I know this is way, way OT, but I also know that this ng has some of the most knowledgeable contributors.
Why, when a laser (say a CD player with the lid off) says it has an output wavelength of 850nm, can you see it as red? And, I have a fibre optic module that I'm playing with at work (again class
1), and you can see a red spot on this too (and it's also supposed to be 850nm).
Yes, you can see way outside the normal "visible" if you crank up the power. Sharp cutoff 'IR' filters that kill all the visible can be quite startling if made into enclosed sunglasses, so you can see the IR that you normally couldn't in bright sunlight.
However, at visible wavelengths, you need to have a very high power to damage the eye, as unless you actually try to stare into the beam, you'll look away as it's too bright.
With wavelengths outside the 'visible', and not too far outside (if the cornea is transparent to them), the eye can focus the light to a small spot, but it does not appear bright enough to cause you to immediately look away, though it's enough to cause damage. You really shouldn't.
And the little blind spots it causes are not obviously noticeable - but are subtely messing up your vision. Don't mess with powerful non visible light.
I got a flash of UV in one eye a long time ago thanks to a missing interlock on a PCB exposure unit. Actually hurt a little and caused a noticeable drop in that eye's ability to render colour. Seems to have recovered (or brain has compensated) now.
It's becoming a problem with computer networking. 10gig over fibre optics and to a lesser degree 1gig are considered dangerous to the vision - both are invisible. It's not actually immediately apparant to the uninformed that shouldn't go looking down random pugs in the computer room - in fact it's a completely natural thing to do (which way round does this go/wonder if the ends are clean?)
Timbo
So how can a class 1 fibre optic module be regarded as safe if the laser can be viewed without needing to dismantle the unit? And why can't you see the infra-red led of a TV remote control? They're at a similar wavelength (880-940nm).
In order for output from a class1 laser to cause eye damage it has to be a) focused and b)stared into for a long time. You stand more chance of causing eye damage by poking yourself in the eye with the fibre.
No one said you can see 880nm, what you can see is the small amount of
750-800 that the laser puts out. You can't see the led on you remote because all of its output is well into the IR (for most IR leds, some do spill into the red).
Intensity. The laser can be some millions of times brighter.
And, actually you may be able to see the remote. Wait in a darkened room for a few minutes, then press a button, while looking directly into the LED.
Well I guess text books being economical with the truth when they say the visable spectrum is "approximately 700 nanometers (abbreviated nm) to approximately 400 nm" Note "approximately". Obviously 850nm is closest to red so red receptor cells are the most likely to respond and that is how it will appear. Obviously the cells won't respond very well at that frequency so the light is far brighter (contains more energy) than it appears and as other have pointed out you may risk damaging your eyesite (although I would guess you have done no damage but I am no expert!). Anyway better to be safe than sorry. Is there a warning on the CD player by any chance? I get the feeling I have seen warnings on them. I assume if damage was being done you would feel pain which would cause you to maybe look away but I am not 100% sure.
>Where I work we build class 3B lasers. I can clearly see the spot on a white card from 1mW of 785 (it looks as bright as a 633nm HeNe laser of ~ 0.1% the power). We also build 830nm lasers, which are _much_ duller/dimmer.
These lasers are moderately pure (i.e. there is no emission in the "visible" spectrum which is 400 to 700 or 750nm depending who you ask) so what you are seeing is the "tail" of the response of the eye into the IR.
Apart from wavelength the light from remote control LEDs is divergent, low powered and diffuse.
Chris
I think you only avert your eyes when you percieve the light as being bright. I only have annecdotal evidence regarding the cd player (a comment from a friend), but the laser module just says class I. And the definition of class I says that it is safe (unless you take it apart).
The laser is supposed to have a spectral width of 0.85nm, so there really shouldn't be much spillage into the 'visible' wavelengths. I guess the eye is sensistive to 800nm+, but it just doesn't appear as bright as it really is. And a further question regarding H&S at work, if there is a danger here, should warning notices be used? (I'm of the opinion that it is labelled as class I, and it is used in accordance with manufacturers recommendations, so it's safe)
Yup, no question that most eyes can see to about 800nm. Pretty much zero response much beyond that though.
There is a bit. For example, I've got some ~650nm laser pointers. If I stare into the beam through a shade 5 welding filter, there is a distinct green beam (no red is visible)
Maybe your filter is absorbing the red light and emitting green in response ;-) Actually, is that as stupid as it sounds?
I've just found out that my fibre module pulses the laser until it establishes that it has a good connection with another module. I guess this reduces the effective power and must be the reason it is class I. Now if I took the case off when it was connected to its other half, then I'd have about 1mW of 850nm laser beam. Of course then it's not class I.
I rest my case that this ng is the place to be.
Well any damge I assume would be caused by heat, I don't know if the retina actually feels pain as such or if the brain interpretates the bright light as pain 'even though it isn't'. Mind you if someone poked a stick in your eye I think you would feel it :OP I am sure you would feel the pain if you put your hand under an infra red heating light. I have not heard of anyone having their eyes damaged by light they 'cannot see' but may I just don't get out much! Of course if you cannot see it you won't be able to focus it unless you know where to look. Also different wavelengths focus at different points so even if you looked at the lights source, the infra red or whatever probably would not be in focus anyway? (So how does the eye and cameras manage it? I think its called achromtic aberration?
A common problem in space flight. The eyes are bombarded by high energy particles which some astronauts see as flashes of light and some don't. But all suffer some deterioration of eyesight after long flights.
In article , snipped-for-privacy@mauve.demon.co.uk says... snip
Look at it on the screen of a digital camera. It appeared white last time I did.
I think so.
Doubling would take it to somewhere around the UV, and two-photon absorbtion basically doesn't happen at the sort of power density in an unfocussed laser pointer beam. (and I suspect you have to try really hard if focussing it).
Well, yes, but that's just because you'r seeing the IR leakage through the various coloured filters, and the colour tells you nothing about the IR light. The camera isn't even close to the eyes responsivity once you get out beyond dark red.
'Fraid so - since green is higher-freq = higher-energy, flourescence just don't happen that way round. Absorbing green and emitting red - yup; absorbing UV and emitting visible - yup (it's what the phosphors on "flourescent bulbs" do for a living); but to jump "up" the energy levels Simply Isn't Done (at least not by single photons - as your man Stirling points out, there are exotic mechanisms for this, but you have to work harder than just shining a beam at a welding filter to make them happen). I'm guessing that what you see therough the welding filter is the small amount of light which the laser-pointer produces at other than the main frequency...
Stefek
I'm glad you added that last bit - I was about to say "I've got a 50mW DPSS @532nm sitting at my desk that begs to differ", but I won't ;-)
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