Increasing the distance of IR

I saw a circuit in an old issue of Nuts and Volts (Dec. 2004).
It talks about an infared transmitter and receiver circiut. The circuit
uses a TSAL4400 for the transmitter and states it's good for 15-feet.
What determines the distance besides testing a built device?
How would I increase the distance?
Thanks.
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Steve wrote:

My guess: the gear described in the article didn't include optics. Adding either convex lenses or parabolic mirrors at (preferably) both ends of the path will increase range a lot. A 2-3" diameter single-element lens is fine: put the LED or photodetector at the focal length. Oh ... and be careful not to point the assembly at the Sun!
I built a pair of IR walkie-talkies using "high-power" LEDs and silicon diode detectors that operated full-duplex (audio bandwidth) with 3" lenses over at least 100 foot range. Also a baseband video link with similar range and hardware. As I recall, the LED was rated at either 5 or 10mw: high power back in 1972.
You might find it useful to do initial setup with a visible-led and then switch to the higher-power IR device.
Roby
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