"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. "Hoffmann Lenses" from the movie THEY LIVE!

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 Big-5 sporting 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.

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

Goggles for All

Years later I finally used up my last small piece of that special 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.