work today i was thinking hard (honest) and staring up at the flourescent lights.

After a few moments i looked away and noticed that i could see a large light spot on the wall, the colour of the light was not white, it was green! with a reddish outline!

So i again stared at the light to see if i could repeat the "vision", and i did! In doing this again i noticed something else. When i was staring at the flourscent light, even before i looked away it began to look a light green.

Why is that?
Afterimage. Fluorescent lamps don't emit pure white light.
Try this experiment: http://faculty.washington.edu/chudler/after.html
« Last Edit: 07/03/2007 19:10:11 by lightarrow »
 

paul.fr

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The changing colours of white light!
« Reply #3 on: 07/03/2007 19:10:13 »
 

paul.fr

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The changing colours of white light!
« Reply #4 on: 08/03/2007 01:20:53 »
Ordinary fluorescent lights are green (you can get daylight balanced ones, but I assume we are not talking about those) - .....

Many thanks for the detailed explination.
 

Offline eric l

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The changing colours of white light!
« Reply #5 on: 08/03/2007 12:33:10 »
This is also one of the reasons why the medical staff in an operation room wears green.  After fixing their eyes on the parts of the open body, generally read, they would see the same objects in green when they look up.  But as everything around is green (or mostly), they can rest their eyes when they look up.
 

Offline lightarrow

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The changing colours of white light!
« Reply #6 on: 08/03/2007 13:02:17 »
This is also one of the reasons why the medical staff in an operation room wears green.  After fixing their eyes on the parts of the open body, generally read, they would see the same objects in green when they look up.  But as everything around is green (or mostly), they can rest their eyes when they look up.
Yes. The colour you see as (negative) afterimage is the complementary colour:
http://en.wikipedia.org/wiki/Complementary_colour.

This effect is well known from professional artists and exploited or reduced. For example, to enhance the contrast between two colours, they put two complementary colours one next the other. Infact, if you draw a bright coloured spot (colour A) in a grey background and look at it without moving eyes, you see the borders of the spot coloured with the complementary colour (colour B). So, if this exactly complementary colour B is drawn as a stripe along the border of the spot, you see B as brighter (with respect to the case you draw it far from the other).

Positive afterimages happens with the same colour, instead of the opposite (complementary) one; in this case the light source you were looking at, have to be very bright. After a few moments, the positive afterimage turns into a negative one. Try to look at the sun directly for just an instant and then close your eyes and cover completely your face with the hands. Then don't move the eyes and oserve how the bright spot you see change colours.

Some times I also enjoy this experiment:
Inside your room, with little light inside, look at the window, during the daylight, for about two minuts, without moving the eyes at all (you may look at a specific point). Then close your eyes and cover your face with hands as I said; do it so that it does't come in any light from outside. After some seconds, and for some seconds, you can see again the window you were looking at before!
« Last Edit: 08/03/2007 18:40:15 by lightarrow »
 

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The changing colours of white light!
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