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Infrared LED's & IR Filters

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I'm researching the active infrared viewing telescopes used by U.S. forces from 1944-1960. Their light assemblies used incandescent lamps with tungsten filaments. They used a glass filter coated with material that blocked visible light and allowed IR light to pass through the filter. The IR imaging tubes view IR at 800nm-1000nm. The filter's we are using were manufactured by American Optical in the mid 1950's and have worked with various different IR light sources.

The exception are the LED's that emit IR light in this spectrum.

If someone could explain why the LED generated IR will not pass through the filters we'd be very grateful.




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It could be the wavelength or, more likely imho, the intensity. Tungsten filament lamps would give out a huge amount of IR light - you can feel the warmth feet away. The IR LEDs on the other hand will be just a fraction of a watt.



New Member
I think some insight into the background of what we're doing is in order. We are historical researchers attempting to reconstruct the history of several of the first U.S. active IR viewing devices. Our primary focus is documenting, sharing and preserving history. We are not equipped nor do we have the knowledge or experience of the professionals common to this forum. We're reaching out for limited assistance from several professionals on a few questions we have.

The end result of the research we do is shared openly on the internet and with anyone having an interest. To include authors and museum curators. We make no money for what we do. Nothing for sale, no advertisements. Personally, I'm not a collector. My background is 40 years of investigative experience. I collect information. The IR equipment I have is destined for two museums and possibly a third. At my own expense.

The IR viewing devices being researched and tested are two groups. The first group is the U.S. Navy RCA US/C-3 IR viewer of 1944 (1P25A IR tube) and the Lewyt Mfg AN/SAR-4 IR viewer of 1954 (6032 IR tube). These were used for signaling ship to ship and ship to shore using ship spotlights and other tungsten filament IR emitting lamps with various IR pass filters. The second group are the U.S. Army Bell & Howell/Cornell-Dubilier Model M2 telescope of 1950 (1P25A IR tube) and American Optics Model M3 telescope of 1952 (6032 IR tube). Both the M2 and M3 were mounted on a U.S. .30 cal. carbine. The M2 could also be used as a handheld IR viewer. These were Generation 0 IR devices of limited abilities compared to today's technology. The design intent was for use in a fixed defensive position in a jungle environment. A Sony CCD camera with IR capability is being used for comparison. The Sony has it's own IR pass through filter with a very low power emitter that is covered up during testing.

The incandescent lamps originally used in these 1944-1954 light assemblies were the GE #1045 (5.9v 5a 29.5 watt) and GE #1077 (5.2v 6a 31.2 watt). They were designed from flange mount auto spotlight lamps of the 1930's and 1940's. The distance of their IR output is anemic compared to modern technology. Both lamps are long since obsolete. Using the filters I have (more on these below) I've tested incandescent lamps 1045, 1077, 1209, 1301, 1501, 1503, 1007, a PAR-46 (used with the first prototypes) with similar ratings. Also several halogen lamp models operating at about the same specs. All have emitted IR light through the filters I'll discuss in a moment. The IR light was observed using the above IR viewing devices.

The IR LED's used so far have been limted due to the nature of what we're doing. The most capable being the Streamlight Super Tac IR flashlight. Streamlight's IR data sheet lacks specifics regarding the LED it uses. A C4 LED at 850nm. From an operational perspective it performs as advertised. Until an IR pass filter is placed over it's reflector. We've had the same experience with the few individual IR LED's we have acquired that emit at 800-900nm.

IR pass filters were going through the same failures and progress in leaps and bounds as the IR equipment they were used for. Those prior to 1952 experienced a number of problems including limited life spans while in storage. While I've located documents identifying the composition of the earlier filters I have yet to locate documents for the later filters we're using. I have the Engineer report numbers and I'm working with their historians attempting to locate copies.

The IR device Technical Manuals indicate these filters have an unlimited lifespan only subject to the environment they are used in. These filters were new and sealed in their Corps of Engineers protective packaging. Which included the original packaging used by American Optical. They were individually boxed then collectively boxed 12 to a carton. The carton was sealed with an approval stamp from the Columbus, OH Engineers supply depot and dated October 1956. They were made from glass coated with several layers of the materials that allowed the IR to pass through while stopping visible light. The glass is 5 3/8" in diameter. I have several other IR pass filters but their origin and composition is unknown.

I've been operating from the operational view of all the other IR emitting lights I have used the filters on performed as expected and advertised. Until the IR LED's.

The heat that passes beyond the filters using the 6 volt 30 watt lamps is minimal. Though greater than the heat generated by the more powerful IR LED assembly used by Streamlight. I apologize for not having more technical information on the filters. The logistics of military records from the 1950's was in turmoil until about 1964. While some survived the path to the holdings of the National Archives many didn't. This is an ongoing research project with efforts involving a number of different historical document resources.

I'm in the process of acquiring several IR pass filters from different reliable sources but the experiences so far have indicated these current filters work with various IR emitters except for the IR LED's.

Any suggestions or assistance is very much appreciated.



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You may be at the upper edge of the filter's cutoff.
Can you get some LEDs with a longer wavelength?


New Member
I'll give the 950nm IR LED's a try.

The 1P25A and 6032 image tubes are rated at "8o,000 +/- 100,000 angstrom" (800 +/- 1000nm).

The earlier IR pass filters had 4 layers Polaroid XR7X-type film bonded to a disk of heat resistant glass. Three layers were blue dyed cellophane, 1 layer was red dyed cellophane. This trasmitted IR beginning at "850 millimicrons" (nm). I have the wavelength charts for the early filters but not for the later filters we're using.

The Streamlight LED and other LED's I've acquired are rate at 8500nm.

The later filter we're using is a black single sided coated glass 5 3/8" in diameter and 4.75mm thick. It has the part number 8247-29-1 in the glass. Edmund Scientific used to carry an IR pass lens with this same part number. I've contacted them to see if they have a spec sheet or know the modern day equivalent. 8247-29-1 was also the Corps of Engineers part number for which a MilSpec would have existed. Unfortunately MilSpecs have a short lifespan when the item they're for becomes obsolete.

Thanks much



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optics isn;t my strong point, but...

I'd start here: https://en.wikipedia.org/wiki/Infrared#page=5 which gives you an idea of a quartz tungsten-halogen light source spectra. It also goes into alignment procedures that needed to be followed.

In our research we needed a cheap stable light source which turned out to 120 VAC ELH lamps until we could afford a $40,000 light source. It turne dout that we HAD to use GE branded bulbs. Sylvania did not have the same spectra, These lamps have a dichroic reflector that puts the IR out the back of the bulb. We were not interested din IR.

The specra above is for quartz-Halogen. Quurtz has better IR and UV transmission. The halogen lamps may not have been invented. At least it give you an IDEA of what your getting. The emission of the actual bulbs would be ideal.

Microscope illuminators use a low voltage flange based lamp. In one application we had to change to a 12 V EYC because of vibrations. Momochometer bulbs are the same way. Those you have a reflector behind and have to image the filament so it fills the gaps and the light is more uniform.

next, take the research here https://en.wikipedia.org/wiki/Infrared and look at the best wavelength for a given purpose.

if your looking for something to replace the lamps with lamps, I;d look at microscope bulbs, e.g. https://www.specialtyoptical.com/product/lighting/77z-15w-microscope-light-bulb/ I don;t have the catalogs I used to have, but it may be an option.

We put an ELH lamp in a fiber illuminator with a Y light pipe for a custom instrument too.

For about a 70 W/m light source we had to run four 300 W ELH bulbs to get about a 5% uniformity over 4 inches. You had alignment issues and diffuser issues (frosted glass) and distance and the thermal issues. As they age, they loose output in the blue.
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