This page is mainly concerned with infrared wavelengths that cameras based on silicon and IR-sensitive photographic films are sensitive to. This means wavelengths over 700 nm and up to around 1100 nm. Some of these wavelengths are slightly visible.
For a narrowed down list of my favorite 13 Digikey-available through-hole ones, go here.
My favorite narrowbeam one of these is Osram SFH 4545, which has a nominally 10 degree beam. I have found its beam to be overconverged, which makes it especially useful for illuminating small areas at close distances. However, it works better than most other narrow beam IR LEDs at any distance. The other 12 have nominal beam widths ranging from 20 to 60 degrees. I have found the Everlight IR1503 to be overconverged with a beam similar to that of Osram SFH 4545.
My favorite one with a beam over 30 degrees wide is the Osram SFH 4546, which has a nominal beam width of 40 degrees.
Please be aware that with these 2 and maybe other Osram LEDs, the longer lead is negative (cathode), opposite "usual convention". However, the Osram ones follow the usual convention of having their flange having a flat spot at the cathode (negative lead).
I have found most of these 13 LEDs to generally be slightly visible in a dark room at 50 mA or more, often even at 40 mA. I have found the Osram ones to be slightly visible in a dark room at 20 mA or more due to their greater output and efficiency.
These 13 IR LEDs can take 100 mA continuously in favorable thermal conditions, which excludes most cluster lamps. Use only 40 mA continuously in a cluster lamp until it is known that the cluster lamp can take more.
For testing a cluster lamp design for safe current, go here.
I recommend designing for a maximum of 70 mA continuous average current in other situations, unless you know you have favorable thermal conditions according to averything relevant on the LED's datasheet.
For some applications, a beam narrower than 10 degrees is desired. One way to achieve this is by placing a convex lens forward of one of the above infrared LEDs. You may want to try this at first with a visible LED whose dimensions and beam characteristics are the same or similar. Otherwise, use a camera that is sensitive to infrared to check your results. Most cell phone cameras and many digital cameras will work. Simply aim your infrared source setup at a wall, and look at the illumination results with the camera.
Cellphone cameras are convenient for not needing focus adjustments when looking at something from a short distance away. This feature can be used to work around their low to moderate resolution.
As an example, the Osram SFH 4545 can be used with a convex lens whose focal length is 150 mm and whose diameter is at least 30 mm, but preferably 40-50 mm. One such lens is Anchor Optical AX27751, a coated convex lens whose focal length is 150mm and whose diameter is 50mm. Most of the output of the Osram SFH 4545 can probably be collimated into a beam 2 degrees wide with this lens. At 100 mA, this means probably at least 40 mW of infrared is within a beam largely representable as 2 degrees wide. Make that about 28-plus mW for continuous operation, due to temperature rise of the LED. A 2 degree circular beam has a "solid angle" of .00096 steradian. This means intensity of 29 watts per steradian continuously at 100 mA, or ~42 W/sr for brief 100 mA pulses.
Incandescent sources:Nowadays, infrared LEDs, even ones of longer peak wavelength such as 940-950 nm, are more efficient than incandescent lamps at producing wavelengths that silicon photosensors and silicon-based cameras respond to. IR LEDs also have higher optical efficiency than incandescents for directing light into desired beam shapes. However, there could be a few applications when low lamp cost and high output power requirement, or a high requirement of ambient temperature, necessitates use of incandescent lamps.
One possible application is distant illumination by an incandescent or halogen spotlight, such as a 12V high candlepower spotlight, an aircraft landing lamp, or a theater type spotlight fixture. Keep in mind that "candlepower" figures for most 12V spotlights are greatly exaggerated, which may be legal now that "candlepower" is not an official photometric beam intensity unit. (The official unit of that is the candela.)
Most reflectorized incandescent lamps of size suitable for use with most better-availability high quality glass filters are MR11 and MR16 types. Keep in mind that many of those have dichroic reflector coatings, which do not reflect most infrared. Also, heating can be a significant problem. Schott glass filters may withstand 400 or more degrees C - but parts of your setup may fail from heating, especially with a filter absorbing wavelengths outside a limited range of infrared wavelengths.
Xenon sources: Xenon flashlamps are surprisingly efficient at producing infrared in the 820-995 nm range. Roughly 30-40% of the camera-useful infrared output of xenon flashlamps, for most applications, is in the range of 820-845 nm. Another roughly 45-50% is in the range of 875-995 nm. There is a little bit of xenon's near-infrared cluster of strong emission wavelengths as far out as about 1020 nm.
Xenon is useful primarily for a very intense pulse of IR illumination, over a wider area than can be economically covered by LEDs.
One reason for some clothes being less opaque to IR than to visible is the fact that most dyes do not absorb much IR. To a camera making use of 940 or 950 nm infrared, most clothes appear white.
Another reason is that some fibers have texture features with width comparable to most visible wavelengths, but small compared to 940 nm. Fabrics with such fibers can have translucency to infrared almost as if they are wet. If such fabrics are wet, use of infrared increases their translucency slightly further.
Wavelengths around 1000 nm may result in even better ability to see through some clothes. Possible light sources may include:
* Incandescent light sources filtered by Schott RG1000 or Wratten 87A filters.
* High pressure mercury vapor lamps, filtered by almost any IR-passing,
visible-blocking filter, but preferably a severe one. Mercury vapor has a
significant infrared line at 1014 nm, and silicon photosensors mostly have
reasonable sensitivity to this wavelength.
NOTE - Availability of ballasts for mercury vapor lamps may be low, and mainly on eBay and similar sites. However, 175 and 400 watt mercury vapor lamps can be powered by many same-wattage metal halide ballasts. Lower wattage mercury vapor lamps can usually be powered by same-wattage metal halide ballasts whose electronics for high voltage starting pulses are removed.
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