IR light for IR LED - doubt

how will IR light from an IR LED propagate ?
like this below



or




what should i do if i need to place two separate IR transmitters and receivers closely and isolated from each other?
please help.. thanks in advance..

The second drawing is true. Use lenses to focus and narrow the beam.

how will IR light from an IR LED propagate ?

Click to expand...

Google IR-led manufacturers datasheet, you find
technical data there
e.g. Relative Radiant Intensity vs. Angular Displacement



example
**broken link removed**

As kak111's post indicates, major IR LED manufacturers make various models to suit different requirements and their radiation characteristics are shown on datasheets. For example, when I need a narrow beam and/or a strong one, I use Vishay's TSAL6100. When I want a broader but still quite strong beam, I use a TSAL6200, also by Vishay. An advantage of using IR LEDs (and other products) made by a major company is that they have detailed datasheets and such datasheets are reliable.

Some time ago, I did a major project in which one section required several IR links outdoors in broad daylight. At first I used no-name IR LEDs from the local market and I could barely get a reliable link at 5m. After some investigation, I found that not only were the LEDs inefficient, but they also focussed their emissions in directions way off the physical axis. Later, I replaced them with TSAL6100 and could easily set up rock-solid links at more than 20m without any change in the circuit.

Majority of 3mm or 5mm IR LED's will have 15-30 degree beam unless modified by optics. If it happens to travel in the way you shown in the first instance, it would have been a laser.
Cheers

Oops, I just noticed that I didn't answer one of the OP's main questions:

what should i do if i need to place two separate IR transmitters and receivers closely and isolated from each other?

Click to expand...

How close do they have to be together? And what's the distance between Tx and Rx? The solution depends a lot on such factors.

You can aim a remote control at various angles from a tv, to get an idea of the radiating pattern. That is for an IR led which is designed for that purpose.

Radio Shack did (or does) sell an IR detector card that can be used to test remote controls. I don't know how well it works. Maybe it will show the dispersion pattern.

Is this for a portable system that will be taken down and set up? Rather than try to aim an invisible light, I have used an LED which the catalog specifically described as narrow-beam 36000 mcd orange or yellow. It projected a spot a couple feet diameter, easily visible, on a wall at a distance of thirty feet.

Before experimenting I thought I could put a small lens directly in front of the sending led, and confine its beam so narrowly that most of its beam would land on a light detector 20 or 30 feet away. I tried but found it is impossible to do with a small lens. The spot it projects is large, big as a dinner plate.

I found the narrow visible beam sending led worked best. I used an inexpensive lens on the detector board to focus the beam on the photodetector. The lens was plastic, 1.5 inch diameter.