IR illumination advice wanted

[hmmwv]

OK - we've already hashed out the LED efficiency issues in the visable range enough - now let me open a new topic - LED (IR) efficiency.

I have an interesting application - a CCTV (IR only) camera is trained on a distant target with a zoom lens (200+ feet away) - at night, the target needs supplemental illumination in the IR spectrum to provide a good picture (background IR does yield a grainy picture, but an IR filtered spot really lights things up nicely without alerting those in the area)

So - given that IR leds are much more efficient than visable LEDs, has anyone compared IR sources against say a tungsten filament with an IR filter?

(also - in this application, focus is critical since the illuminator can't be closer than 150' to the subject - so a second question is how much beam divergence does an IR LED have compared with a parabolic reflector based IR source)

(and no- IR lasers are not suitable due to their LACK of divergence - i've played with them to some extent, and they are useful to put a spot in the picture at night which can be watched on a PC running "guard" software to do frame grabs and look for the spot moving - but otherwise they don't diverge enough to illuminate 2-3 people.)

 

[The_LED_Museum]

Most IR LEDs have a beam divergence similar to that of most visible models.
This value ranges from 10° to 100°, which is roughly the same range you find visible LEDs in. Getting one to fire a 150' beam could be tricky.

 

[Brock]

In the theatre I work in we use IR cameras to "see in the dark". This allows stage managers to make calls when scenery can safely move without hitting actors. It also allows us to see if we are set to go in a true "blackout". The problem is IR LED's just don't throw that far. One company came through with about a 150 LED array and it was ok, but still didn't compare to the standard IR illuminators we use. The only advantage was there was very little visible light, unlike the standard IR unit that emit a dim red glow. The standard ones we use are very bright in the IR range and run about 300w to 500w at 120v.

 

[Harrkev]

Here is an idea. This is not guaranteed to work, but you could try it.

1) Take a small box, and paint the inside white or silver.

2) Mount the LEDs in the box

3) Take a piece of clear plexiglass and sand both sides until you get a milky appearance.

4) Mount the plexiglass on the box. Now you have a nice diffuse source.

5) Obtain from a local office supply store (I got mine al Wal-mart) a page-size fresnel lens. This is a thin plastic sheet that acts as a magnifier.

6) Mount the lens in some sort of rigid frame.

7) Using your camera in the dark, turn on the LEDs and move the lens back and forth in front of the box until you get a good sized focused spot.

8) Mount the lens permenantly at this point.


LED's are far easier. If you want to use incadescent, keep in mind that the infrared filter material can be expensive and hard to find.

Some developed photographic film will work wonderfully as an IR filter. Some developed film leaders appear to be black, but are transparent to IR. I used to have a friend-of-a-friend who worked in a photo lab, and gave us a bunch of black leaders. I was able to take two layers of this stuff and cut it to fit in a 2-AA mag-light. You could see a *very* dim red glow, but the IR was being blasted out! Unfortunately, I cannot tell you which brands work well (or at all). It could be exposed print film, or unexposed slide film. Since it was black, it was hard to read the label. If you have friends into photography, then getting some of this stuff should be free!

I hope that this helps.

 

[Glow Bug]

ITT Night Vision sells a small infrared illuminator with a ir led about the size of a mini-mag light with a sliding head with a magnifing lens in it. You can adjust the beam width by sliding the head back and forth. I do not know if it would br enough light for your application though. It would vary upon the sensitivity of your equipment to ir. Here is the link>>> http://www.ittnv.com/itt/Active/ConProductPages/NightQuest/AccessoryPage/20

Surefire makes an infrared unit also. Heres the link>>> http://www.surefire.com/cgi-bin/main.pl?pgm=co_disp&func=displ&carfnbr=237&sesent=0,0

 

[PeLu]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by hmmwv:
OK - we've already hashed out the LED efficiency issues in the visable range enough - now let me open a new topic - LED (IR) efficiency.
<HR></BLOCKQUOTE>

We discussed only luminous efficency. I think, quantum efficiency was never mentioned in this forum (why should we?). This makes it a very easy topic to discuss:
IR LED have a luminous efficiency of 0% .-)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>
So - given that IR leds are much more efficient than visable LEDs, has anyone compared IR sources against say a tungsten filament with an IR filter?
<HR></BLOCKQUOTE>

You have to tell which wavelength and about which power range you want to discuss.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>
and no- IR lasers are not suitable due to their LACK of divergence
<HR></BLOCKQUOTE>

I have an IR laser for almost the same purpose. It has an adjustable lens in front of it and spreads the beam as necessary. No sidelight, very nice.

 

[PeLu]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Harrkev:
Some developed photographic film will work wonderfully as an IR filter.
....
It could be exposed print film, or unexposed slide film. Since it was black, it was hard to read the label.<HR></BLOCKQUOTE>

It is unexposed (but processed) colour slide film. We use it to trigger slave flashes (slave sensors are sensitive to IR and sometimes you do not want to have the camera's flash light the foreground).