# Safe distance to look at a 950mn 5w Infrared LED from?

Discussion in 'LEDs and Optoelectronics' started by Voltaic, May 8, 2015.

1. ### Voltaic

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May 6, 2015
Can someone give me a guide or make a estimation of what is a safe/unsafe distance to look at a 940nm 4.3w Infrared LED?

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
"from within large concrete bunkers some thirty-seven miles away"

Seriously, it all depends on beam width.

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82
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May 6, 2015
4. ### Voltaic

82
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May 6, 2015
So the beam width is just defined by the leds, the module is not lensed or lasered outside of the actual leds themselves.

Last edited: May 8, 2015
5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Lets assume that the beam width is 60 degrees. Let's also assume that at 1cm the entire power is captured in a 1cm^2 aperture.

It works out then at about 5w/cm^2 at 1cm, and it will follow an inverse square law from there.

Now, what is a safe level of exposure? According to this http://www.researchgate.net/profile...ic_devices/links/0fcfd50fefcdad89c3000000.pdf you're looking at maintaining less than 80mW/cm^2 on the retina.

For safety sake, lets assume that this means less than 1mW through your pupil. I'm basing this on the fact that a 1mW visible laser is not powerful enough to damage your shift in the time it takes you to take reflexive protective action (about 250ms). Note that since you can't see IR, you won't take that reflexive action.

Assuming a7mm pupil diameter, that gives an aperture of about 0.4cm^2.

Thus at 1 cm, your eye would be exposed to about .4 * 5W, or 2W.

2W is 2000 times to much energy, and since the energy is proportional to the square root of the distance, you need to be about 45 cm away.

HOWEVER this only allows you to look at the LED for 0.25 second. If we assume that we need to be far enough away to look at it for 30 seconds, then we would have to be 11 times further away, or about 5 metres.

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6. ### hevans1944Hop - AC8NS

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Jun 21, 2012
This looks like an infrared illuminator for a night-vision application. As a general rule, never look directly into a solid-state light source whether it be diode laser or LED. If there is any doubt about it, wear a set of protective goggles appropriate to the wavelength emitted. Vision is a precious thing that should be protected at all times by whatever means necessary. That said, I wouldn't be afraid to walk in front of that thingy when it was on, but I sure wouldn't risk getting up close to peer into the LEDs.

Years ago, when I was a lot younger and more foolish, I was working with a Sylvania CO2 laser emitting five watts at 10.6 μm wavelength. The laser was mounted to an optical table with an assortment of mirrors, lenses, beam-splitters, etc. The on-off switch (a beam-dump solenoid) was positioned on the other side of the beam path. I reached across the table to turn the beam on and received the fully collimated beam on my right forearm as the beam-dump mirror more or less instantly retracted out of the way. I didn't have time to realize my mistake and turn off the beam, so my arm received a nice little burn as, by reflex, I lifted it out of the beam. Lesson learned: know where the laser beam is (or will be) at all times. Another engineer, at about this same time period, leaned over a 300 watt CO2 laser beam to make some sort of adjustment. His tie flopped down into the beam and immediately burst into flames (what was left of it). I wasn't there to see that, but he brought the tie in for everyone to see. Back then CO2 lasers were new and most of us didn't know much about infrared radiation. We learned real quick though. Others were not so lucky. One fellow (not where I worked) lost an eye in a microsecond because of a glint off a metal object when the beam produced by a Q-switched visible laser went astray. That could have been prevented if he had been wearing protective goggles with band-stop attenuation specific to the wavelength of his laser.

Moral of these stories: better safe than sorry. You wouldn't stare into a welder's arc, would you? Don't stare into powerful LEDs and lasers either.

Hop

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7. ### Voltaic

82
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May 6, 2015
Thanks, wow 5 metres it's quite powerful then.

Also would there be a simple way to diffuse the beam a lot so there is no strong point source (by mounting something infront of it)?
Or would an array of lots of very low powered ir leds be safe?

Is there something I can put in front of it to diffuse the beam so much that it makes it safe?
Otherwise I think I'll scrap it, or put it in an enclosed box project.

Last edited: May 8, 2015
8. ### hevans1944Hop - AC8NS

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Jun 21, 2012
What does this even mean? I don't see "5 metres" mentioned anywhere. The datasheet (scroll down to the bottom of the page you linked) for your infrared LED says it produces a maximum of 1000 mW for 10 W power input, an optimistic 10% efficiency.

Infrared is tricky. Most plastics are transparent to infrared. You can use a cell phone camera, most of which respond to 950 nm near-infrared, to check this out. Point an IR remote control at the camera and press some buttons to make sure the camera "sees" the IR emission. A lot of low-power IR LEDs are safe. They are used frequently with CCD TV cameras as illumination sources good out to about ten feet or so.

I wouldn't scrap it. Find some plastic sheet about a quarter-inch thick and test it for infrared transmission using the cell phone camera and remote control. If it's transparent in the infrared, you can double-check it with the cell phone camera and your LED source. Hold the plastic and the camera about two feet away when you do this. Avoid looking directly at the LED array. Assuming everything is copacetic, now you have to roughen one side of the plastic to create a diffuser. Try 000 or 0000 steel wool or some medium-grit sandpaper. You could also try using acetone to "fog" the plastic, but mechanical roughing will produce a better diffusion surface. Periodically test your diffuser with your LED source and the cell phone camera until satisfied with the results.

Hop

9. ### Colin Mitchell

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Aug 31, 2014
Do you mean: "look at" or "look into."

10. ### Voltaic

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May 6, 2015
Look at briefly, I had thought of using it for either an enclosed box heat lamp for heating things (like sprouts in winter), or a ceiling heat lamp for near infrared, like the incandescent heat bulbs, but it doesn't sound safe to use as a ceiling heat lamp if you can damage your eyes just from looking at it from metres across the room so I've scrapped that idea.

Hope it wasn't bad when I glanced at it briefly from couple of metres away when I was testing it before :/

Is it possible that a thick layer of crystalised salt could make it safe to look at? Such as a salt crystal lamp several cm's thick?

Last edited: May 9, 2015