[optics] Those weird blue Christmas lights

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Why can't our eyes focus on that one type of deep blue Christmas lights? It always appears fuzzy from a distance, even if another bulb right next to it appears in focus.
 

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  • #2
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I can't say I've ever had that problem. So I don't think it's "our" eyes.
 
  • #3
Andy Resnick
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Why can't our eyes focus on that one type of deep blue Christmas lights? It always appears fuzzy from a distance, even if another bulb right next to it appears in focus.
I've noticed the same effect with deep-blue (say 405 nm) laser light. Not sure why, I wonder if it has to do with the fact that only the blue cones get stimulated, and they only make up around 2% of the detectors in our eyes. There's some interesting information here:

http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html

Apparently, the blue cones are usually defocused with respect to the red and green cones.
 
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Andy, I took the time to read your link. Very informative!
 
  • #5
DaveC426913
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I'm not sure if this is related, but many of the new LED lights, especially the blue ones, have a pronounced high frequency flicker to them. You can sense this if you dart your eyes around while looking at them.
 
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Redbelly98
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I've noticed the same effect with deep-blue (say 405 nm) laser light. Not sure why, I wonder if it has to do with the fact that only the blue cones get stimulated, and they only make up around 2% of the detectors in our eyes. There's some interesting information here:

http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html

Apparently, the blue cones are usually defocused with respect to the red and green cones.
I had noticed the same effect with 390 nm lasers. A faint violet, hazy blur appeared around the mirrors where the beam would hit. But as I would have to remove my uv-blocking glasses to notice this, even normal objects did not appear entirely sharp to me.

The hyperphysics article also mentions the higher refractive index for blue & violet wavelengths, which had always been my assumption for explaining the effect.
 
  • #7
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I'm obviously looking at the wrong type of light. I have honestly not experienced this.

Interesting articles though.
 
  • #8
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this explains why red text on a blue background makes my eyes "vibrate". they can't both be in focus at the same time.
 
  • #9
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I remember "cats eye" taillights on old cars, like in the 1930's, I think they were accessories, bought at auto stores. The normal red taillight lens had a small deep blue lens imbedded in it, and at night the effect was kinda strange, a sort of fuzzy blue haze mixed with the red, quite distracting. I vaguely remember hearing that they were illegal. Haven't thought of them in years...
 
  • #10
rcgldr
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Blue and violet light are diffracted the most by a lens, so it can end up being more out focus than other colors, espeicially if it's dark and more of the lenses in a person's eye is used to focus the lightl.
 
  • #11
DaveC426913
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Blue and violet light are diffracted the most by a lens, so it can end up being more out focus than other colors
What exactly does it mean for a light to be "out of focus" by its own optical components? Focusing phenomena usually occur at the receiving end of the light path.

eg. there are LED lights that are designed for area illumination, so they have lenses that diffract their LED light to span a very broad angle. But that doesn't make the light look "out of focus".
 
  • #12
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What exactly does it mean for a light to be "out of focus" by its own optical components? Focusing phenomena usually occur at the receiving end of the light path.

I'm guessing that the deep blue light is selectively scattered by material in the eye.
 

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