Building an IR lightsource

This sounds fine. Don't forget to limit the current in each 3 LED chain to about 10mA. If you apply 5V and drop 4.8V, you are left with .2V, .2/.01=20 Ohms. Dissipation is 2mW so a quarter watt resistor will be fine for each chain.

I browsed CONRAD.NL with "camera" search line, found there at least 5 with IR leds, nicely cased, or OEM print, for about 50-150 euros. Try one like it in your country, leds in line with the camera works very well.

V Sun, 30 Sep 2007 21:29:37 -0500, Lord Garth napsal(a):

Do you know if common acrylic is transparent to IR? If I need to do one of those "ships in a bottle" projects where I have to drill holes in the tube and then push the LEDs through things might get a bit more interesting.... :-)

--Yan

Try with your TV remote. See if it works trough 5 or more layers of the plastic a few meters away from your TV.

It is. For that matter, anything that is "clear" to visible light also passes pretty much everything between visible and maybe 1400 nm or so, and usually even longer. (Water starts to block IR at about 1400 nm and blocks IR a lot past 1500 nm, and is more IR-blocking than most other transparent materials). The main exceptions are found mainly where their use is blocking/reflecting infrared.

LEDs all-too-often don't do too reliably well when connected directly to fixed voltage source.

Better would to make a chain of 2 LEDs and a resistor (39 ohms for just under 50 mA, 33 ohms for just over 50 mA, 18 ohms for about 100 mA, 22 ohms for a little under 100 mA).

Experiment and measure voltage drop across the resistor, divide square of that by resistor value assuming the resistor is at low end of tolerance range. Pick a resistor wattage that exceeds this, preferably by a factor of at least 5/3 for really good reliability.

I have some experience with military grade 1/4 watt resistors that resembled commercial grade 1/2 watt resistors, also with a couple 1/2 watt resistors failing while dssipating about .35 watt.

- Don Klipstein ( snipped-for-privacy@misty.com)

All common transparent plastics are transparent to all of the usual wavelengths of IR LEDs. Same is true of all common grades of glass - cheaper ones of pale blue-green or pale greenish tint will attenuate IR a little bit, worse than for visible red, but still pass most of the output of the usual IR LEDs of nominal wavelength up to 950 nm - probably usually block about half at most any wavelength in the IR-A range (past visible up to 1500 nm).

Keep in mind that most transparent colored plastics largely pass most IR in the 850-1500 nm range. So does cobalt blue glass and "Woods glass" (UV-pass, visible-block, dark violet color, used for "blacklights"). The most-IR-blocking kinds of colored glass appear to me to be turquoise-cyanish to greenish and I suspect are tinted by iron or copper or both.

Transparent greenish plastic I expect to pass most 850-1500 nm IR unless it is a plastic filter made for use in welding goggles or a welding mask.

- Don Klipstein ( snipped-for-privacy@misty.com)

That's fine so long as the LEDs really do drop exactly 1.6V each. You only need a 2% increase to get 4.9V (half the current) or a 2% decrease for

4.7V (50% more current).

I'd suggest making each chain use 2 LEDs and a 180 Ohm resistor. That will be far more robust against slight variations in LED voltage.

The usual IR LEDs do well with a lot more than 10 mA. I would go for a good 30 mA. With 5 volts and 1.6 volts across each of 2 LEDs, 30 mA means a 60 ohm resistor. The nearest standard values are 56 and 68. Keep in mind that at 30 mA, IR LEDs usually drop less than 1.6 volts. Then again, if you use a 56 ohm resistor and get a little more than 30 mA, I see little chance for harm - IR LEDs tend to be rated for at least 50 mA.

Watch for heat buildup in a cluster lamp however.

- Don Klipstein ( snipped-for-privacy@misty.com)

to

fine

of

:-)

This would be easy to test as many video cameras see in Ir. This is especially true of B/W cameras. Just get one LED lit up and drop some acrylic in the path. I suspect it will attenuate to some degree but much will pass through.

so then why do thermal imagers have such a bad time imaging people (heat signatures) thru glass? standard/non filmed or IR protected. both seem to be barriers to heat signatures

sides,

You are talking about a completely different part of the IR spectrum. IR LEDs generally produce light in the 800 to

1000 nm range, while thermal imagers work with wavelengths about 10 times that long. This is the difference between shining near IR light on stuff and seeing what they look like in the light they reflect, and treating the objects being viewed as light sources (by the energy they emit because of their temperature).