# Why is it so difficult to focus on blue LED lights?



## Bahiri (Aug 28, 2012)

Blue LED lights have appeared in recent years on anything from Christmas tree lights to police cars. However, they're incredibly difficult to focus on compared to any other light source, often appearing somewhat of a blur. Why is this?


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## mvyrmnd (Aug 28, 2012)

Your eye can't see blue very well. You evolved to see red ( blood, berries) and green ( trees, broccoli) well, but not blue (all that's blue is the sky... Why do you need to see it well?). This is why you can see so much better with warm tinted lights than cool tinted lights.

This is, of course, a gross oversimplification. Many smart people will come to explain further.


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## Yoda4561 (Aug 28, 2012)

And this doesn't just apply to LED light, any monochromatic deep blue to visible violet light is difficult for the human eye to focus with. Physically speaking it has something to do with the refractive index of the human eye lens at that wavelength or some jibber jabber like that.


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## anuragwap (Aug 28, 2012)

Deleted!


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## ShineOnYouCrazyDiamond (Aug 28, 2012)

I hope it's in keeping with the OP to keep this conversation going with a blue led comment that's not entirely related to the issue of focusing on blue LEDs, but more of a comment about there overuse in electronics. 

When you directly into the axis of a blue led it is really bright and hurts my much more than any bright yellow, orange or red LEDs used in technology. My AT&T U-Verse cable box has a super bright blue led that shines right in your eye when you are trying to watch TV - I've had to cover them all with a small piece of black tape. I had a HP laptop once that had a blue HDD led just beneath the monitor and it would blind you with the HDD access flicker. My Keurig had all blue LEDs, some digital clocks have blue LEDs to illuminate them, the power button on my NAS is crazy bright blue. 

I just think its overkill.

On the OP note though, I actually don't find it very hard to focus on the blue LEDs. They are pretty clear to me.


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## alpg88 (Aug 28, 2012)

i never had problem focusing blue leds any more than white or red, i seem to see just fine with blue light, color rendering is totally different story however. but as far as lens or reflector focusing it, same thing. how your eyes see it is also different story.
also i have a 30mw blue laser, form 10 feet and up, the "dot" appears as a bright star, not a dot as you see green or red lasers. but it is optical illusion, if i leave laser pointed, and walk to the dot, from closer than 10 feet it looks like a dot.
i once shined blue light at bbq on the grill, it did look pretty scary, wasn't sure i wanted to eat it for a minute or two.


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## MikeAusC (Aug 29, 2012)

mvyrmnd said:


> . . . You evolved to see red ( blood, berries) . . . .



But only VERY recently. Very few animals have Red sensors e.g. dogs only have Green and Blue cones.


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## sqchram (Sep 3, 2012)

I recall reading something about red-shift / blue-shift when choosing headlight color temperatures. Why fog lights are yellow, and lights are yellow in some countries.


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## MichaelW (Sep 3, 2012)

A combination index of refraction of vitreous humor & limitations of focusing ability of the eye.
They are especially annoying, because your visual cortex will try to focus, even though it will not have any success, and will try again... It is kind of like an auto-focus camera stuck in a loop.
Damn those x-mas decorations with royal blue! with its peak intensity at ~450 nanometers [rebel brand]. I don't think police cars use those, just the slightly longer plain 'blue', but with the intensity level they use, I'd prefer the use of cyan, as it would be less oppressive to the eyes/visual cortex.


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## Anders Hoveland (Sep 4, 2012)

Just try focusing on the dot from a violet 404nm laser. It appears very blurry. 404nm is about the shortest wavelength (highest frequency) of light there is that is still (not very well) vissible to the human eye.
Be sure the background does not have fluorescent optical brighteners in it though (like many types of white paper), because often when this laser beam hits something it becomes converted to other frequencies besides violet, and so the dot just appears white.


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## Canuke (Sep 4, 2012)

Combination of factors:

1. Red and green (which are, from the evolutionary standpoint, the same frequency, as the red and green cones are recent variants of one another while the blue cones are distinct, they use a different photoreactive enzyme) are the dominant *resolution source* for human vision, while blue cones are much more important for expanding our ability to detect color (chrominance). It's why we have more red and green cones than blue ones (IIRC the ration is 10:1, though that may be way off).

Since focus is much more important to spatial resolution (details) rather than color resolution, the human "autofocus" is biased to favor the red-green band. Blue gets short shrift because of its relatively minimal importance to resolution. An experiment: if you mess around with the three color channels of an image in a photo program like Photoshop, messing up the blue channel affects the final color image the least, given the same amount of "messing" (like noise or deresolution).

Last but not least: as noted, chromatic aberration means that the different colors won't quite match up. It's not normally visible, likely because our brains auto-compensate when combining the different cone inputs into the final image (Canon digital cameras recently acquired a similar ability.) Not everyone is going to notice the effect: this is probably because blue focusses "shorter" in terms of lens focal length than red. So, you are much more likely to notice "blue fringing" if your unaided vision tends towards the nearsighted side (like mine does), given the focus center on red/green -- and you'll see it worst with distant blue sources than near ones. If you know someone who has worse nearsighted vision than yours and wears glasses (and neither of you have astigmatism) try using their glasses and see what happens.


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## EricB (Sep 7, 2012)

I take it this is also why on less saturated blue light sources (especially incandescents), a second red image appears to form behind the blue, and IIRC, slightly offset to the right?


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