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[white foam head wearing black welders goggles]

INFRARED GOGGLES
FOR UNDER $10

A Human IR Vision Experiment
Sept. 14, 2002 Bill Beaty

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DANGER! POSSIBLE SAFETY HAZARD! I've worn the IR goggles in for hours at a time in bright Seattle sunlight. I'm not blind yet. The dark goggles make your pupils open up wide, but then the goggles don't block all that IR sunlight. Is this harmful? It hasn't hurt me yet, but I don't know if it's totally safe. My particular goggles contain glass for UV filters. Is this enough? I DON'T KNOW! At the very least, don't ever stare at the sun while wearing the goggles, you might get a nasty suprise later on (called Snow Blindness. Your cornea surface rots and sticks to your eyelids. Ewww.) DON'T STARE AT THE SUN!!!!
NEW: photo gallery      

  1. The original brainstorm
  2. Goggles for everyone
  3. Making your own
  4. Rude t-shirts
  5. Congo Blue or better
  6. Goth-ray vision
  7. Deeper IR
  8. Cheap IR floodlight
  9. How they work
  10. Frequently Asked Questions FAQ
  11. Spectrum graphs
  12. LINKS
  13. Photo Gallery


Brainstorm!

"Congo blue" filters pass infrared light!

If you lay three layers of Congo Blue on one layer of Primary Red, you form an opaque black filter which is very transparent to IR!!!!!

Oh, so you're not impressed? Well, read on.

Back in 1994 when I was working for Eaton photocontrols, we had all these big sheets of opaque black Lexan plastic which was IR-transparent. It was intended for curved, vacuformed covers for the infrared sensors in truck's backup-alarms. Viewing this plastic through an infrared-converter scope was very strange, since the IR-scope changed the opaque black sheets into glass-clear Lexan. Some black boxes had been built for custom test equipment, and the IR scope turned them transparent, so you could see all the circuitry inside.

And if I put these black sheets in a window, human eyes saw opaque black plastic, while IR scopes (and monochrome security cameras) would see right through it, as if it was glass. The IR scopes thought the black window was totally transparent. Other tricks were possible. I could hide lettering and artwork behind those opaque black squares against a wall, and the IR video cameras would see the hidden art or message.

At one point I started wondering just how much IR light a human eye could see. After all, if the infrared light was EXTREMELY BRIGHT (such as the IR of a sunny day,) human eyes might still detect it. And remember, if 30KHz ultrasonic sound is loud enough, you will hear it. Same basic idea. I took a small piece of our black IR filter-plastic and cut it into 2in. oblong disks to fit over the eye-depressions in my own skull. I taped them onto my face with black electrical tape. Yes, I looked odd, but it worked! After I became accustomed to the darkness inside the filters, I could see through them. Going outside on a sunny day was stunning. The sky was almost black, while the trees and shrubs were all frosty pink. The grass looked like fluorescent red cherry Koolaid powder. Different colors of human skin were always the same light grey. People's eyes looked very black, and certain dark clothing looked white. I was afraid that I might damage my eyes, since the IR sunlight was very bright, and my pupils were wide open. (After years of playing with these, I still haven't hurt my eyes, so they're PROBABLY somewhat safe to use.)

I couldn't see anything indoors though, and I could barely see anything during a cloudy day. This "IR vision" requires a sunny day in order to work. YOU CAN'T USE THESE FILTER-GOGGLES FOR NIGHT VISION! BRIGHT SUNLIGHT IS REQUIRED. In darkness the goggles do nothing. If you want to actually convert IR to visible, or to amplify light, get a CCD security cam, or go buy a cheap night-vision scope from Harbor Freight Tools instead. Or try putting an IR filter over the lens of a $20 Casio monochrome wrist camera

Later I found some $7 welders' goggles and cut some of that black "IR plastic" into 2in. disks to fit the lenses. This worked great. I could stagger around in the noonday sun while observing the strange twilight-dark world of the near infrared spectrum. Frosty white trees against a black sky. Driving with the goggles was dangerous: I could see just fine, and cars' non-LED tailights looked abnormally bright, but red traffic lights were totally dark (here in Seattle they use red LEDs for stoplights which lack the IR output of the original incandescent bulb.) Some sorts of car tail lights, the LED kind, were also dark.

Goggles for All

Years later I finally used up my last small piece of black IR plastic. I still had several pairs of the goggles, but it was sad that I couldn't show others how to make their own. Perhaps some #89 Kodak Wratten filters would work, but that stuff is wicked expensive, and I never tried it.

But then one day I was messing with a Rosco filter booklet and happened to hold a red filter over the IR filter-goggles and look outdoors. The view was different. The red Rosco filter seemed to remove a greyish-violet color that I hadn't really noticed before. The IR goggles pass lots of infrared, but for some reason they also seemed to pass some deep blue. Why would IR filters let some blue light through? Heyyyyy... which of the Rosco filters does the same thing? Which filter passes a spike-spectrum of deep blue, but it also lets through lots of IR? CONGO BLUE! Maybe our IR plastic supplier cheated. Maybe when Eaton ordered some visible-opaque, IR-transparent plastic, they actually gave us Congo Blue plastic with lots of extra dye added to the mix (so barely any visible blue light would get through.)

Sure enough, when I stacked several sheets of Congo Blue filters and stuck them in my welding goggles, they acted much the same as those Lexan IR-pass filters. I could go out in bright sunshine and see the familiar pink-tree, dark-sky world. A few small pieces of Congo Blue filter costs about $0.50... which means that ANYONE can make these goggles now. No expensive Wratten filters or exotic custom-ordered IR Lexan is needed.

Make Your Own


Needed:

  • Bright sunshine or many hundreds of watts of 725nM IR floodlights
  • Cheap welding goggles (w/removable filters) (looks like these #702020 from staples.com, or maybe these flip-front types )
  • Sheet of "Congo Blue" filter gel (Lee #181 from leefilters.com, or Rosco #382) costs maybe $9 for 24" sheet
  • Optional: sheet of "Primary Red" filter gel (Lee #106 or Rosco #27)

Search Amazon.com for "Congo Blue" filter material. Or try bulbamerica.com #9181 $1.80 for 10in. square congo blue. One sheet makes LOTS of goggles.

Find yourself a pair of inexpensive welding goggles: the ones with round, unscrewable lenses with circular filter-disks are only $7 at my local welding supply shop. If you wear glasses, buy the larger green version with the removable rectangular filter-window. Buy some "Congo Blue" theatrical filter gel, and for later experiments get some "Primary Red" as well. (I got mine from pnta.com, PNTA theater supplies here in Seattle.) Remove the dark-green filter disks from the goggles and use them as guides to cut out twelve disks of Congo Blue filter plus two disks of Primary Red. Stick six layers of Congo Blue into each goggle eyepiece. Don't use the dark green disks that came with your goggles. Use only the filters you've made. Wait for a sunny day, strap 'em on, and go for a walk outdoors.

What will you see? The whole world looks blue-grey with deep red highlights. But then you start to notice some strange things. Get away from the buildings and look at grass, bushes, and trees. Look at different plants with the goggles, then take them off. Many plants look frosty-whitish-pink with the goggles, but for normal human eyes they look green or greenish black. Sometimes you can see birds moving around deep inside the frosty white bushes and trees, yet normal human eyes would see nothing, just a dark green shady bush.

Look at people's clothing and skin color w/the goggles, then take them off and look again. Many items of clothing look white in the infrared, yet they look black or dark blue to normal eyes.

If you use Congo Blue filters alone, and don't include the Primary Red, then the filters will let some blue light through also. This is useful, since whenever there is too little IR light to be seen, you can still stumble around using the remaining blue-grey visible light. Without the Primary Red filters added, the world appears dim blue-grey, and the IR scenery appears bright red. Place one or two Primary Red in each lens of your goggles and this gets rid of the blue. It lets you see purely Infrared light. I've come to enjoy the blue/IR mixture, since an all red world is less interesting, even though it's an all-Infrared world. A dim blue world with bright red patches is cool, since those bright red patches are actually the Infrared scenery that normally would be invisible.

Will these goggles let you see IR lasers and LEDs? Maybe. The LED or laser must make some 730nM light. The goggles don't amplify. The goggles work by cutting out the bright background light. They let you see the dim IR light that remains. It's as if they "turn off" the visible room lights so that you can see the bits of infrared. They don't work any better than using a black background or going into a darkened room. So, take your IR laser or LED into a pitch black room. Can you see its dim reddish light without using the goggles? If yes, then you probably can see its light if you use the IR filter goggles in a brightly-lit room. However, if you find that can't see your IR LED by eye in a darkened room, then these IR goggles won't help, since they don't amplify the light. They work by making the whole world into a "darkened room" while still letting the bright IR light get to your eyes.

Making secret messages and IR t-shirts

Here's a trick. Take a sheet of Congo Blue filter and overlay it with Primary Red. It looks black to your unaided eyes. Now wear your IR filter goggles and observe those "black" filter sheets under incandescent light (or take them outdoors into sunlight.) You'll find that the sheets of Congo Blue plus Primary Red now appear to be transparent! They look a bit grey rather than totally clear, so you might want to try using a different colored filter instead, for example use a couple of layers of Roscolux #385 "Royal Blue" Unaided eyes think the red/blue filter stack is totally black, but your IR goggles let you SEE RIGHT THROUGH the filter sheets. Write a secret message on a piece of paper and cover it up with the "black" IR filter stack. Normal humans will see nothing but a shiney black square. But with your goggles you can see the secret message. Make IR-only signs. "No Cyborgs beyond this point." "Human infants taste terrible!" "Chlorine-breathing reptoids out of US Congress!" Any eyes that possess, ahem, enhanced longwave response will see your hidden message, but all of the "normals" will just see a featureless black square.

Here's another way to do the same thing. First use the goggles to look at different kinds of dark clothing. Find some cloth that looks light grey in the infrared, but looks dark black when you take the goggles off. It's easy to make a secret message with this cloth. Just write on that black cloth using black magic marker. Human eyes can't see the black-on-black. But if you wear the goggles on a sunny day, then the black writing will be clearly visible against the light cloth. (Most black magic markers have ink which is black in both the visible and the IR.) You can draw anything you want to on your black clothing. Only people with IR goggles (or IR cameras) can see it. [NOTE: I made some signs like this, and I found that I can still see the lettering by eye if large block letters are used. The large regions of "sharpie marker" ink is still visible on the black cloth. Run the black cloth through the wash to reduce this problem. On the other hand, thin writing is still invisible. It's only the big black blocks that can be seen by humans if they're paying attention.]

Test for better filters

Here's a trick that demonstrates that you're really dealing with IR. If you have a "swatch pack" of Lee color filters, find the Congo Blue #181 and the Peacock Blue #115. To normal human eyes, Congo blue appears almost opaque black, and Peacock blue looks transparent sky-blue. Now wear your IR filter goggles and look again. (You'll need sunlight or an incandescent bulb for illumination.) You'll find that the Congo Blue filter is no longer opaque! It now looks transparent... but now you can't see through the Peacock Blue. In the IR band, their roles are reversed. The Peacock blue filter is a black absorber under IR light, while the Congo Blue is transparent. Look at other filters in the Lee filter swatch-pack. You're really seeing the IR transmission of these filters, and as with Peacock Blue, the ones which look black in the near-IR are often very transparent in the visible.

Goth-ray Vision

Remember that I mentioned that certain clothing looks black for human eyes, but looks white in the IR? Certain dark blue dyes act this way. Some new blue-jeans look white in the infrared, while black work-pants appear black, but in the visible spectrum they both look very dark. Find yourself a black windbreaker which appears white in the infrared. Use carbon-based ink to put a nasty message or some disturbing artwork on the back, and only IR cameras (and IR filter goggles) will see it. Do "Goth Warchalking", where you write on bluish-black paper with black magic markers, and the resulting messages are only visible to these weirdos who go around wearing black-lensed mad scientist goggles.

Now I need to find some IR-absorbing spray paint and magic markers. I want to do the opposite to the above. I want some kind of paint which looks totally water-clear to human eyes, but looks totally black at 720nM infrared. Why? Because then I can put IR graffiti all over everything, and nobody can see it unless they're equipped with IR goggles. I'll draw "crop circles" on lawns and city streets that only IR cameras can see! Maybe get some huge nasty tattoos on my face which are invisible to mundane eyes. Hmmmm, I wonder if anyone is already doing this. If I keep a lookout while wearing goggles maybe I'll find secret messages on city sidewalks written by the MIBs. Search google on "infrared paint."

A view more Infra-reddly

Congo blue filers give your eyes a peak sensitivity of around 720nM. That's definitely into the IR band which starts at 700. If you want your vision to be much deeper into the IR, you can use a different Lee or Rosco filter, one with an even deeper IR cutoff. One such filter is Lee #120 "Deep Blue." This filter passes much more blue light than Congo blue, so you'll need to use three or four layers of Deep Blue, plus two or three layers of Primary Red.

The result is different than the congo blue goggles. With these goggles you can barely see anything at all, even in brightest daylight. But after about 15 seconds your eyes grow used to the dark. And then the sky looks far more black, and the plants and trees are even whiter. Humans are boring: they're all just grey-red, including clothing and hair. But human faces are weird because everyone's eyes look huge and dark.

Ditch the goggles, make an IR floodlight

In a dark room or during a moonless night these goggles are worthless. Their whole purpose is to block the background light from the environment, and if there IS no background light, then you don't need any goggles to see a bit of IR. So, if you want to experiment with direct viewing of IR LEDs or (dangerous!) IR diode lasers, just go into a well-darkened room and observe IR sources directly. But that leads to another idea: don't put filters on your *eyes*, instead put the congo blue layers over a white light source. If you have a simple theatrical floodlight that blocks any spill from the rear, and can take a colored filter in the front, then you can make a high power near-IR floodlamp. Give it a few layers of Congo Blue and one or two sheets of Primary Red to cut out the blue leakage. This is NOT the same as an 850nM LED floodlamp used with security cameras. In a dark room it looks fairly strange; appearing as dim red light until you aim it at a human face and find that their skin is translucent, their hair is wispy grey, and their eyes are alien-looking black. As usual, certain types of black cloth instead look grey (so your black Sharpie-marker artwork suddenly becomes visible.) Also, a sheet of congo blue looks nearly transparent when held in your hand. If people wear the IR goggles, the filters don't look very dark, and you can see their eyes. And psychologically its very eerie, since these effects are occuring, yet you're not wearing any goggles on your face. It might be pretty cool if used to light an "infrared art gallery" with black-on-black velvet paintings. Or if used to create incredibly intense 900nM illumination (and if this doesn't damage human eyes,) then spandex clothing worn in the gallery would appear transparent.

Speaking of art, here's an idea that requires a bright outdoor environment (such as Burning Man.) Build a booth out of transparent plastic. Cover the entire thing with layers of Congo Blue and Red. Make sure the door gives a good light-seal. Perhaps add a ventilation fan, since it'll get hot in there. Now climb inside, get used to the dark, and look around. The entire world will look like "IR goggles-view!" But that's just the first part. Now build one or two more of these booths and place them about ten feet apart. The outside observers see black shiny monolith booths, but a person inside a booths think the *other* booths are nearly transparent. Wave to the people in the other booths. Only they can see you, yet you might be surrounded by a clueless crowd outside the booths. It's almost like being invisible. Now do other things that might spring to mind. Go wild. But remember: I'M WEARING IR GOGGLES, so the "opaque" booths are transparent to me as well.

How do they work?

These IR goggles are simple: red filters block blue light, and blue filters block red... yet both colors of stagelight filters happen to pass the invisible IR light. If you stack up some blue and red filters, you get black. But it's not QUITE black, since they only block the "visible light" which has wavelength shorter than 700 nanometers. Together the two filters create an IR-pass or "lowpass" color filter.

On the other hand, human eyes are highpass filters. When you combine a lowpass filter with a highpass filter, you get a bandpass filter. When you place an IR-pass filter on human eyes, the edges of the filter responses overlap to form a pass band or sensitivity peak. The frequency of this peak is in the IR spectrum. Your eyes normally have a tiny bit of sensitivity in the IR band, but usually the bright sunlight washes it out. Wear these goggles to block out the "normal" sunlight. Your eyes have been converted into IR light sensors. Your view will be dim, but you will be seeing actual infrared light.

"Congo blue" in fig. 1 passes a hump of blue light while killing all the green, yellow, and red, but it also passes lots of IR above 700nM wavelength. "Primary red" in fig. 2 kills all the yellow, green, and blue wavelengths, but it passes IR just fine. Human eyes themselves are like a "filter" which passes green light best, but sees from violet through red, plus a tiny bit out past 700nM. Stack them all up in figure 4, and the red and blue parts get removed since the red filter absorbs blue, and the blue filter absorbs red. Now add lots more layers of congo blue, and the sloping edge of the IR band gets much sharper, so only "invisible" light from above the 710nM wavelength gets through. Use two or three layers of red filter to make sure all the blue light is suppressed. Multiply all these curves together and we get the curve in figure 5. It's a small peak, with the center frequency a little past 710nM in the infrared band. Figure 5 shows that your eyes have been converted into infrared sensors. The gain is terrible, that's why you need full sunlight in order to see any infrared scenery. Whaddaya want for under $10 bucks?!

 


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DANGER! DON'T STARE AT THE SUN!!!! I've worn the IR goggles for hours at a time in bright sunlight. I'm not blind yet. The dark goggles make your pupils open up wide, but then the goggles don't block all that IR sunlight. Is this harmful? It hasn't hurt me yet, but I don't know if it's totally safe. At the very least, don't stare at the sun while wearing the goggles, you might get a nasty suprise later on (called Snow Blindness. Your cornea surface rots and sticks to your eyelids. Ewww.)





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