IR LED direction accuracy

It might be worth thinking about a packaged IR laser diode. They cost the earth, but the built-in optics offer a fairly narrow beam. This is an example that I found by a bit of googling - I've no idea if you can actually buy it, or how much it costs. Ostensibly similar parts cost around a hundred euro.

-- Bill Sloman, Nijmegen

Most lens-front LEDs do this, fire light in random directions. Chip placement isn't very accurate. The best way to focus LED light is to use a flat-face chip and add your own lens.

No. Even though the directionality graphs seem to peak at 0 degrees, real parts will usually fire in odd directions. More datasheet creativity.

Why not use a laser, maybe a laser pointer?

John

I was not really thinking of focussing the LED output. My approach (so far theoretical) is to use an array of LEDs for higher intensity. Their combined IR emission would be impossible to focus into a parallel beam with a single lens. The lens will rather be at the receiving end.

My concern with the direction of emission has to do with ease of aiming the emitter at the receiver. According to the datasheet, the Vishay TSAL6100 I mentioned has high intensity due to the narrow angle: 130 mW/sr typ compared to 20-40 mW/sr for wider angle types, and still has 90% intensity at +/- 4 degrees. 4 degrees is 7' at 100', or 28m at 400m. It shouldn't be difficult to aim the beam with that degree of accuracy, provided that the direction of peak emission is approximately along the axis of the device.

However, if the direction of peak radiation for individual diodes is unpredictable, it will also partly defeat the aim of combining the IR outputs from several devices.

I've also been considering that and have not completely rejected it. The only laser device I can get hold of without too much hassle is a pointer.

On a sunny day (Mon, 10 Jan 2011 23:04:11 +0530) it happened "Pimpom" wrote in :

Old CD burner, maybe DVD burner, even Blu-ray burner.. Those lasers are much more powerful.

Interesting. But I might have given the wrong impression by mentioning the beam width of the TSAL6100 LED. There's no reason to confine the beam to a narrow angle except insofar as to concentrate the intensity at the receiver and thus extend the usable range. The TSAL6100 has a stated radiant intensity of 130 mW/sr at 100mA compared to 20-40 mW/sr for wider angle types.

I was thinking of using several LEDs, something like 25 of them in a 5x5 grid, and a lens at the receiver. But the effect will be partly cancelled if they don't all radiate in the same direction, at least within a very few degrees.

"Pimpom" wrote in news:igfh9o$m8k$ snipped-for-privacy@news.albasani.net:

Look at the Data-sheet those LED's are meant to be pulsed and are rated as such.Radiant intensity at 1A pulses goes up to 650 to

1000mW/sr.

What you could try is a light pipe this could be as simple as a bic pen with the ink part removed.This would help to increase the focus of the beam and improve the range, other then that you need a lense.

Ah yes, a burner laser. I'd forgotten about them. Thanks for bringing it up. My son and I started a project using those last year, but never finished it because other things took priority over it. I wonder how practicable is the idea of projecting a laser beam onto a receiver from 400m, especially since the system has to be set up quickly, used for a day and then dismantled. An advantage of using IR is that the receiver can be a standard remote control receiver module.

On a sunny day (Mon, 10 Jan 2011 23:56:17 +0530) it happened "Pimpom" wrote in :

Pointing the laser will be difficult. But sending IR to a remote control receiver over 400 m will be next to impossible, at least I had big problems with much shorter distances. At least with a laser in the visible range you can see where it hits (points). I have tried no more then 50 meters with a helium neon laser though, YMMV.

Are you using some kind of telescope at the receiving end, does it pass IR?

PS there is also a safety issue with lasers, the risk of shining somebody in th eyes, somebody may topple your transmitter, anything, aircraft.. I would use a radio link if I were you. Cheap about 430 MHz modules in a free band are available from many places, with digital IO too. Most optical link announcements I have seen over time always stayed just that: announcements.

The datasheet does not directly give a duty cycle vs. pulse duration vs. current rating. The radiant intensity figure for 1A is only for a single 100us pulse. The current vs. pulse duration curves are drawn up to 1A for duty cycles up to 0.05, so I assume that's the maximum permissible level. I'd like to use a duty cycle of at least 0.25 with 25usec pulses. Interpolating the curves implies that this would be safe only at around 0.25A max.

The problem with trying to focus the beam with a lens is that I don't think a single LED will be powerful enough to cover the distance, and a single lens will not be able to focus radiation from multiple LEDs into a parallel beam. OTOH, a lens at the receiving end will be able to focus the radiation from several LEDs onto a single receiver.

When I first started thinking seriously about this project, I also quickly discarded the idea of using IR at such distances. I felt that RF was the only realistic option. But then I got to thinking about IR again and came up with this:

I've been using cheap no-name, no-datasheet receiver modules and LEDs (see my opening post) successfully at distances of around

10m outdoors under bright sunlight. With high-intensity LEDs and sensitive receivers, both with known characteristics, I hope to get at least twice that range, i.e. 20m. The target distance is 20 times that or 400m.

An array of, say, 25 LEDs, will give 25/20^2 = 1/16th of the radiant intensity at 400m.

The receiver's sensitive area is about 0.2" in diameter. A 2" lens will catch x100 radiation. This gives a theoretical received radiation of 100/16 = 6.25 times that with a single LED at 20m.

Factors that will inevitably reduce that figure in practice are -

1. Atmospheric absorbtion: No idea except a wild guess. At least there's only minimal pollution at the site. There may be slight seasonal haze.
2. Partial opacity of the lens glass to IR: Most sources found with a quick google search for ordinary glass gave only vague statements like "fairly transparent to near infrared", "passes near IR very well", and so on. The one curve I found for window glass gives a transmissivity of about 0.86 at 940nm. (I wonder if a cheap magnifying glass will be better than a coated lens).
3. Imperfections in alignment and focus: If the Rx is somewhat offset from the focal plane of the lens, it will reduce efficiency, but will be more tolerant of alignment errors.

If all these factors reduced the received radiation to 0.2 of the theoretical figure, it will still be in the same range as that for a single LED at 20m. Now please feel free to point out any flaws in my reasoning.

Outdoor 'laser' Paintball by any chance ????

Sorry for the, perhaps, silly question. But why not use red LEDs? At least then you can see them.... use your eyes to line things up. And though it may not be important, you can turn the red on and off faster than the IR diodes.

Is there some advantage to the IR diodes I don't know about? Are they brighter?

George H.

On a sunny day (Tue, 11 Jan 2011 01:55:21 +0530) it happened "Pimpom" wrote in :

I have read of some amateur project that used a parabolic reflector and visible light (light bulb) on the transmission side, telescope on the other end. It is possible to slowly modulate a light bulb, or perhaps faster with some LCD shutters in front (from some LCD 3D glasses). Then you could use a powerful car headlight :-) LCD glasses can do 50 on / off flips per second at least mine can. Kerr cell ? IIRC. Hey I just invented that LCD thing. Signal lights like that are as old as the world, and with a normal telescope can bee seen far away, and are not dangerous.

IR has more attenuation than radio, but it could work.

Sounds OK, but then again, the proof of the pudding is in the eating. My experiences with IR LEDs are not that hope giving, but it is many years (30 or so) ago. I was interested in this for transmitting simultaneous translation to a group of people with receivers from a big IR LED panel. Only indoors. These systems exist. But outdoors you will also get a lot of noise from daylight and other sources. Give it a try, I am curious. Anyways, for outdoors we went to radio.

SNIP

Because IR detecters are the 'preferred' reception device.... You can use 632nm laser, (class1e.g. ) and a fibre optic array to collect the beam.... as used by LaserRunner systems.

There's no spec for accuracy of beam alignment. Getting the body aligned would require snug holes in a plate or something like that.

Or just throw a 10W or 20W white LED at the problem (modulated and with higher peak power), or perhaps a 3W with a lens.

Don't know if they're brighter or not. But IR has at least one advantage: I can choose from a range of receiver modules that already incorporate amplification, AGC, interference suppression, etc.

If necessary, I guess I could use a digital camera to monitor the IR from the transmitter at the receiver's location and direct an assistant to adjust the transmitter's direction. However, I don't think that will be necessary as even the narrow-angle TSAL6100 LED outputs 90% of the axial intensity at 4 degrees off center. 4 deg is 7' at 100'. Yesterday, I marked out these distances in my compound and decided that it shouldn't be difficult to aim the transmitter with that level of accuracy.

All of this, of course, is *IF* the LED's peak radiation is along its axis, at least approximately. Which was really my original question.

You have to be careful with massed IR LEDs, as they can cause eye damage pretty easily! Be very careful with them...

Charlie

Yes, at least you can see how (literally) blinding a 20W white LED is, and avoid it, as when looking at the sun.