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Why doesn't intense infrared radiation from a fire harm your eyes?

by DaleSwanson
Tags: eyes, harm, infrared, intense, radiation
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DaleSwanson
#1
Jan8-12, 10:21 PM
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The other night I was watching a campfire and the infrared light from it was intense enough that it warmed my face quite a bit, almost uncomfortably. I must have been receiving quite a bit of invisible light in my eyes, but there was no damage (and I've never heard any warning about watching campfires being dangerous to eyes).

On the other hand, light from the Sun would not warm my skin as much as the fire did, yet looking directly at the Sun would damage my eyes. Also, I know that infrared lasers can be quite an eye hazard because your eyes don't respond to limit the light at all.

This leads me to believe the following conclusions: A. Infrared light can damage your eyes. B. The intensity of light received from a campfire can be higher than from the Sun. C. Even if there is more intense light from a campfire, it is safer than light from the Sun.

Why isn't light from a campfire a danger to your eyes?
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DaveC426913
#2
Jan8-12, 10:29 PM
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Quote Quote by DaleSwanson View Post
A. Infrared light can damage your eyes.
B. The intensity of light received from a campfire can be higher than from the Sun.
I don't see how you arrive at these conclusions. I don't see how they follow from any of the points you made.

But the short answer to your question is two-fold:
1] The sun is many orders of magnitude more intense than any campfire. Your eyes adjust so that the campfire seems bright, but that's entirely subjective (your pupil compensates for the low light). Try using your camera. Then you'll see the huge difference.
2] IR light is low energy compared to UV light, which is much of the damaging light that comes from the sun.
DaleSwanson
#3
Jan8-12, 10:42 PM
P: 351
I arrive at conclusion A. (that infrared light can damage your eyes), based on the fact that infrared lasers can damage your eyes.

Conclusion B, may be confusing because I used 'light' when I should have said all electromagnetic radiation. I realize my eyes are receiving much more visible light from the Sun than from a fire. However, since the fire warms my face more than the sun, that makes me believe there is more total wattage of EM radiation hitting my face (and thus my eyes) from the fire.

Bobbywhy
#4
Jan8-12, 10:45 PM
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Why doesn't intense infrared radiation from a fire harm your eyes?

Despite being inside of your eye, there are cells that normally never see light. When you go from a low light area to a bright area, your pupils are still dialated, the light entering is not focused onto the cells in the back of your eye that are designed for seeing, and light falls on these other sensitive cells causing the pain that you feel.

Of course, it depends on what wavelength of IR you are talking about. The near IR (800 nm to 1400 nm) is focused on your retina just as if it were visible light. The only difference is your retina does not have receptors that detect it. If you look at a visible light that is too powerful, you will damage your eyes. If you look at a IR light in this range that is equally powerful, you will do nearly equal damage. And you do not have pain receptors on your retina to know you are burning it. Because you don't detect it, so you wont blink or look away like you would with visible light, IR light in this range is actually more dangerous than visible light. It does not matter that the photons are less energetic, since when you talk about light power, you talk about total energy and not energy per photon.

If you look at light over 1400 nm, it will be absorbed primarily by your cornea. Since it is not focused by the lens (like the near IR is), it is much harder to get the energy density needed to damage your eye. Also, you can sense pain in your cornea, so you will know that your eye is getting burnt. And the cornea is one of the fastest regenerating tissues in your body, so damage to the cornea is not as catastrophic as damage to the retina. So, unless you are looking at a super powerful light source in this range, it's probably not a concern. They do, however, make lasers that are plenty powerful enough at these wavelengths to do damage.

In general, it is hard to find light sources in the IR that are powerful enough to do damage, in either wavelength range. Diffuse sources, like the ones used to heat things, are not going to be powerful enough. And things in consumer products (laser mice, remote controls, etc) are going to be made weak enough to not cause damage for legal reasons. But if you are talking about IR lasers or powerful night illumination systems, eye damage is certainly an issue and IR lasers cause more laser eye injuries than all other types of lasers.
Drakkith
#5
Jan8-12, 10:55 PM
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The radiation emitted from a campfire is nowhere close to the Sun. Looking directly at the sun puts not only Infrared, but also massive amounts of visible light into your eyes, as most of the energy put out is in the visible range. A campfire puts out a very tiny fraction of its energy as visible light. Most of it is infrared. The amount of infrared light entering your eye is not enough to damage it unless you get very close to the fire, at which point I would be more worried about my face burning off.
DaleSwanson
#6
Jan8-12, 11:46 PM
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Quote Quote by Bobbywhy View Post
If you look at light over 1400 nm, it will be absorbed primarily by your cornea. Since it is not focused by the lens (like the near IR is), it is much harder to get the energy density needed to damage your eye.
I think this is the answer I was looking for, thanks. So NIR is focused optically, but other wavelengths aren't. Thus it doesn't matter that there is much more total EM radiation if it is in non-optical wavelengths. Campfires output mostly light in wavelengths longer than NIR (and thus non-optical).

Quote Quote by Drakkith
The radiation emitted from a campfire is nowhere close to the Sun. Looking directly at the sun puts not only Infrared, but also massive amounts of visible light into your eyes, as most of the energy put out is in the visible range. A campfire puts out a very tiny fraction of its energy as visible light. Most of it is infrared. The amount of infrared light entering your eye is not enough to damage it unless you get very close to the fire, at which point I would be more worried about my face burning off.
Yes, clearly the Sun puts out more total light than a campfire. However, what matters is watts/m2, and at the range I was talking about the watts/m2 for all forms of EM radiation was higher for a campfire (as evidenced by the greater warming on my face from the fire). As Bobbywhy pointed out, most of those wavelengths don't get focused and thus don't damage the eye.
Drakkith
#7
Jan9-12, 12:18 AM
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Quote Quote by DaleSwanson View Post
Yes, clearly the Sun puts out more total light than a campfire. However, what matters is watts/m2, and at the range I was talking about the watts/m2 for all forms of EM radiation was higher for a campfire (as evidenced by the greater warming on my face from the fire). As Bobbywhy pointed out, most of those wavelengths don't get focused and thus don't damage the eye.
Yes, that does seem to make more sense than my explanation!
Lsos
#8
Jan9-12, 02:04 AM
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I am just talking based on my gut feeling here, but I would say that the infrared light doesn't damage your eyes simply because your eyes aren't sensitive to it. Your eyes are sensitive to visible light, so an excess of this will quickly damage them...much like loud audible noises will damage your hearing. So it's not so much the "wattage/m^2" from the sun that damages your eyes, but the simple fact that you are overloading them.

I'm guessing an infrared laser DOES damage your eyes based on the "wattage/m^2" mechanism, but I'll also guess that the laser's "wattage/m^2" is many orders of magnitude greater than a campfire's or even the sun's.
DaleSpam
#9
Jan9-12, 05:09 AM
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Prolonged exposure to IR can damage the eyes and cause "Glass blower's cataracts".
http://www.optometry.co.uk/uploads/a...oke1990521.pdf
http://www.ilo.org/safework_bookshel...t&nd=857170571
Ryan_m_b
#10
Jan9-12, 05:24 AM
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I think that IR must be very intense i.e. laser-level concentrated to cause damage to the eye. Wiki has some graphs and information detailing power levels and exposure times for various wavelengths http://en.wikipedia.org/wiki/Laser_s...sible_exposure
Dickfore
#11
Jan9-12, 06:03 AM
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The intensity of a laser beam is much higher than both Sun's radiation and campfire. The Sun has spectral components in all wavelengths, and not just the IR.
russ_watters
#12
Jan9-12, 06:45 AM
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Quote Quote by Ryan_m_b View Post
I think that IR must be very intense i.e. laser-level concentrated to cause damage to the eye.
This is the key to all three examples. It isnt the total heat absorbed, it is the intensity. For example, a small campfire that shows you a 1 foot cross section from 5 feet away is 530x bigger than the sun. If it warms you as much as the sun, it is 530 times less intense, meaning an individual cell on your cornea recieves 530x less light/heat.
DaleSpam
#13
Jan9-12, 06:59 AM
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Prolonged (occupational) exposure can cause damage at relatively low intensity. Much less intense than lasers.
Andy Resnick
#14
Jan9-12, 07:37 AM
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Quote Quote by DaveC426913 View Post
<snip>
2] IR light is low energy compared to UV light, which is much of the damaging light that comes from the sun.
I think this is the essential answer.
russ_watters
#15
Jan9-12, 08:28 AM
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No, I'm pretty sure it isn't. It's all about intensity. The OP captured his error in the title: he thinks a fire is intense, but it isn't. See:
russ_watters
#16
Jan9-12, 08:36 AM
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Quote Quote by DaleSwanson View Post
Yes, clearly the Sun puts out more total light than a campfire. However, what matters is watts/m2, and at the range I was talking about the watts/m2 for all forms of EM radiation was higher for a campfire (as evidenced by the greater warming on my face from the fire). As Bobbywhy pointed out, most of those wavelengths don't get focused and thus don't damage the eye.
What matters in eye damage isn't the watts per sq meter hitting your body, but the watts per sq m hitting a tiny spot on the back of your retina. Because your eye focuses the light, the size - and thus intensity - of that spot is a function of the angular size of the source. So a fire can heat up your body as much as the sun while at the same time appearing 500x less intense to your eye.
DaveC426913
#17
Jan9-12, 12:08 PM
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Quote Quote by russ_watters View Post
a tiny spot on the back of your retina. Because your eye focuses the light, the size - and thus intensity - of that spot is a function of the angular size of the source.
And - if I understand correctly - your cornea/lens does not focus IR light, so it won't appear as a spot on your retina, it will be diffuse IR on your retina.
russ_watters
#18
Jan9-12, 12:16 PM
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It may not focus as well, but I'd be awfully surprised if it was so far out of focus as to affecect the intensity of something so large. I'll look into it though.


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